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Posted By: Magnetoman Restoring a Rotating Armature Magneto - 07/29/12 11:04 pm
Restoring a Rotating Armature Magneto

Table of Contents
(Note: I added this Table of Contents after uploading the final installment of this thread. Clicking on an underlined link will open a new window with the individual post)

Table of Contents, Background, Introduction

Initial Inspection and Initial Tests

Disassembly and Detailed Inspection

Disassembly and Detailed Inspection (Continued)

Disassembly and Detailed Inspection (Continued)

Repairing Damage Done to Armature by Previous Restorer

A Sidebar About Screw Threads

Repairing Damage Done to Armature by Previous Restorer (Continued)

Preparing the End Cap
-- Removing the Bad Condenser
-- Testing the Armature's End Cap

Tests and Repairs of the Electrical Components
-- The Slip ring
-- The Coil
-- The Condenser
-- Sidebar About Replacement Magneto Condensers

Tests and Repairs of the Electrical Components (Continued)
-- Reassembling the Armature
-- Testing the Reassembled Armature
-- Brushes

Tests and Repairs of the Electrical Components (Continued)
-- Brush Spring Pressure
-- Contact Breaker Points

Mechanical Components
-- The Bearings

Mechanical Components (Continued)
-- The Cam

Reassembly, Remagnetizing, and Extended Stress Tests
-- End Float
-- Contact Spring Pressure
-- Magnetizing
-- Sidebar about Magneto "Chargers"

Reassembly, Remagnetizing, and Extended Stress Tests (Continued)
-- Strobotac

Reassembly, Remagnetizing, and Extended Stress Tests (Continued)
-- Long-Term Tester
-- Elevated Temperature Test

Final Tests
-- Low Speed Test
-- Distributor Tester
-- Disassembly, Inspection, Remagnetization, and Reassembly
-- Timing a Magneto Using an Inductance Meter
-- Performance of the Magneto on the Road

-- The Major Problem Areas in Restored Magnetos
-- Tools
-- Magneto Repairers
-- Armature Winders
-- How Much Should it Cost to Have Your Magneto Rebuilt?
-- Cosmetics
-- Final Comments

Appendix I: Post-WWII Magneto Condensers

Appendix II: Replacement Condensers for Post-WWII Magnetos

Appendix III: Anatomy of a Post-WWII Lucas K2F Magneto

Appendix IV: Diagnosing a Failed Aftermarket Slip Ring

Appendix V: An Alternative Replacement Condenser

Appendix VI: Adapter to Attach Points Plate to Condenser

Appendix VII: Rebuilding a Lucas KNC1 Competition Magneto

Appendix VIII: Rebuilding a Lucas Magdyno

This "bike project" will be different than most on this site, in that it will be about just one component. However, the magneto is a component that is of interest to owners of a variety of classic American, British, and Continental machines.

In addition to Harley-Davidson and Indian (and others), Bosch magnetos also were used on a number of very early British bikes as well as ones made on the Continent. Also, all Lucas and BTH rotating armature magnetos are direct copies of the Bosch design, which was appropriated by the Allies at the start of WWI. For these reasons, my restoration of a c1920 Bosch ZEV might be of interest to a variety of motorcyclists. Because of this, although the "original" will be on BritBike Forum (, I have given permission for it to be cross-posted on and However, those are the only locations that have permission to post copies of this material.

While I won't be able to reply to individual emails, people who are reading this on other than BritBike Forum can send questions or comments to [email protected] and I will address them in future posts if they are of sufficiently broad interest. To save people time, no, I am unable to recommend someone to restore your magneto. Sorry.

Recently a good friend purchased a 1923 Harley-Davidson Model F twin to use for an event. He shipped the bike to someone to rebuild for him but, despite it having a freshly rebuilt Bosch ZEV magneto, he asked me to check it out while the engine was being rebuilt. Unfortunately, the magneto was non-functional when it arrived at my house even though it appeared to have had 0 hours on it since it had been "professionally rebuilt."

Although I finished rebuilding it a few weeks ago, only now am I finding the time to start converting my notes to prose. I'll post this in installments as I complete each segment, and will try to write it as if it were a "real time" narrative, but it probably will end up in both present and past tense.

Because my friend had specific plans for his motorcycle, I was working to a hard deadline to get this magneto fixed, so I didn't always interrupt my work flow to photograph every step. However, so there wouldn't be gaps in the visuals, I later made additional photographs to reconstruct the missing steps. I've noted these where ever they occur, such as the following of a single-cylinder Bosch ZE1.

[Linked Image]

[Linked Image]

The ZE1 shares the same construction and many parts with the V-twin ZEV, so the magneto I rebuilt looks almost identical to it.

Although I try to work only on my own magnetos, I've disassembled about a dozen "professionally restored" magnetos over the past fifteen years. Without exception, what I found inside each of these failed magnetos in no way deserved the term "professional." I'm not going to name names, but two of those failed magnetos were rebuilt by people who many riders of classic British motorcycles would recognize (I don't know/remember who restored the others). However, it's important to add that I've never looked inside a restored one that was functioning properly, so what I've observed cannot be taken as representative of the work of all magneto restorers.

My goal in restoring any magneto, including this one, is simply to return it to the condition it had when it left the factory. No better and no worse. In some cases improvements are required because original components or materials are not available, or there is some intrinsic defect that has to be addressed. For example, I use a modern resin on the coil rather than whatever old resin was used 50+ years ago, because those old resins had several undesirable characteristics.

The steps I will describe in subsequent posts are the ones my experience tells me are necessary to return a magneto to its original performance. To do less would be to produce a magneto that is less reliable, has a shorter life, and/or produces a lower output. As I think will be clear from my subsequent posts in this thread, the magneto repair and testing equipment I routinely use to accomplish this are well beyond those employed by people who do this for a living. Whether someone could find customers willing to pay what it would cost to restore a magneto as I will describe is not an issue, because I am my only customer.

I'm guessing that this magneto restoration tale will take a dozen installments to tell, not counting responses to any questions, and it might take two months to get to the end. So, please be patient.
Send questions or comments to [email protected].


When I did my initial inspection of the Bosch ZEV I found the lock nut on the fixed point was loose and the gap was ~0.025", which is about 0.010" too large. A shim under part of the points assembly was in the wrong position resulting in the points being offset from each other by ~1/4 their diameter. I didn't photograph it that day, but the next day I reconstructed it roughly as it had looked, which is shown in the next photo.

[Linked Image]

Since the slip ring was new, and its condition indicated the magneto could not have run for more than an hour at most, it's possible the previous owner tried adjusting the points when the magneto failed and this is why they were loose. However, whether they were left this way by the restorer (which seems unlikely), or by the person who sold the motorcycle to my friend (more plausible), I don't know.

Another problem with the points that is not apparent from the above photograph is that the ID of one of the insulators was too small to fit over the OD of another, leaving a space between it and the brass housing. It's a little difficult to see, but from right to left at the bottom of the next photograph is the head of the screw that mounts the fixed point assembly to the rotating base, a brass washer, a flat insulator (with the gap mentioned above between it and the brass assembly), the assembly itself, and a cylindrical insulator that keeps the mounting screw from shorting the assembly to the rotating base. The OD of that cylindrical insulator is larger than the ID of the other insulator, resulting in the gap. Because this can be very easily fixed by slightly enlarging the ID of the flat insulator with a round file, the fact the restorer didn't notice it is the first of the direct evidence we have about the "quality" of his work.

[Linked Image]

The magnets didn't feel as strong as they should have felt, but this is hard to be sure of without an actual measurement. However, it wasn't worth the time it would have taken to measure the field strength with my magnetometer since I already knew I would be completely disassembling the magneto. Although the Alnico magnets in post-WWII magnetos retain enough of their magnetism for the magnetos to still function (although, at reduced capacity) after a restorer removes the armature and replaces it, that isn't the case for steel magnets. If it does have reduced magnetism in the present form it likely means the restorer did not understand this essential point about pre-Alnico magnets, or didn't have the facility to remagnetize them. Or both. But, taking it apart means I will have to remagnetize it no matter what.

The brass inside the points plate was not tarnished, indicating the magneto was restored sometime relatively recently (i.e. within the last decade or so). I assembled everything in the points cavity in their proper positions, set my test rig with the minimum gap (so it would spark even with weak magnets), and fired it up to see what things were like. The magneto exhibited classic bad condenser sparking at the points almost immediately (it probably had already failed before I even looked, but it took maybe 30 sec. to make sure everything was properly spinning before I looked at the points).

The next photograph shows the rig I use for extended tests. I built it around a 1/2-hp, 1725 rpm motor that operates either clockwise or counter-clockwise, and it has a board with adjustable gaps for magnetos of up to six-cylinders. Adjustable plates and brackets allow all types of platform- and flange-mounted magnetos to be easily attached.

[Linked Image]
Originally Posted by Alex
Nice writeup. Thanks.
Thanks very much. I'll be spending a lot of time on airplanes this month, but some of that time should allow me to get material in shape for some of the upcoming posts in this restoration thread.
Posted By: Mattsta Re: Restoring a Rotating Armature Magneto - 08/03/12 7:11 am
Originally Posted by Alex
Nice writeup. Thanks.


We are lucky to have someone on the forum prepared to share this kind of information
Originally Posted by Mattsta
We are lucky to have someone on the forum prepared to share this kind of information
Thanks for the comment. I'll be away on a ride this weekend, but should have my next post ready to upload by early in the week.

I think people will find the information in the upcoming posts pretty useful. I have never seen anything as comprehensive as what I will be posting published anywhere before. Even for someone who doesn't plan to rebuild his own magneto, my hope is that having the mystery removed will help raise their level of expectations and, as a result, the quality of restoration work. As long as so many motorcyclists accept having their rebuilt magnetos fail, and even regard it as somehow inevitable, the bar will remain set pretty low for work done on these devices.
Send questions or comments to [email protected].


After my initial inspection and test to characterize the condition of this magneto when it arrived, I disassembled it, making notes of what I found to refer to when I started the actual restoration. Disassembly was quite easy. Two screws hold each HT pickup, and a bolt presses the points assembly into a taper in the end of the armature. If this were a later BTH or Lucas magneto there also would have been a safety gap screw to remove (failing to remove it accounts for the characteristic piece broken from so many slip rings).

The photograph in the first of these posts shows there is fairly large gap between the points assembly and the housing so I tried gently prying at the edges and it immediately popped off. Had it not come off so easily, and rather than prying with ever-bigger screwdrivers, I would have taken the time to fabricate an appropriate "pusher" to slip under the assembly and that used the housing to push against (a two-jaw puller would have to push against the screw threads in the armature, which would be fine as long as a the assembly released before the threads had too much force on them). BTH and Lucas thoughtfully provided threaded holes in their later points assemblies that are used to push them off the armature using the same bolt that holds them in place. After removing the points assembly, and the four screws holding the front plate, the armature was free to be removed.

Overall the components were in reasonable condition for their age, although there were some issues to think over before attacking the rebuild:

1) It has a new slip ring, which looks good. However, some aftermarket slip rings were made with inappropriate plastic that is partially conductive so this requires measurement with a megohmmeter.

Had the slip ring been chipped I have a Teflon mould I use to repair ones on BTH and Lucas magnetos using a non-conductive epoxy with 3000 psi shear strength. If I had needed a mould for this restoration, but had this one not been the correct size for the Bosch, I would have made one specifically for it. The photograph shows the mould on the left, a new Lucas slip ring on the right, and the chipped slip ring that is on a spare Bosch ZEV armature. However, for a slip ring as damaged as the one in this photograph I would fabricate the missing portion on my lathe and attach it to the existing piece rather than build it up entirely with epoxy (unless it were an "emergency" field repair).

[Linked Image]

To remove the slip ring on this armature would have first required removing the bearing race. Although I didn't need to do this, I will describe in a later post how it is done.

2) The bearings appear to be new, but I will inspect the races and the balls with a microscope for any evidence of Brinelling.

3) Although I suspected the leads were of the cotton resistance type, because they felt so limp, their resistances were less than 1 Ohm so they must be made with copper wire. However, even though the insulation looks a bit old I will have to leave it to the engine rebuilder to replace these leads once the magneto is installed, because I can't guess the correct lengths to use. This will be in the "installation notes" I will send to the engine rebuilder when I return the rebuilt magneto to him. I also attached a yellow tag (from an office supply store) with a note to one of the leads to be sure it wasn't overlooked.

4) The HT pickups are old, but look to be OK. I made a note to do a more careful inspection of them later, but all three brushes in the magneto looked like they were new.

Unfortunately, there are aftermarket brushes that are much harder than OEM, and these would grind a groove into the slip ring if this is what is in this magneto. An example of this is shown in the next photograph, which is on a BTH armature that was in a box of parts I bought a few years ago.

[Linked Image]

While the above photograph shows the damage hard brushes can cause, soft aftermarket brushes also are a problem. These slough off carbon too fast and can cause an internal short in less than a few hundred miles. Since there is no sign this magneto ever has functioned since being rebuilt, there is no way to know from the appearance of the slip ring itself whether the brushes have the proper hardness. Since this is problematic, I will replace all three with NOSs Lucas brushes if the diameters allow that to be done. If not, and although I am equipped to measure Rockwell C hardness and could convert to the Shore scale used for materials like carbon, the Rockwell test shocks the sample with a pointed indenter so it is not used on brittle materials. Since this measurement itself could destroy the brush, if the Lucas brushes won't fit, I have my own tests I will make. Although not as precise as a proper hardness test, it still allows me to determine whether they are appropriate to use or not.

[detailed inspection to be continued]
Posted By: Tiger100 Re: Restoring a Rotating Armature Magneto - 08/10/12 9:52 am
Totally interesting article MM, I look forward to the next section.
Originally Posted by Tiger100
Totally interesting article MM, I look forward to the next section.
Thanks very much. Because of my travel schedule, it looks like I will be posting sections roughly one per week. The best is yet to come.

5) The magneto has a rewound coil in it, as well as a new condenser (although, as I wrote in a previous post, the latter already had failed). The electrical properties of the primary and secondary with the armature in the magneto were:

Primary___: R= 0.58 Ohms; L= 3.40 mH
Secondary: R= 4.67 kOhms; L=16.38 H

The inductances would be somewhat lower if outside the housing. For comparison, the primary of one of my two spare ZEV armatures is still good, and it has essentially identical values of R=0.57 Ohms and L=3.52 mH (next photograph).

[Linked Image]

Although I don't have a functioning Bosch ZEV secondary to measure, the resistance of the secondary is reasonable, although the inductance is about 50% higher than that of a comparable Lucas armature. My conclusion was that the person who rewound this coil neither made it with significantly too many windings of too fine wire, nor too few of too coarse.

--------- Sidebar (not in chronological sequence) ---------
I am writing the following three paragraphs after the fact. I realized after having sent the magneto back that I should have measured the transformer ratio, which would have told me if the secondary has the correct number of turns. All that is required is to run a small AC voltage from a signal generator through the primary and measure the voltage developed in the secondary. This transformer ratio is ~35 for similar Lucas and BTH magneto armatures. The actual ratio of turns is 50, but the transformer loss due to generation of eddy currents is ~30%.

The reason the number of turns in the secondary concerns me is that, not having x-ray vision, I have no way of knowing if the rewinder used insulation between its layers. Not doing so would result in a less robust coil. The R and L values for the secondary are consistent with wire that is ~20% too large in diameter, but with ~50% too many turns. Combined, that would give the correct resistance, but an inductance that was 50% too high. The resistance and the inductance of the primary are as they should be, consistent with the primary being made up of the correct number of turns (which determines L) of the correct diameter wire (which determines R).

In order for a coil with too many turns of too-large diameter wire to fit in the volume available means interlayer insulation likely would have been omitted. Omitting this insulation makes it easier to wind coils, which is all the more reason to think it might not be there. The secondary winding would have about 30 layers, and in normal operation the coil output would be limited by the spark plug to no more than 6 kV. This means there would be ~200 V between adjacent layers, which the insulation would be able to handle without problem. But, if a plug wire fell off and the voltage rose to 18 kV, adjacent layers of the secondary would have to withstand ~600 V. That still is within the limit a good quality insulation can withstand. But, I don't know the quality of the insulation, or if the coil was vacuum impregnated so the insulation can't abrade. Unfortunately, the limited time I had to get this magneto back to the engine builder made me overlook this test that I otherwise would have thought to make, so this will be a lingering uncertainty.
--------- end of sidebar ---------

Unfortunately, the restorer got carried away with his use of epoxy on the coil. The coil itself is not only potted with epoxy, it is firmly attached to the armature by excess. However, what is not possible to tell is if only the outside of the coil had been slathered with epoxy, or if it had been vacuum impregnated. I made a note of this because it is a concern, but I will have to think about how I want to deal with the uncertainty.

The reason vacuum impregnation is an issue is that the motion of any wire through a magnetic field subjects it to a sideways force. If the armature coils are tightly wound, there is no place for any of them to move, so that force wouldn't matter. However, if there is any room for movement at all, the oscillating motion due to the field changing direction twice every revolution can result in the insulation on a wire slowly abrading away, eventually shorting it to its neighbor. Such shorts do not necessarily cause the armature to fail completely, but they do degrade the output. Vacuum impregnation using an appropriate low viscosity epoxy or other potting compound fills the voids, making motion of the wires impossible.

Winding an armature coil is a tedious job, and I don't begrudge the ~$150 that people charge for doing this. However, I do begrudge the quality of some of the coils that are produced. Proper coil winding is a craft, and not all magneto coils I have seen were wound by people who mastered it. I'm reminded of something I learned from an instrument maker 40 years ago: "There's a difference between handmade, and homemade." Too many coils I've seen over the past 15 years are of homemade quality (and, again, I've only seen the insides of magnetos that have failed, so this is not a representative sampling).

Magneto armatures were not my first exposure to coil winding, so my disappointment with the quality of rewound armatures is based on first-hand knowledge of what is possible to do oneself. In any case, I felt I had no choice but to purchase the equipment needed to do this in my own workshop. Having a coil winder meant I was no longer dependent on anyone but myself for any aspect of a restoration. Anyway, I have my own German-made coil winder as well as equipment for vacuum impregnation of potting compound. However, the short time to get this magneto restored, interrupted by the trips I have to take before it has to be shipped back, means I would much rather use the existing coil if it survives my stress tests (to be described later).

I haven't used my coil winder for a few months so the area around it has accumulated some clutter. But, the photograph shows it with the left half of the pair of brackets I use to hold armature like the one in the Bosch (and Lucas and BTH). A mating piece is held by the tailstock whose dead center can be seen at the edge of the photograph. The very thin wire of the secondary is fed from the spool through a spring-loaded tensioner to help keep it from breaking (partially shown at the top, with a pulley at the end of a pink rod that is connected to the tensioning mechanism), and is automatically layered by an auto-reversing mechanism whose pitch is adjusted to match the diameter of the wire. The speed is controlled by a rheostat in the foot pedal to allow starts and stops to be made smoothly, and a counter keeps track of progress.

[Linked Image]

After winding a coil I pot it with an appropriate low viscosity compound and use a vacuum pump to impregnate the liquid into the voids between the windings. I use thick-walled plastic containers to hold the compound and the armature under vacuum, allowing me to see what is going on without the risk of implosion if a glass container cracked.

I set up the next photograph using a partially-disassembled armature in the plastic container to show the scale. I am not being coy about not naming the potting compound I use, but recently I've been experimenting with several new ones and don't want to "endorse" any until my tests of those are completed.

[Linked Image]

The excess epoxy on the present coil would be OK (except, see below), except part of the coil is slightly too large and has rubbed on the inside surface of the body. I can't tell at this point in the restoration if it is simply excess insulation that is rubbing, or if wire is involved. But, it's possible to check the output of the coil with my Merc-o-tronic tester. The output of the coil can be seen sparking across a 5 mm gap in the window in the tester, so it passed this test (however, I'll do a longer test of just the coil itself before declaring it to be good).

[Linked Image]

As an aside, I have at least a dozen magneto testers, but I almost always turn to a Merc-o-tronic 98 or 9800 first when checking a coil. Although these can make six types of tests, all but one of them -- the current required to generate a spark across a 5 mm gap -- can be better made with more modern instruments. However, if I only could have one instrument for testing magnetos, this is what I would choose.

6) As long as I can remove the end cap that contains the condenser, removing the epoxy that encases it, although a headache, and replacing it with a proper capacitor should be straightforward (note, the photograph above was taken during a later test I made after I had removed the end cap).

Send questions or comments to [email protected]

[detailed inspection to be contined]

7) In the next photograph the protrusion at 7:00 in the taper locates the points block with respect to the armature to ensure the magnetic flux reversal coincides with the opening of the points. The missing chunk at 11:00 in the taper appears to be damage caused by some heavy-handed restorer. The black cylinder at 4:00 is a carbon brush that completes the secondary circuit back to earth when the points open. To do this, the carbon brush has to make good contact with the surface it rubs against.

[Linked Image]

Unfortunately, as can be seen in the next photograph, the surface the earth brush rides against is in pretty poor condition. Although the brush can make a good electrical connection to such a surface, its roughness will wear away the brush more rapidly than it should, eventually resulting in failure of the magneto. I will have to turn it on my lathe to make it smooth.

[Linked Image]

Since this surface needs to be parallel to the back surface of the plate holding it I tapped four holes in a piece of scrap Al, faced it in the lathe, then mounted this plate on it in order to face the surface for the earth brush.

After turning any rubbing surface on the lathe I use my surface roughness meter to verify it has the correct smoothness. I didn't take a photograph after turning the ZEV surface, but the following one shows the roughness meter being used on the ring used for the earthing brush on a Lucas armature from my box of spares. I had the meter set to display the roughness in micro-in. (59 micro-in. is 1.5 um). In fact, a "perfectly smooth" surface is not desirable since some abrasion of the carbon during operation is required to ensure good electrical contact. Industry practice for the roughness of commutators calls for values 1.5-2 um, so the earthing ring on this Lucas armature happens to be fine as-is.

[Linked Image]

I measured the earthing ring on a BTH armature several years ago when I restored the magneto for another friend. It had failed shortly after he paid quite a bit to have it professionally restored by someone whose name would be familiar to many riders. The roughness measured 190-210 micro-in., which is nearly 3x too high. The grooves the restorer left in it by using a too-sharp lathe tool would have abraded the earth brush too rapidly had the inappropriate condenser he installed not failed. In fact, the grooves were so pronounced that it was easily possible to determine the feed rate he used on his lathe. I still have this armature because it was easier for me to rebuild an unrestored armature I had than it would have been to spend the time to first undo what the restorer had done to it.

8) The condenser in the Bosch armature is completely potted in the end cap using some red epoxy that I'll have to remove. If you look closely you can see that the clear epoxy used to pot the coil also overlaps with the armature, firmly attaching it. This excess epoxy is sloppy workmanship and would significantly add to the effort required to prepare this armature if I had to wind a new coil on it.

[Linked Image]

9) A problematic aspect of the armature itself is that some of its laminations have suffered at the hands of the restorer (or an even earlier restorer -- there is no way to know when over the past 90 years some of the damage was done). Armatures are built up from laminated steel plates that are insulated from each other to minimize eddy current losses that otherwise would severely reduce the output from the magneto, especially at low rpm. Someone beat quite a bit on one end of the plates on one side, and there are two holes drilled 180-deg. apart in them as well. I don't have any prior experience with Bosch ZEV magnetos to know whether or not these holes were drilled at the factory for some reason, but neither of my spares have them. Also, although the hole in the above photograph appears to be round, the one on the opposite side is fairly crude.

I have two Bosch ZEV armatures of my own, both with their original, rotten coils, that I could press into service for winding new coils if needed. But, no matter which armature I end up using, there are issues to consider. I'm going to think about this for a day before proceeding. The armature I am repairing is in the first photograph, and one of my own is in the second.

[Linked Image]

[Linked Image]

In contrast to the armature I am working on, the photograph of mine shows what an unmolested one looks like (complete with 100 years of patina). However, the coil in mine is original, looking about as fresh as the wrappings on a mummy in the British Museum.

The dents in the armature will have shorted the outer edges of those laminations together. Also, the holes someone drilled through the laminations in the center will have shorted those laminations that are in contact with the holes. Although these are electrical shorts, each increases the eddy current losses rather than stopping the output entirely, so it's a judgment call at this point whether their cumulative effect on the magneto output at low rpm will be unacceptable. All things considered, my experience tells me it is worthwhile proceeding with this armature despite the damage to the laminations.

Despite the issues noted in the past several posts, detailed inspection has not revealed any problems that would keep me from restoring this magneto to full operation. Unfortunately, its poor condition means it will take longer than the couple of days I had hoped would be the case had it been in good shape, since first I will have to undo all the damage done by the person who restored it. But, now have to decide how to proceed (e.g. use the present armature if tests show it is good, or wind my own replacement coil using one of my old armatures). Send questions or comments to [email protected]
Posted By: Magnetoman Re: Restoring a Rotating Armature Magneto - 08/28/12 11:59 pm

The story of what the professional restorer did to this magneto just keeps getting better…

Although the end cap came off without difficulty, often they are a very tight fit. Because of this, I designed a set of separators consisting of four pieces held in place with hose clamps that lets me gently press armatures apart by turning two screws. The next photograph uses one of my spare ZEV armatures to show how it works.

[Linked Image]

The next photograph shows the end containing the condenser of the actual armature I am rebuilding, with the four screws holding this end cap in place. Or, so it appears. The photograph also shows a screwdriver set with bits in small increments of thickness and width to perfectly match the slot in any screw head, as well as a small impact driver for those bits if any extra encouragement is needed (which it wasn't in this case). Depending on circumstances I use a 1/4"-drive air impact driver (35 ft-lb. max.), but this 1/4"-drive manual one also is perfect for "emergency" repairs away from home.

[Linked Image]

Even though the heads had been staked to keep them from coming loose, the screws came out easily. Too easily. Normally, staking them would seem to be the mark of careful work, except....

...three of the screws were broken!!! The staking held the heads in place so it looked like the screws were as they should be, but they weren't. Comparing the screws to those in my spare ZEV armatures shows the three broken ones are the originals, while the one screw doing all the work is a modern replacement. But, that's not the end of the bodge.

[Linked Image]

The only screw from the set of four whose diameter and pitch I can accurately measure is the unbroken one. As can be seen, it's threaded all the way to the head, whereas the three broken ones are not, showing the one good screw isn't original. This screw's OD is close to the fairly uncommon U.S. size 6-40, but according to my pitch gauges it's close to, but not quite, 38 tpi. It can't be metric, because its diameter is ~3.6 mm, and its pitch would correspond to an implausible ~0.66 mm… Damn!, after breaking three screws the magneto repairman crammed a British 4 BA screw in the one good hole, and this is all that was holding it together!! The condition of the slip ring indicates the magneto couldn't have worked for more than a few minutes after having been rebuilt, which is a good thing because this single screw would have been the only thing keeping the armature from flying apart while it was spinning at up to ~2000 rpm.

This motorcycle lived on the British Isles in recent years, which is why it didn't take me too long to check for BA threads. This is truly an international magneto: designed in Germany, manufactured in America, and bodged in England.

Luckily, I have a mill, so getting the three screws out shouldn't be too much of an issue, although it would take time. First, I had to fabricate a fixture with 2.000" ID at one end of it to hold the armature precisely vertical in the mill. I turned this on my lathe, as well as a base with a 5/16" hole to centrally locate the end of the armature in the fixture, and slit the fixture on the mill. Send questions or comments to [email protected]

[Linked Image]
Although pitch gauges were indispensible for identifying the threads used in this magneto, so were three types of microscopes. Because of the broken screws I had to spend a few hours researching threads before I could repair them, and what I found was that this magneto is filled with bastard threads, some of which are badly made. The interesting thing is that, notwithstanding the BA screw, this seems to be how it came from the factory.

Although the magneto was designed in Germany, it was made in the U.S. and was worked on in the UK so there is a mix of fasteners on it. There are components on the magneto that are definitely of German ancestry, but others that are American. For example, the diameters of both ends of three armatures are 2.000" +/-0.003". I would have to get my hands on a ZEV that I knew was made in Germany to be sure but, presumably, it would be 50.0mm = 1.969". Also, since it was made nearly 100 years ago, I can't assume the fasteners were manufactured perfectly in the first place, so both the diameters and pitches could vary slightly from what they should be.

There are at least 13 different functions performed by screws on this magneto (not 13 different thread forms, though). I've analyzed 7 of them, requiring three microscopes and several precision thread gauges to come to the following conclusions. It wasn't straightforward getting there, and I doubt many, if any, other people know what I'm about to tell you. First, though, a tiny bit of history.

What we now know as "metric" threads is an invention of c1965, with various national metric standards used before then. In some cases the fasteners made to the old standards are a good fit with modern ISO metric, but it some cases they are not. It happens that c1890 the Germans adopted the Loewenherz standard for small screws for use in instruments. This is the same function numbered screws serve for the U.S. (e.g. 6-32) and BA for the British, the latter of which was a standard adopted straight from the Swiss Thury system with one trivial modification.

Anyway, the smallest screw I measured was one that holds the earth/ground lead for the condenser in one of my spare ZEV armatures (an unknown screw was in this hole in my friend's armature). Although it is 2.50 mm in diameter, it has a pitch of exactly 0.45 mm, making it a slightly inaccurate Loewenherz 2.6x0.45 (the next size smaller in the series is an L2.3x0.40, whose pitch makes it an impossible match).

Next up in size are the screws for the HT pickups. I started with these, and they caused me no end of grief before I came to a conclusion. Their diameter is 0.159", whereas 4.0 mm would be trivially smaller at 0.1575". However, if they are L4 they should have a pitch of 0.75 mm. Unfortunately, as I finally concluded, pre-War factory standards must have slowly degraded by the time this magneto was made nearly a decade after Bosch's U.S. assets had been seized, resulting in a bastard version of the L4. The pitch is slightly finer than 0.75 mm (which would be 33.86 tpi), and it is also finer than 34 tpi, but coarser than 36 tpi. I could find no reference in an old edition of Machinery's Handbook to any standard U.S. screw ever being made with 35 tpi. But, since less than 1/4" of thread engages in the housing, even if the holes they go into were made with a perfect L4x0.75 tap, and the pitch of the screws were as far off as far as being 35 tpi, the total mismatch would be only ~1/4 thread, so they would go in with only a little difficulty at the end of their travel. However the fact the factory didn't just adopt the nearest American screw for the armatures indicates that after WWI they instead continued working with ever-more-worn German tooling and using ever-more-out-of-spec replacements.

Next up in size are the armature screws, with diameter 0.1345". Here both 3.5 mm (0.1378") and the old U.S. 9/64" (0.1406") are potential candidates. An L3.5 mm screw has a pitch of 0.6 mm (42.33 tpi), but also should have flat roots and crests each of width 3/8 of the pitch, and a 53-deg. 8 min. included angle (vs. 60-deg. for the American thread). Here, the (in)accuracy of pitch gauges were not up to the task of distinguishing between threads differing by less than 1%. Because these screws were broken and I needed to find a suitable tap to repair the armature, I had to make careful measurements using three types of microscopes. The illumination is from below in the following micrograph taken in a metallurgical microscope (i.e. the screw at the bottom, appearing as black).

[Linked Image]

Although the 55-deg. angle of these screws is close to that of the Loewenherz thread form, the roots and crests are not flat as they should be if properly made. To accurately determine the pitch I used a traveling microscope as shown in the next photograph. This instrument uses a precision micrometer thread to translate a 21x microscope over the specimen, allowing lengths up to 10 cm to be measured to a precision of 0.01 mm (0.0004"). This is shown in the next two photographs using a 6-32 screw for scale.

[Linked Image]

[Linked Image]

My measurements determined these screws are 42.0+/-0.1 tpi, not the almost identical 0.6 mm (42.33 tpi). However, the engagement in the armature is only 1/2" so the mismatch in this length would only be 0.16 of a thread even if the factory had used a perfect L3.5 tap. So, like the previous example, my conclusion is that this screw is a poor tolerance imitation of the German Loewenherz 3.5x0.6 thread, as opposed to being an American substitute.

Next, larger in size are the screws holding the covers. These are definitely 7/32"-24, which was an American standard at the time. The nearest Loewenherz screw has a diameter close enough, but the pitch differs by too much (28.22 tpi).

Finally, the four holes in the base that are used to mount it to the motorcycle are American 5/16"-18.

What the above shows is that when this magneto was made in the 1920s it left the factory with a mix of American and metric Loewenherz fasteners of dubious quality. Because of this, I couldn't make any assumptions about any thread without carefully measuring it. Send questions or comments to [email protected]

After determining the correct size of the armature screws (Loewenherz L3.5x0.6), I mounted the armature vertically in the mill's vise using the fixture described in a previous post that I had made for this purpose (shown in the third photograph, below). I then used a centering microscope to locate the positions of the three broken screws to +/-0.001" and recorded the x,y coordinates of the mill's Digital Read Out (DRO) so I could return to each of them after replacing the microscope with a carbide center drill. Meanwhile, I had ordered an M3.5x0.6 tap because this is an uncommon size and wasn't in my set of metric taps and dies (more on the difference between an M tap and an L thread below). There was plenty to do, so waiting a few days for the tap to arrive didn't cause any delay.

I didn't take photographs at the time when I was using the centering microscope, so I set it up again a few days later using a 6-32 screw, which has nearly the same diameter as the L3.5 mm.

[Linked Image]

[Linked Image]

Each of the concentric rings on the microscope's reticule differs from the next by 0.010", so it should be apparent that it is easy to use this microscope to center the screw directly under the spindle of the mill to quite a bit better than this (specifically, to within 0.001"). Depending on how the photograph is displayed on your screen, you may be able to see the 0.001" marks on the horizontal and vertical bars.

Having determined the screw sizes, and having located their centers to 0.001", I then drilled out the three broken screws. To do this I used a carbide center drill to start the holes on the jagged surfaces followed by a 7/64" drill, which is 5 thou. smaller than the tapping drill for M3.5x0.6. I used this drill because it's easier to make a too-small hole larger, if necessary, than vice versa.

[Linked Image]

As can be seen from the next photograph, enough of the threads were exposed by the drill, followed by use of a dental pick, that a tap could properly engage the old threads and clean them out. At this point I could have taken the time to modify a 60-deg. carbide threading insert to the 53-deg. 8 min. of a Lowenhertz thread and used it to make my own "perfect" L3.5 tap. However, from what I had found thus far it was likely the actual internal threads were an imperfect variation of L3.5 anyway. So, although the small difference in thread form between L3.5 and M3.5 means a tiny amount of material from the armature itself (rather than just the fragments of broken screws) might have been removed, in the overall scheme of things I think my use of an M3.5 tap should count as a perfect fix of these holes.

[Linked Image]

To answer a question asked by several people offline, at this point we're less than halfway through this magneto restoration, so there is quite a bit more to come. Send questions or comments to [email protected]

Removing the Bad Condenser

Even though the condenser the previous restorer used already had failed when I received the magneto, the failure mode of many condensers is to develop a relatively low resistance but to retain the capacitance value. The condenser in this magneto was 0.22 uF, while the original ones still in my two armatures are 0.12 uF. So, not only did the restorer use one that was inadequate to survive the high current pulses (which is a very common problem with the condensers many restorers use in magnetos, accounting for a high percentage of the failures of rebuilt magnetos), it had a capacitance that was too high.

As an aside, the reason the condensers in my two spare Bosch ZEV armatures are still operational after nearly 100 years is they are made of mica. This material is a very stable dielectric (the resistances of both condensers are still over 20 MOhms), but today mica is used only for specialty applications because of the high expense of fabrication as well as it having a relatively low dielectric constant (so the capacitors are large). However, in addition to it being difficult to remove these mica condensers from their cavities in the end caps in order to transfer them to another armature, I can't send my friend out on the road with a condenser that is nearly a century old. Although the mica is very stable, the thin metal electrodes do corrode. So, it was time to come to terms with the epoxy. An hour with my lathe and mill dealt with it, although creating quite a mess.

[Linked Image]

[Linked Image]

Before anyone mentions it, normally using a Jacobs chuck to hold an end mill is like using a Crescent wrench instead of a proper spanner. But, because epoxy is soft it only imposes a small side load on the chuck, and precise concentricity with the spindle wasn't important, so there was no reason to use proper collets. Also, I knew I would be using end mills of several sizes to remove the epoxy, and a Jacobs chuck made this messy work go faster.

Testing the Armature's End Cap

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It is not immediately obvious, although it is somewhat betrayed by a narrow moat of red epoxy, but the lower 2/3 of the base of the cavity in the photograph is a separate platform sitting on a thin sheet of insulating material. This platform is electrically connected to one side of the contact breaker points via the central mounting bolt that is threaded into the platform. Since the insulator has to withstand ~200 Volt pulses across the condenser every time the points open, I tested it with a high voltage megohmmeter to be sure all is well. As can be seen in the next photograph, the resistance between the platform at the bottom of the condenser cavity and the rest of the armature cap was 8.9 GOhm at 1000 Volts. This is 4000x higher resistance than is needed, and at 5x higher voltage than it will experience in operation.

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Although I've described quite a bit of work in my posts up to this point, keep in mind that everything I've done thus far has been just to undo the damage done by the "professional restorer." This is why I cringe every time I open a magneto and discover it already has been "restored." It is also why I concluded some years ago I had no choice but to restore them myself. Only after having undone the damage can I now start the work to make this magneto functional again. Send questions or comments to [email protected]
--------- Sidebar (not in chronological sequence) ---------
Spoiler Alert! -- don't read this sidebar if you want to wait until the final post to find out if this story has a happy ending.
The Bosch ZEV whose restoration I'm describing in this series of posts just finished the 2012 cross-country Cannonball Motorcycle Run. Unfortunately, the bike's rebuilder was able to finish it only a few days before it had to be transported to the start in New York so there was no time for my friend to give it a proper shakedown. The engine suffered a variety of problems that would have been easy to fix under different circumstances, but that resulted in the bike covering only ~800 miles over the course of the Run. However, the restored magneto was trouble free. Added to the ~1000 simulated miles I subjected it to before shipping it back to the engine builder (to be described in a later post), this restored 90-year old magneto has "travelled" nearly 1800 miles without problem thus far. I have every reason to expect it to be good for many thousands more.
------------- end of sidebar ---------------------


At this point everything that needed to be taken apart has been, all the damage done by the restorer repaired, and the parts required to rebuild/restore are in hand. What remains is to test the coil separately, and then rebuild the armature with a proper condenser. After that I'll put it back together, subject the full magneto to a lengthy series of stress tests, partially disassemble it so I can see if any problem areas had developed (e.g. excessive wear of the carbon brushes), and put it back together again.

The Slip Ring

[Linked Image]

To make sure the material used for the aftermarket slip ring isn't conductive I wrapped a number of turns of stainless steel wire around it and measured the resistance between the wire and the armature housing. I used this wire because I have a large spool of it and it isn't insulated, so it made contact with the metal on the slip ring. Because the slip ring in turn is connected to the coil, this test also measured whether there was any leakage from the coil to the armature. The resistance was over 11 GOhms with a 2500 volt potential applied, which is comparable (i.e. about half) to the voltage it will be subjected to in actual operation. This amount of leakage is completely negligible, so the material in the slip ring is good, as is the insulation between the coil and the armature.

The Coil

I use an Eisemann tester for extended tests of coils because it uses a simple DC motor and set of points to produce the spark (plus I use a modern power supply instead of a 6V battery). If anything in it wears out it will be much easier to replace than it would be to repair the circuitry of a tester like one of my Merc-o-tronics. I ran the coil for an hour on this tester with the gap set to 5 mm (6 kV). The coil works fine so I will use it instead of winding one of my own. Although it is important to test the coil at elevated temperature, because of time constraints I will wait to do this test on the finished magneto. I do have a concern about whether the coil was vacuum impregnated or not, but I am running out of time. Even if it was not vacuum impregnated, that does not mean the coil will fail, but this is an uncertainty I will have to accept given the limited time available.

[Linked Image]

The Condenser

----------- Sidebar About Replacement Magneto Condensers -----------
For at least 30 years there have been recommendations for a variety of replacement condensers and, figuratively speaking, for just as long the sides of roads have been littered with motorcycles whose magnetos have failed because of using them. Part of the reason for this is that manufacturers typically do not design capacitors for applications where they will be subjected to high current, high voltage pulses while being repeatedly cycled over a wide temperature range in the presence of moisture, organic vapors, ozone, and vibration, so they do not test them under these severe conditions.

Unfortunately, relying on recommendations -- from motorcyclists, suppliers, or professional restorers -- in selecting a replacement is quite risky. For example, several club magazines have recommended completely inappropriate capacitors from stores like Radio Shack that would fail within the first few moments of operation. For a number of years one well-known supplier sold replacement condensers with the claim "Modern substitute, very high specification, zero failure." Despite this claim, they failed in service. Still, it took a number of years before enough motorcyclists complained about failures for the supplier to cease selling them. Currently another supplier is advertising replacement condensers that are of a type not rated for pulsed high current applications, and whose dielectric layers are made of a porous oxide. Even if they survive the current pulses of a stressed magneto, no extended environmental testing has been done of them, so there is no way to even guess how long they might last in the ozone-rich atmosphere near the points before delamination or breakdown of the oxide might set in. For quite a while one well-known magneto rebuilder sold condensers to other well-known rebuilders at a high price that he claimed were custom manufactured to his rigorous specifications. Not only were they just inexpensive capacitors from which he had removed the markings, they also failed in service. A number of magnetos used in the 2010 cross-country Cannonball Motorcycle Run, all rebuilt by the same well-known restorer, failed due to bad condensers.

Despite the examples given above, and many others, condensers that later failed in service always were claimed at the time to function quite well. To paraphrase Bruce Springsteen's 'Magic', as far as replacement condensers are concerned, you are well advised to 'trust none of what you hear, and less of what is claimed.'

Condensers are not mysterious components that are doomed to failure, but a proper replacement definitely does have to have the right combination of electrical, mechanical, and materials properties in order to survive in a magneto. The Fall and Winter 2011 issues of The Antique Motorcycle magazine contain a two-part article by physicist Dr. Charles Falco explaining the science of why the wax paper condensers that Lucas used in their post-WWII magnetos have a limited lifetime, and detailing extensive electrical and environmental tests he made of particular Panasonic film/foil capacitors, concluding from those tests they would last under the harsh operating conditions of a magneto for the equivalent of at least 40 years or 140,000 miles. As well as Dr. Falco having no connection with any supplier that possibly might affect his recommendation, to the best of my knowledge these are the only replacement capacitors that have been subjected to any such battery of tests designed specifically to mimic actual operation. Because they have the proper electrical specifications and survived a year-long set of accelerated electrical and environmental tests conducted by an independent party, these are the ones I use. Luckily, I have a large stock of them, because they are now no longer in production. Until someone like him conducts tests to identify another, in-production replacement, if you use any capacitor other than these Panasonics it is at your peril.
----------- End of Sidebar -----------

The next photograph shows a complete condenser pack that is ready to install in a Lucas armature, consisting of two 0.082 uF Panasonic film/foil capacitors soldered in parallel to give 0.16 uF total. My notation on it shows I tested the pack after assembling it and found it had the correct capacitance and that its leakage resistance was greater than 20 GOhm when measured with 500 Volts (i.e. 1000x more than it needs to be, at a voltage at least 2x higher than it will experience in use). The pack I used in the Bosch was the same as this one, minus the mounting bracket, and I installed it in the end cap using the minimum amount of 3000 psi epoxy necessary to insure it would stay in place.

[Linked Image]

Unlike BTH and Lucas magnetos, the recess in the end cap of the Bosch is deep enough that I could have used a single 0.18 uF capacitor from the same Panasonic family instead. They are sufficiently fatter than the 0.082 uF that they can't be used in BTH and Lucas magnetos, so two of the smaller ones have to be connected in parallel instead. However, when I reached this point in restoring the Bosch it was more convenient to use one of the pre-soldered pairs that I already had on hand.

Although earlier I said that the original Bosch mica condensers have a slightly smaller value, at 0.12 uF, than the 0.16 uF I used, that's a bit too low for the properties of this rewound coil. The inductance of the original ZEV coil I have is such that 0.12 uF was not optimum for them, either. However, the mica condensers completely fill the cavity so it would have been impossible for Bosch to make their capacitance any higher than this.

Send questions or comments to [email protected]

Reassembling the Armature

Although I have every reason to expect this magneto to function for at least a few decades with only minor maintenance (e.g. new points and brushes at ~10,000 miles), it is good practice to avoid doing anything during any restoration that could not be easily undone in the future. Although I could use Loctite on the armature screws without technically violating this, instead I use a torque wrench followed by pinning the heads of the screws using a small punch on the surrounding brass, as was originally done to secure them. This is illustrated in the next two photographs, using as an example the head of a screw in one of my spare Bosch ZEV armatures, with the original pinning that was done by the factory.

[Linked Image]

The torque specifications for fasteners depend not only on the quality of the material used to make them, but also on the shapes of the heads. Because of this there is a fair amount of variation in the values quoted in handbooks for 3.5 mm steel screws, and in the end I decided on 10 in.-lb. as reasonable for these flathead screws of unknown quality steel. This value falls nicely in the range of one of my smaller torque wrenches.

[Linked Image]

Testing the Reassembled Armature

After reassembling the armature with the new capacitors I checked the output using one of my Merc-o-Tronic 9800 testers. The blue arc in the window of the tester shows the operation of the coil is reliable at a test current of just over 1 Amp supplied to the primary. This is as it should be, so the completed armature passes this test.

[Linked Image]


As I mentioned in an earlier post, many aftermarket brushes in circulation are too hard, and many are too soft. Since I don't know the origin of the brushes that already were in this magneto, I made a note during my initial inspection to replace them with NOS Lucas brushes if possible. Luckily, the Lucas brushes (0.193"-0.194" diameter) fit without problem in place of the 0.185"-dia. brushes that were in it, easily taking care of this issue.

Pursuing this a little further, I subsequently conducted a test to see how soft the brushes were. Pressing down with the pressure I would use to write with a pencil, I made a half-dozen lines side-by-side using a pencil and four brushes, as shown in the next photograph. Since I don't know how well the subtleties will reproduce in image, following it are my observations:

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-- No. 2 pencil (approximately the same as an HB pencil): This serves as a convenient hardness standard others could use for comparison with their own measurements.
-- "Bosch ZEV": Aftermarket brushes that were in the ZEV. These lines are perhaps a tiny bit lighter than the pencil, but comparable.
-- Lucas 451260 HT pickup brush: Significantly lighter than the aftermarket "ZEV" (note: the lines were so light that I pressed harder when making the middle line, which is why it is darker than the ones on either side of it).
-- Lucas 200737 dyno/generator brush: Same as aftermarket "ZEV."
-- Lucas 451260 magneto earthing brush: Same as aftermarket "ZEV."

The differences might be more easily seen in the following composite micrograph, where from the left the three lines are the No. 2 pencil, the aftermarket "ZEV" brush, and the Lucas HT brush. Although it may not be apparent in the photograph, in a stereomicroscope it even can be seen that the Lucas brush was hard enough to tear small fibers from the paper that are standing above the surface.

[Linked Image]

Although the above test isn't quantitative, it definitely shows that the brushes the "professional restorer" had installed were significantly softer than the ones Lucas supplied for this application. Also, although the other two Lucas brushes I tested are about the same hardness as those that came in the ZEV, they are meant to run on metal surfaces, not phenolic. My speculation is the reason many aftermarket HT brushes are soft is they were manufactured using the (incorrect) specifications for brushes intended to be used on copper or brass. Although phenolic is softer than these metals, it is more abrasive (because of this, carbide rather than high speed steel is commonly recommended for machining it). Send questions or comments to [email protected]
Overall, a very interesting and informative series of posts, Magnetoman. A few comments/queries:-

Originally Posted by Magnetoman - #450359 - 08/22, 2012, 4:13 am
The black cylinder at 4:00 is a carbon brush that completes the primary circuit back to earth when the points close. To do this, the carbon brush has to make good contact with the surface it rubs against.

Could you explain this peculiarity of the Bosch ZEV? In all other models of rotating-coil magneto I've examined, that black cylinder (the earth brush) is part of the secondary circuit, and it's doing its business primarily when the points are open to provide continuity for the plug current from the magneto body (and therefore the sparking plug body) back to the armature body bypassing the armature bearings and their insulators.

Originally Posted by Magnetoman - #455839 - 09/25, 2012, 10:03 am
To make sure the material used for the aftermarket slip ring isn't conductive I wrapped a number of turns of stainless steel wire around it and measured the resistance between the wire and the armature housing. ... Because the slip ring in turn is connected to the coil, this test also measured whether there was any leakage from the coil to the armature. The resistance was over 11 GOhms with a 2500 volt potential applied, which is comparable (i.e. about half) to the voltage it will be subjected to in actual operation. This amount of leakage is completely negligible, so the material in the slip ring is good, as is the insulation between the coil and the armature.

I'm not so sure you should draw the conclusion from this test that the slip ring is good. 11 gigaohms at 2500 volts can certainly break down to a far lower resistance at the higher operating voltage of the slip ring. Breakdown of aftermarket slip rings is indeed a problem, but in my experience it often only rears its head at elevated temperature. A slip ring may test good at room temperature, but start breaking down at operating temperature.

I am also concerned that, with this test, you are applying the 2500 volts not only to the slip ring conductor and the HT tail of the HT winding, but also to the whole of the HT winding and more worryingly the LT winding. In operation, the insulation around the LT winding sees voltages of perhaps 250 V, and it might be fair enough to test it to three times that voltage, but testing it to ten times that voltage is, IMHO, asking for trouble. Even if it passes the test, the test may itself trigger the onset of a later breakdown. If you were a professional restorer and if you were to do that, I think you ought, as you have said in the past, to give your customers their money back.

Originally Posted by Magnetoman - #455839 - 09/25, 2012, 10:03 am
I use an Eisemann tester for extended tests of coils ... I ran the coil for an hour on this tester with the gap set to 5 mm (6 kV). The coil works fine so I will use it instead of winding one of my own.

The Eisemann tester looks like a nice piece of kit. But does it get the armature up to operating temperature, or do you heat the armature elsewhere and then test it on the Eisemann while it's still warm?

Originally Posted by Magnetoman - #455839 - 09/25, 2012, 10:03 am
Currently another supplier is advertising replacement condensers that are of a type not rated for pulsed high current applications, and whose dielectric layers are made of a porous oxide. Even if they survive the current pulses of a stressed magneto, no extended environmental testing has been done of them, so there is no way to even guess how long they might last in the ozone-rich atmosphere near the points before delamination or breakdown of the oxide might set in.

If you are going to have yet another tedious sleep go about the Brightspark EasyCap, it might help people if you named names instead of leaving them wondering. The fact is EasyCaps have undergone extended environmental testing; they carry on working; they are guaranteed; if one were to fail, it is a ten minute job to replace it smile without the need to pull the armature apart and mill out the old one frown as you have described in detail above; and last but not least the customers and their bikes are happy.

Originally Posted by Magnetoman - #455839 - 09/25, 2012, 10:03 am
The Fall and Winter 2011 issues of The Antique Motorcycle magazine contain a two-part article by physicist Dr. Charles Falco explaining the science of why the wax paper condensers that Lucas used in their post-WWII magnetos have a limited lifetime, ...

Yes, that was a very interesting article. One thing I did find puzzling about it was Dr Falco's reasoning that a breakdown of the waxed paper dielectric caused an increase in the equivalent series resistance of the capacitor, when schoolboy physics would suggest it causes a decrease in the equivalent parallel resistance of the capacitor. Did you understand that?

Originally Posted by Magnetoman - #455839 - 09/25, 2012, 10:03 am
... if you use any capacitor other than these Panasonics it is at your peril.

Crikey, that's not much help then, since they're out of production. Do you know whether Dr. Falco has any plans to test any capacitors which are in production?

Originally Posted by Magnetoman - #455839 - 09/25, 2012, 10:03 am
The pack I used in the Bosch was the same as this one, minus the mounting bracket, and I installed it in the end cap using the minimum amount of 3000 psi epoxy necessary to insure it would stay in place.

Yes, if securing a capacitor in the end cap with resin, it is important to use the minimum necessary. We have seen some where the resin totally fills the end cap, bridges the gap between it and the windings, and encapsulates the tails of the winding. Removing a dud capacitor without also damaging the winding insulation or the tails is then, ummm, difficult to say the least. Some people use a blob of bathroom sealant, which seems to work quite well.

Originally Posted by Magnetoman - #456910 - 10/02, 2012, 9:49 am
After reassembling the armature with the new capacitors I checked the output using one of my Merc-o-Tronic 9800 testers. The blue arc in the window of the tester shows the operation of the coil is reliable at a test current of just over 1 Amp supplied to the primary. This is as it should be, so the completed armature passes this test.

Again, do you recommend a room temperature test or a hot test?

But as I said at the start, a very interesting and informative series of posts. Keep up the good work! clap

Brightspark Magnetos
Originally Posted by Ken Tee
Originally Posted by Magnetoman - #450359 - 08/22, 2012, 4:13 am
a carbon brush that completes the primary circuit
that black cylinder (the earth brush) is part of the secondary circuit, ... bypassing the armature bearings and their insulators.
Thanks for catching the miswording.

Originally Posted by Ken Tee
Originally Posted by Magnetoman - #455839 - 09/25, 2012, 10:03 am
To make sure the material used for the aftermarket slip ring isn't conductive ... The resistance was over 11 GOhms with a 2500 volt potential applied ... This amount of leakage is completely negligible, so the material in the slip ring is good, as is the insulation between the coil and the armature.
I'm not so sure you should draw the conclusion from this test that the slip ring is good. 11 gigaohms at 2500 volts can certainly break down to a far lower resistance at the higher operating voltage of the slip ring. Breakdown of aftermarket slip rings is indeed a problem, but in my experience it often only rears its head at elevated temperature. A slip ring may test good at room temperature, but start breaking down at operating temperature.
Your experience certainly is different than mine. I've used both an IR thermometer and a thermocouple to measure the external and internal temperatures of a number of magnetos, and my measurements found the highest temperature a slip ring will see under the worst of conditions is well under 50oC. There is simply no physical mechanism I am aware of that could increase the electrical conductivity of whatever impurities are in a phenolic-like plastic by many orders of magnitude over that limited range. And, even if there were, the failure mechanism for a slip ring is completely different than that of a condenser that is warm. In the case of a slip ring it would be dielectric breakdown of the material, not increased electrical conductivity of the material itself.

I don't know what measurements you made that found slip rings that are good at room temperature break down 20-30oC higher. All I can guess is that either the slip rings you measured started out with very much lower resistances than 10 GOhm, or you misinterpreted whatever observations you made that led you to this conclusion. Perhaps arcing through a track of carbon that you hadn't noticed. This is why I test them at 2.5 kV, to reveal any such issue.

Originally Posted by Ken Tee
I am also concerned that, with this test, you are applying the 2500 volts not only to the slip ring conductor and the HT tail of the HT winding, but also to the whole of the HT winding and more worryingly the LT winding. In operation, the insulation around the LT winding sees voltages of perhaps 250 V, and it might be fair enough to test it to three times that voltage, but testing it to ten times that voltage is, IMHO, asking for trouble. Even if it passes the test, the test may itself trigger the onset of a later breakdown. If you were a professional restorer and if you were to do that, I think you ought, as you have said in the past, to give your customers their money back.
You have a fundamental misunderstanding of the electrical circuit of a magneto. When the 2.5kV was applied to the coil in that test all of the wires of the primary and secondary were elevated to that potential. There was no voltage whatever between one layer of windings and the next, so the insulation on the LT (and HT) wires is irrelevant.

The only places where there was a large electric field during this test was between the operating surface of the slip ring (where I had wound the stainless wire) and the nearest piece of metal on the armature, and between the bottom winding of the primary and the core of the armature over which it was wound. Had the person who made this coil wound the first layer of the primary directly on the armature, rather than properly having a layer of insulation there first, the primary certainly would have broken down since whatever insulation is on the LT wire itself would have failed at less than 1 kV maximum. Had the 2.5 kV induced arcing of the primary to the armature, it would have been a good thing, since it would have revealed the fact the coil had been defectively wound. This is why this is an important test to conduct and, rather than giving someone's money back for having conducted this test, quite the opposite.

Originally Posted by Ken Tee
Originally Posted by Magnetoman - #455839 - 09/25, 2012, 10:03 am
I use an Eisemann tester for extended tests of coils ... I ran the coil for an hour on this tester with the gap set to 5 mm (6 kV). The coil works fine so I will use it instead of winding one of my own.
The Eisemann tester looks like a nice piece of kit. But does it get the armature up to operating temperature, or do you heat the armature elsewhere and then test it on the Eisemann while it's still warm?
Good question. I should have written in that section that the only way to be sure the coil would not fail in operation would be to test it while at elevated temperature. This is what I would have done at this point in process of rebuilding one of my own magnetos. However, in the case of this Bosch, I knew I would later test the entire assembled magneto for an extended period at elevated temperature (to be described in a future post), so I bypassed testing the coil alone at elevated temperature. In several places in my posts I explicitly point out where I took shortcuts because of time constraints forced by the deadline, and I should have mentioned that here as well.

Originally Posted by Ken Tee
Originally Posted by Magnetoman - #455839 - 09/25, 2012, 10:03 am
Currently another supplier is advertising replacement condensers that are of a type not rated for pulsed high current applications, and whose dielectric layers are made of a porous oxide. Even if they survive the current pulses of a stressed magneto, no extended environmental testing has been done of them, so there is no way to even guess how long they might last in the ozone-rich atmosphere near the points before delamination or breakdown of the oxide might set in.
If you are going to have yet another tedious sleep go about the Brightspark EasyCap, it might help people if you named names instead of leaving them wondering. The fact is EasyCaps have undergone extended environmental testing; they carry on working; they are guaranteed; if one were to fail, it is a ten minute job to replace it smile without the need to pull the armature apart and mill out the old one frown as you have described in detail above; and last but not least the customers and their bikes are happy.
I had written all that I planned to about condensers, so if you consider the following text tedious, you're responsible for prompting it:

Your website is not the easiest to navigate, but the only information I could find on "extended environmental testing" is a page in your FAQ section where you describe finding no degradation after one of your capacitors spent a day in a pan of hot water. That might be a fine test for sugar cubes, but says nearly nothing about the long term survival of your capacitors in a magneto. I can only infer that the fact you devote a page by itself to this test, as if it were meaningful, that it indicates you actually think it is meaningful, and that it somehow counts as "extended environmental testing." It does not. Further, your capacitors are located in the points housing, where there is an elevated level of highly reactive ozone created by the sparks from the points. I could find nothing on your site about testing whether the porous oxide of your capacitors degrades in the presence of ozone.

Given the long and checkered history of inappropriate capacitors being sold for use in magnetos, the onus falls on the supplier to conduct appropriate tests to indicate yet another new capacitor isn't going to fail. As far as I could find on your website, you have not conducted such tests, which are needed independent of whether or not you offer a free replacement if/when they fail. There is also the issue of the degraded performance of the magneto itself as a result of removing the armature to install any condenser, including yours, which I will deal with in a quantitative fashion when I come to the point of magnetizing the Bosch in a future post.

Originally Posted by Ken Tee
If you are going to have yet another tedious sleep go about the Brightspark EasyCap, it might help people if you named names instead of leaving them wondering.
I searched the archives and can't find that I've ever made a single negative comment about your EasyCap. The sidebar about capacitors in my previous post is the first time I've said anything about your capacitors, and even then not by name. Unless I've missed something, if you say people are having a "tedious go" about the product you are selling, it means people in addition to me have issues with these capacitors.

Originally Posted by Magnetoman - #455839 - 09/25, 2012, 10:03 am
Yes, that was a very interesting article. One thing I did find puzzling about it was Dr Falco's reasoning that a breakdown of the waxed paper dielectric caused an increase in the equivalent series resistance of the capacitor, when schoolboy physics would suggest it causes a decrease in the equivalent parallel resistance of the capacitor. Did you understand that?
Schoolboy physics doesn't have much to say about the breakdown of capacitors. But, if it did, it would say that the use of an equivalent series resistance (ESR) is a very widely used approximate model found to be quite useful in describing the electrical behavior of capacitors when used in a circuit. If you Google "equivalent series resistance" (in quotes) you will get over 800,000 hits, indicating how widely used this model is. On just the first page of hits you will find good explanations for why it is so useful to lump together all the complex internal physical phenomena into a single ESR

Originally Posted by Ken Tee
Originally Posted by Magnetoman - #455839 - 09/25, 2012, 10:03 am
... if you use any capacitor other than these Panasonics it is at your peril.
Crikey, that's not much help then, since they're out of production.?
The same criticism certainly could leveled about the Lucas HT brushes I suggest using, since they've probably been out of production for 40 years. And yet, it's possible to find them.

Originally Posted by Ken Tee
Originally Posted by Magnetoman - #455839 - 09/25, 2012, 10:03 am
installed it in the end cap using the minimum amount of 3000 psi epoxy necessary to insure it would stay in place.
We have seen some where the resin totally fills the end cap, bridges the gap between it and the windings, and encapsulates the tails of the winding. Removing a dud capacitor without also damaging the winding insulation or the tails is then, ummm, difficult to say the least.
Yes, this is one of the reasons I cringe when I discover a magneto has been "professionally restored," since this is one of the atrocities I sometimes find that has been committed. However, although it is a headache, I've found it to be not all that much worse than the one I had to deal with on this Bosch. It's easy to tell where the coil ends, so the epoxy holding the coil to the endcap can be milled away by mounting the armature tilted slightly from horizontal (or by carefully using a Dremel).

But, the broader issue is whether there is an easy way out. One that avoids having to deal with whatever condenser is currently in the magneto, but that still restores the performance to the as-new level. I would love it if there were. If there were, I wouldn't have had to waste an hour of my life milling away all the epoxy the guy used who restored the Bosch. However, the fact I went to all that trouble isn't because I enjoy cleaning up wads of epoxy chips. It's because my experience tells me there is no proven-reliable capacitor I could use other than the Panasonic. I wish there was an easier way to restore proper performance, but I am unaware of any such easy way out.

Originally Posted by Magnetoman - #455839 - 09/25, 2012, 10:03 am
Some people use a blob of bathroom sealant, which seems to work quite well.
The use of bathroom RTV sealant is definitely the sign of someone who doesn't know what he is doing. If the RTV is still in place, it only means the electrical leads on the inappropriate replacement capacitor were strong enough to hold it in place for the short time the magneto was used before the capacitor failed, because the shear strength of RTV is too low to have done any good. Do not use bathroom sealant.

Originally Posted by Ken Tee
Originally Posted by Magnetoman - #456910 - 10/02, 2012, 9:49 am
After reassembling the armature with the new capacitors I checked the output using one of my Merc-o-Tronic 9800 testers. The blue arc in the window of the tester shows the operation of the coil is reliable at a test current of just over 1 Amp supplied to the primary. This is as it should be, so the completed armature passes this test.
Again, do you recommend a room temperature test or a hot test?
To briefly repeat the answer I gave earlier, about the test on the Eisemann tester, I "always" (except for this Bosch) run this test at elevated temperature. A future post will describe the extended test I ran of the assembled magneto at elevated temperature, which it passed.

Brush Spring Pressure

In addition to hardness, spring pressure is the other important parameter for a brush. I determined from measurements of the depth of the recess in the HT pickup that compressing the spring until the bottom end of the brush is 5 mm above the surface is a good value to use to simulate the average pressure on a brush when it is partially worn. Since Pressure = Force/Area, if a spring has the incorrect spring constant it will exert either too much or too little force on the rubbing surface of the brush (Force = -spring constant x displacement). The next photograph shows how I made this measurement using an electronic scale with a resolution of 0.01 g:

[Linked Image]

What I found was the brushes that were in this magneto had springs that are ~25% stiffer than those on Lucas HT brushes. This means that not only were the "KEV" brushes the restorer used significantly softer than proper ones (so they would have worn away faster), they were subjected to 25% greater pressure on the slip ring (wearing the brush away even faster still). Had I not replaced these brushes with proper ones the slip ring soon would have been covered with a conductive coating of carbon that likely would have resulted in an internal short, bringing the motorcycle to a halt.

Contact Breaker Points

Although the points in this magneto were fine, had they not been, and if there were no replacement readily available (e.g. cannibalized from my ZE1 magneto), I would have had to rebuild them. This can be done by silver soldering short lengths of pure tungsten TIG electrode rod over the ends of the worn points. Tungsten wets easily with certain silver solders, and a single 7" piece of 4 mm-dia. pure tungsten rod is enough to restore a lifetime's worth of magnetos. The end of one of these TIG electrodes is shown in the next photograph using my unrestored Bosch ZE1 for scale.

[Linked Image]

Note that if you rebuild your own points you need to use pure tungsten TIG electrodes, which are painted green on one end, as in the photograph. Do not use rods with either red or yellow ends because they contain 1-2% radioactive thorium. Because of the nature of the radioactivity, in solid form this isn't an issue, but you don't want to breathe the dust and have it permanently settle on the surface of your lungs. Obviously, it is difficult to form the W pieces into the necessary shape without creating dust. Also, the reason for thorium is that it makes it easier for the TIG electrode to emit electrons, and that is the opposite of what you want in contact breaker points. When the points separate, you would like it to be as difficult as possible for an arc to form.

Because platinum oxidizes only very slowly, Pt points (actually, Pt alloyed with Ir, Ru, and/or Os) were used in the earliest magnetos before it was realized laminated armatures significantly improved the low speed operation for which it had been felt Pt provided an advantage. Perhaps because of this, and citing a "lower surface resistance," shop manuals often say Pt should be used for the contacts in magnetos intended for higher performance motorcycles. However, despite what is commonly written, my measurements show that the difference in surface resistance between the two metals is negligible unless the magneto sits unused for some months in a humid garage, in which case tungsten can benefit from a light burnishing to remove the resistive oxide.

The next photograph shows that the resistance across the tungsten points in a Lucas KNC1 magneto is only 9.4 milliOhm (actual values varied between 2 and 11 mOhm when measured after successive make/breaks when rotating the armature by hand). I found the same range for the W points in the Lucas KVF beside it. An important fact is that both of these magnetos were given new tungsten points 16 years ago and have been sitting unused inside the house ever since. Even after this extended time, the oxidation of the W was negligible.

[Linked Image]

Since 10 mOhm is less than 2% of the resistance of the primary coil itself, the contact resistance of these points will only reduce the primary current, and hence the output of the magneto, by ~2%. What this means is, even if the resistance of Pt points were zero, the output of two otherwise identical magnetos with Pt and W points in good condition would differ by a negligible amount.

Although Pt will resist oxidation in a humid environment better than W, it reacts with organic vapors to form PtC, which is both resistive and rapidly erodes the contacts. The choice of materials for magneto points is addressed in Chapter V 'The Magneto-Type Ignition Systems' of Electrical Contacts Data Book: Materials, Designs, and Applications (P.R. Mallory & Co., 1945): "… because of the location of the magneto and its design, the contacts may be subjected to oil mist. In such cases it has been the practice to use tungsten for the contacts, because platinum and its alloys have a tendency to become brittle… [causing] high contact resistance and a corresponding high rate of electrical erosion." Since organic vapors typically waft about on old motorcycles, my conclusion from such reference books as well as my own measurements is that tungsten is the better choice for motorcycle magnetos in general, and for this Bosch ZEV in particular. Send questions or comments to [email protected]
Originally Posted by Magnetoman
You have a fundamental misunderstanding of the electrical circuit of a magneto. When the 2.5kV was applied to the coil in that test all of the wires of the primary and secondary were elevated to that potential. There was no voltage whatever between one layer of windings and the next, so the insulation on the LT (and HT) wires is irrelevant.


Just an observation here. I'm not a magneto expert, but I do work with electric motors for a living, and I'm having trouble understanding the above statement in your previous post. In my experience, if a coil of wire is conducting a current, there will be a difference of potential between the wires in different layers of that coil, because any length of wire will have resistance. This is why a coil can short circuit to itself- a turn to turn short. In my industry, a turn-to turn short is one of the more difficult faults to catch, and is usually caused by overhandling the wire during coil winding. Vacuum impregnation of potting varnish helps, but if two wires are in contact with each other, and their varnish coating is degraded, it will prove to you quickly that there is a definately a voltage between the layers of a coil.

Perhaps I'm not understanding a subtle difference between a magneto and an electric motor.

This is not meant to detract from a thoroughly enjoyable and informative thread. I'm looking forward to the next update.

Originally Posted by L.A.kevin
In my experience, if a coil of wire is conducting a current, there will be a difference of potential between the wires in different layers of that coil, because any length of wire will have resistance. This is why a coil can short circuit to itself- a turn to turn short. In my industry, a turn-to turn short is one of the more difficult faults to catch, and is usually caused by overhandling the wire during coil winding. Vacuum impregnation of potting varnish helps, but if two wires are in contact with each other, and their varnish coating is degraded, it will prove to you quickly that there is a definately a voltage between the layers of a coil.

Perhaps I'm not understanding a subtle difference between a magneto and an electric motor.
This is a very reasonable question. In the context of your question, there is no difference between an electric motor and a magneto (although, it actually is a transformer). However, this particular test is different than the one you normally would run on an electric motor. In the test I ran there is no current being conducted by the wire.

First, assume all the insulation on the wires themselves are fine, and a layer of insulation thick enough to withstand 2.5 kV was applied to the armature before the primary was wound on it. That is, assume the coil was properly wound. Since none of the leads from the coil are attached to the body of the armature during this test, applying 2.5 kV between the slip ring (which in turn is connected to the HT lead) and the armature body will cause a tiny current to flow for a moment until the potential of all of the windings are elevated to 2.5 kV with respect to the armature. Since all of the wires in the coil are connected together, once this potential is established there is 0 V between any two layers of the coil. Therefore, even if the insulation were missing from some of the internal coils, and two adjacent coils were shorted together, no current would flow between them -- in this test -- because there is no voltage difference to drive the current.

What may be confusing you is this isn't quite the test you would normally do for an electric motor. In an electric motor you would actively drive current through the coils by connecting your ohmmeter to the leads. In this case, current would flow between coils that were shorted together.

What the test I did checks is if the various insulators between the coil and the armature body are good. It says nothing about whether there are internal shorts between the coils which, as you say, is something of great concern. To test for internal shorts requires a dynamic tester, like the Eisemannn or Merc-o-tronic. If there were internal shorts, but not ones so bad as to make the coil totally inoperable, that would be revealed by needing to apply a larger AC current to the primary. You can't do this dynamic test for shorts on an electric motor because it only has a "primary," so you can't measure the AC current required to create a spark from the "secondary."

Although it gets a bit off topic, for testing for internal shorts on a dynamo (which is completely equivalent to your electric motor), I have an Evoibi coil tester. It measures the losses in the dynamo at two frequencies. Since the frequency dependence of the losses from internal shorts is different from the basic coil itself, this reveals whether there are any such shorted turns. By the way, these testers were developed for testing electric motors, where the market is a bit bigger for diagnostic equipment than it is for old motorcycle dynamos. Unfortunately, the large core losses of a magneto's armature makes it impossible to use such testers to determine if there are shorted coils in them. When working on motorcycle electrics, one can't have too many testers...
Originally Posted by Ken Tee
Charming! Pot calling the kettle black? I didn't say anything about the "insulation on the LT wires" and I wasn't thinking at the time about any inter-layer potential differences because, as you agree, there wouldn't be any; instead I wrote of the "insulation around the LT winding." Yes, one place where that sees a large electric field with your test is between the copper of the innermost LT layer and the cross-piece of the bobbin. Another place is between the copper of the live LT tail and the channel in the bobbin wing along which it runs.
There is no need to get your back up. If ever I answer a different question than the one you intended to ask, all you have to do is to point that out. The wording of what you wrote in your previous post is consistent with the same misunderstanding expressed by someone else in a question in a subsequent post, the answer to which I provided. Fine that you say this isn't what you meant.

Originally Posted by Ken Tee
What I admit I don't understand, however, is:
[*] given that you didn't know how the armature had been wound, and given that insulation around the LT winding that can withstand, say, 1 kV long term would be perfectly adequate, why do you apply 2.5 kV with the consequent risk of blowing your friend's armature's insulation.
As I've written in a few places, I had no way of knowing if this coil had been properly wound, or if it had been vacuum impregnated. If it had been properly wound, there would be a layer of insulation around the core of the armature that easily can withstand 2.5 kV. If not, my test would have destroyed the coil and I would have had to spend a day winding a new one.

Had the coil been directly wound on the armature, and even if it had been vacuum impregnated, not all of that resin might have made its way as far as the first layer of windings. In operation, those windings are subjected to a cyclic electromagnetic force as the armature rotates, and relative motion caused by this force can and does abrade insulation from windings. Even if the coil passed my other tests, there still would be the non-negligible possibility it would fail due to this abrasion after some unknown number of miles.

Even though the coil already had passed one dynamic test with a Merc-o-tronic tester, that test cannot determine if the primary was improperly wound directly on the armature. Again, the reason for doing this test is a properly wound coil will pass it, and an improperly wound one will fail.

Originally Posted by Ken Tee

[*] given that slip rings see 6 kV in operation, and given that slip rings can exhibit faults only when exposed to voltages higher than 2.5 kV, why do you conclude that a slip ring that passes the 2.5 kV test is good.
My experience is that no slip ring has passed this test but failed in operation. The resistance was 10 GOhm (i.e. ten-thousand-million Ohms, to emphasize the point) at that voltage, and either the resistance would have had to fall by many, many orders of magnitude between 2.5 and 6 kV to fail (which is physically implausible), or something in the slip ring that survived 2.5 kV would have to suffer dielectric breakdown at 6 kV. Amongst the many things I might worry about happening on a magneto, this is way down on the list. This is why I concluded it was good.

Originally Posted by Ken Tee
[*] given that your insulation tester has a crocodile clip that you could have clipped directly onto the live tail of the LT winding or the earth tail, why do you go to the trouble of winding a length of stainless steel wire around the slip ring and then connecting your insulation tester's crocodile clip to that. (It just struck me as a very odd thing to do.)
I do it this way because it simultaneously tests whether there is insulation on the armature and whether the operating surface of the slip ring has a sufficiently high resistance so as not to be a concern. Attaching the tester to either of the primary coil leads wouldn't test the latter.

Originally Posted by Ken Tee
given that you have an Eisemann tester at your disposal, why use an insulation tester on the slip ring at all.
Already answered.

Originally Posted by Ken Tee
On the point about environmental testing of our condensers, you originally wrote, as if it were fact, "no extended environmental testing has been done on them." Our own two main test bikes have clocked up tens of thousands of miles without any degradation of their EasyCaps. We have sold many hundreds of EasyCaps, and we're unaware of even a single failure of an EasyCap in service to date. That is environmental testing, in a real-life environment, ozone and all.
As I wrote previously, there is a long and checkered history of condensers that have been sold with seller's written assurances as strong as the following: "Modern substitute, very high specification, zero failure." Despite that seller's unqualified assurances of not a single failure, an unreasonable percentage of them failed in service. I have no way to confirm your own claim that there hasn't been "even a single failure" of yours, nor do I even care. What I do know is I have enough legitimate concerns about your capacitors, some of which I have expressed, not to have any interest in relying on them myself.

However, an important point is, it is not up to someone else to demonstrate your capacitors do have problems in service. It is up to you to provide credible test data to indicate they do not. Having them sit in a pan of hot water for a day, or telling us that they have had zero failures, is not sufficient.

Originally Posted by Ken Tee
Looking at the alloy and steel that are exposed to this ozone, I'd have thought that these levels of ozone are insignificant. But if they are significant, I am surprised that the eminent physicist Dr. Falco didn't mention the ozone issue in his article previously referred to. Did he do any ozone tests on the Panasonics, do you know?
I'm not going to take the time to look for the quote, but at least one treatise on magnetos explicitly comments on the importance of not sealing the points housing because of the buildup of ozone levels. That's why even a Lucas Wader magneto has a way for water to get in -- in order to let the ozone out. When a capacitor is placed in the cavity at the other end of the armature, the reactive ozone created by the points, or any of the brushes, has to make at least a few right angle turns in contact with metal to get as far as the condenser, which itself is covered by epoxy as well as its own coating. In contrast, your condenser is directly adjacent to the points, where the ozone level is highest, and not protected by anything other than itself.

Originally Posted by Ken Tee
On the ESR point, the simplistic model of a real capacitor being merely a perfect capacitor in series with a perfect resistor is, as far as I can see, totally insufficient to describe the faults that are found in magneto capacitors. One fault is the inability to hold a charge. I can't see how that can be modelled with an ESR, but without an EPR (equivalent parallel resistance). Another fault is dielectric breakdown. I guess an EPR and ESR might perhaps be able to model that at one particular operating point, but not over a wide range. Anyway, that's by the by. I was just puzzled by what Dr Falco had written.
The simplified, not "simplistic," ESR model was used in an article in a motorcycle magazine to explain to that audience the essence of why Lucas wax paper condensers failed with time. This present thread is about restoring a c1920 Bosch magneto, and has nothing to do with failure modes of various condensers. However, since so many types of replacement condensers touted by so many people over the years have failed, I included a sidebar about capacitors because I believe it is important for people reading this to understand why I used the one I did in order to have high confidence it would survive long term. You have not reported any tests on the capacitors you sell that would be needed to give me the necessary confidence to even consider using them.

At this point I think we have covered capacitors in far more detail than anyone reading these posts cares to know about them, so please direct any future questions to any aspects of this restoration other than capacitors. I'll be happy to discuss any and all issues, other than why I don't use the capacitors you are promoting.

Since the electrical issues seem to be under control, it's time to turn to the mechanical components.

The Bearings

The bearings and races were new and of reasonable quality, as shown in the next photograph.

[Linked Image]

The next photograph is of one of the races, taken through the eyepiece of a stereomicroscope. The track for the balls that runs vertically is approximately 0.1" wide. Because of physical interference, it was impossible to place either race in my metallurgical microscope without removing it from the armature, so I couldn't make a higher magnification micrograph (since the races were new, I didn't want to remove them since the stereomicroscope already had revealed what I needed to know). The important thing is, the bearings pass this microscopic inspection.

[Linked Image]

Had I needed to remove the races from the armature, I made brass clamps for the two common sizes of magneto races. Although it may not be easy to tell from the photograph, the inner surface of each clamp is machined with a convex profile to make contact within the track of the race around the entire circumference to ensure there can be no damage caused whatever. Once clamped on the race I use a two-jaw puller along with a brass plug at the end of the armature for the screw to push against.

[Linked Image]

In addition, for "emergency" use away from home I machined a standard bearing puller to give it an inner surface that matches the track of the race of the larger of the two bearings. When used on the smaller race it does not make full contact everywhere on the circumference, but I calculated that the maximum pressure it exerts is far below the yield strength of the bearing steel so it still is fine in spite of the imperfect fit.

[Linked Image]

Pullers are sold for magneto races that use four radial screws to clamp on the groove of the race. I definitely do not recommend using them since the tips of their screws subject four small regions of the race to very high pressures when removing the race. Even a small mark made in a track can seriously reduce the lifetime of the bearing.

Another use of the modified bearing puller is to remove slip rings. Slip rings are made from brittle and fairly fragile materials (e.g. Bakelite, Tufnol, or similar phenolic). Further, they are tapered, so the outer edge that one is tempted to pull on is the weakest. However, by loosely clamping this bearing puller inside a slip ring I can gently apply uniform pressure to the stronger inner circumference using a two-jaw puller. While the modified bearing puller serves double-duty as a slip ring puller when away from home, the next photograph shows an aluminum puller I made specifically for this purpose.

[Linked Image]

Reinstalling races can be done with a socket of an appropriate size and an arbor press, but I made a "tool" specifically for doing this. I haven't included a photograph because it looks pretty much like a socket. For "emergency" use a hammer can be used instead of an arbor press, but I much prefer to use a press whenever possible.

Both of the outer races are insulated from the housing by thin paper "cups" to eliminate them as paths for the current that flows between the coil and earth during the spark. Ideally, the resistance of the cups would be infinite so no current at all could flow. Such a current would electro-etch the bearings over time. However, as long as the resistance of the cups is significantly larger than the 10 mOhm or so of the earth brush, only a small fraction of the current will take the path through the bearings. For example, even if the resistance were as little as 100 Ohm;, only ~0.01/100 = 0.01% would flow through the bearings. The resistance of the paper cups under both races was greater than 10 kOhm;, so I did not have to remove either outer race of this Bosch ZEV to replace the insulators.

Often races are held quite firmly in the housing, which is a problem when it is necessary to remove them. A blind bearing puller is perfect for this, except one of the races is located deep inside the main housing where it is difficult to reach. These pullers work by expansion, which requires holding the "nut" that is in the middle of the puller while turning the "bolt" at the outer end to expand the tapered mandrel onto the inner surface of the race. Since the "nut" is otherwise inaccessible inside the housing, I fabricated a clamp small enough to hold it while still fitting inside the housing. This puller and clamp is shown on top of a Lucas Magdyno housing in the next photograph, with the expanding end positioned at approximately the depth of the inner race. A socket and 3/8"-to-1/4" reducer is on the "bolt". Send questions or comments to [email protected]

[Linked Image]

The Cam

Although a Harley-Davidson is a "45-degree V-twin," the HT leads of the Bosch ZEV magneto don't fire 45-degrees apart. The configuration of this engine is equivalent to a vertical twin (e.g. BSA 650) whose cylinders have been split to be at 45-deg. rather than 0-deg. with respect to each other. As a result, lobes one and two on the points cam need to be 157.5 degrees apart. The reason for this is, since the magneto operates at half engine speed, 157.5-deg. corresponds to 315 degrees of rotation of the crankshaft. Thus, if one cylinder fires at 0-deg., the second fires after the engine turns 315-deg. further, i.e. 45-degrees less than another full revolution (360-315=45). It then takes another 45-deg. for the crankshaft to get back to the first cylinder, which is now on its exhaust stroke, plus another full rotation before that cylinder is ready for another spark, which is 405-deg total (45+360=405). So, the angular separation between the second lobe and the first needs to be half that, i.e. 202.5-deg. Since 157.5 + 202.5 = 360, this all works out.

[Linked Image]

I used my Talyrond roundness tester to check the contour of the cam itself. The resolution of this instrument is overkill for what is needed, since it lets me center and level components to within 10 micro-inches, measure heights to 10 micro-inches, and determine angles to arc-seconds . However, if I didn't have the Talyrond, the 15 arc-sec. resolution of my mill's rotary table, along with a mechanical dial test indicator, would have been fine for these measurements (although not as fast or convenient to align as the Talyrond). Given how bad other aspects of the restoration were, I was afraid I might find the cam to be a poor tolerance attempt to be 45-deg., or even a 42-deg. cam for an Indian. If this were the case, I either would have to quickly find a replacement (that might turn out to be no better), modify it, or machine one from scratch. However, I was pleasantly surprised to find the angular separation between the lobes at the positions where the ramps would open the points by 0.012" to be precisely 45.0 degrees (or, rather, 157.5 and 202.5 degrees).

Unfortunately, the contour of the cam alone does not determine where this magneto will fire, since the OD of the cam ring locates it in the ID of an adjustable housing which in turn is a sliding fit in an annular slot in the magneto. Because of this, there is no way to know without measurement how the various tolerances add up to affect the final timing. So, once I knew that at least I was starting with a good cam profile, I assembled it in the magneto, centered the axis of rotation of the armature precisely on the axis of rotation of the Talyrond, and again measured the cam profile with respect to that axis.

[Linked Image]

I found when the cam assembly is installed on the body of the magneto the tolerances do add up to cause some deviation of the timing from the 157.5/202.5 values. However, most or all of the problem is intrinsic to the design of the magneto itself, which allows a few thou. side to side motion of the cam because of the need for a sliding fit of the assembly for advancing and retarding the ignition. Because of the slope of the ramps on the cam, a sideways displacement of a few thou. has a significant effect on the timing. However, the deadline for getting this magneto finished and shipped back to the guy who is rebuilding the engine is rapidly approaching, so there isn't time to re-engineer the design and fabricate something with tighter tolerance than it had when it left the factory. I will proceed with it as it is but, if I find the timing too far off in my later dynamic test, I will have no choice but to devise a solution.

Concentricity of the Armature Components

To be sure the components of the armature are properly concentric after I assembled it, I used a precision bench center with four dial test indicators of resolution 0.0001" (ten-thou.). I reconstructed the setup for the next photograph a few days later using a different armature and only three of the four indicators.

[Linked Image]

In fact, this is not how I hold armatures in place for these measurements, since the races determine the axis of rotation, not the ends of the shafts. However, the fixtures I designed to locate armatures with respect to the races in this bench center would completely obscure what is going on, so what I show here is more of a schematic to illustrate the important points. The indicator at the left is shown measuring one of the races. In this setup, if the race were perfectly concentric with the dimple in the end of the shaft, the indicator would not waver as the armature was rotated. However, what is important are the races, not the dimple, so the fixtures not shown here are designed to use the races as the reference surfaces. Similarly, the middle indicator checks for the run out of the slip ring with respect to the race, the third indicator would check for the run out of the earth ring if this were a more modern BTH or Lucas, and the four indicator (not shown) would check for the run out of the second race. The Bosch uses a different surface for the earth brush, so actually I used an indicator on the fixture located on the second race rather than the armature housing.

Another subtle alignment issue that also needs to be checked is if the shafts at each end are coaxial with each other. As an example of why this could be a problem, imagine if while reattaching the end cap that I caught a metal chip between it and the body. This would cause one shaft to be tilted with respect to the other. While such an alignment problem may not be likely when carefully assembling an armature that left the factory as one piece, it certainly cannot be relied on when dealing with a rebuilt armature that might be a mongrel made up of components from several others. The operational consequences of this would be that it would misalign the bearing races (causing premature wear).

Since the shaft at one end of the armature is stubby, checking alignment accurately presents a measurement problem. Although I used the Talyrond for this measurement, I could have done it more precisely than the way I did, albeit only if I spent more time setting it up. Instead, it was faster to check if the shafts were coaxial by installing the points block in the taper of the armature's shaft and measuring the run out of the back face of the block near the outer edge. When I did this I found the run out was several thou., but it also varied by a few thou. each time I removed and reinstalled the block. In any case, this is small enough not to be a concern, so finding the source(s) of the small run out and reducing it further wasn't necessary.

Although the armature itself was in good alignment, I determined that there was almost 0.01" of run out of the surface of the slip ring, which would cause unwanted motion of the brushes in their holders. So, I mounted the armature in the lathe using a collet to hold the race at one end (again, because the race is the surface that determines the axis of rotation), and a high precision live center at the other end, as laid out schematically in the next photograph using a different armature. The total indicated run out (TIR) of the spindle of my lathe is less than 0.0002", and the TIR of this precision center is 0.0001", so this machining operation results in an armature that is at least as good as it was when it left the factory c1920. I then polished the track to achieve a measured roughness of just under 1.5 um.

A critical observer will notice the lathe collet clamps on the outer surface of the race, not the track the balls run on. However, I've verified with several races on the Talyrond that these two surfaces are concentric to at least 0.00002" (20 millionths), and that on this particular race they are to at least 0.0001". The lower precision is because I measured it using a mechanical indicator in the assembled configuration on the bench center rather than with the electronic indicator of the Talyrond. Also, it would have been more precise if the live center somehow could have been fixed to the race at that end of the armature rather than in the inner taper of the shaft. However, to do this would have required machining another precision fixture, and any added precision from going to this effort would have been negligible anyway. That race is ~4" from the center of the slip ring, which in turn is only ~0.5" from the race held in the collet. Because of this ~8:1 leverage, the ~1 thou. run out I measured for the race at the tailstock end with respect to the ID of the shaft as held by the precision center added a "wobble" of only a ten thou. to the slip ring when mounted on the lathe. This is comparable to the TIR of the lathe's spindle, and is at least 10x better than it needs to be.

[Linked Image]

Send questions or comments to [email protected]
Originally Posted by Magnetoman
Another subtle alignment issue that also needs to be checked is if the shafts at each end are coaxial with each other. .... The operational consequences of this {shaft misalignment} would be that it would misalign the bearing races (causing premature wear) ....

Originally Posted by Magnetoman
.... as well as the points block (causing the rubbing block to wander in and out on the cam as it rotated).
Why would the rubbing block wander in and out? (apart of course from the effect of the cam.)
Posted By: johnm Re: Restoring a Rotating Armature Magneto - 10/22/12 12:46 pm
Thankyou for posting this detail on the mechanical side of magneto restoration.

In my early attempts at classic racing I ruined two sets of pistons before I realised the asymmetry of my ignition timing due to poor alignment of the armature.

While checking to this level of detail may seem over the top for everyday use it is exactly this sort of accuracy that makes the fastest bikes go fast!! (and reliable!!)

If you get every setting - ignition, cams and valve timing, carbs exactly right with a blue printed motor you will gain at least 5 bhp on a 500 cc bike. 8 bhp on a 750.

Originally Posted by Ken Tee
Why would the rubbing block wander in and out? (apart of course from the effect of the cam.
Thanks for noticing this. I'm editing with iPhone so easiest to just delete that clause, which I just did.
Originally Posted by johnm
While checking to this level of detail may seem over the top for everyday use it is exactly this sort of accuracy that makes the fastest bikes go fast!! (and reliable!!)
Thanks very much for your note. It's nice to know the information is being read and appreciated. While h.p. wasn't a concern for this particular restoration, it comes as a free added bonus with reliability and smooth running.

An impression of "over-the-topness" might come from my description of this restoration being spread out over four months by the time I'm done. However, although I didn't keep track of the time spent actually working on the magneto, what is taking four months to describe probably added up to the equivalent of about two days of work on it. The magneto was actually in my hands for 29 days, during which time I made two trips across the country, plus worked at my normal job.

If the level of attention I used on this magneto does strike some people as over the top, all I can say is, it's not. It certainly is possible to get away with a much worse restoration than the one I'm documenting here and still have the magneto function, just as it is possible to do a poor rebuild of an engine using improper clearances and ill-fitting aftermarket parts and still have the engine run. Also, while I am equipped with some over the top instruments, e.g. the Talyrond of my most recent installment, in all such cases there are "normal" pieces of equipment that could have been used instead (a mill's rotary table instead of the Talyrond). Some tools are specialized (e.g. a Merc-o-tronic tester), but specialized tools also are needed to rebuild an engine (e.g. a cam pinion extractor). The point is, the tools I used would be needed by anyone who is capable of doing magneto repairs properly, and their work would be done to the level of detail I'm describing.
Posted By: johnm Re: Restoring a Rotating Armature Magneto - 10/23/12 7:22 am
Yes it is a game to catch young and not so young players.

I measured 8 degree difference on an old Lucas K2F mag due to bad armature alignment. I kept trying to fix it by grinding the cam ring and ruined a second set of pistons before I finally realised. I was being a bit stupid really and should have figured it out earlier if I had not been diverted by other issues.

On my race bike I set ignition to within 0.5 degree both sides and have experimented with the dyno to get the best setting for petrol and methanol.

Many magneto servicing places address the electrical issues but not the mechanical issues
Originally Posted by johnm
I measured 8 degree difference on an old Lucas K2F mag due to bad armature alignment... Many magneto servicing places address the electrical issues but not the mechanical issues
This is a very important point, and you clearly understand the issue.

I didn't measure the slope of the ramps on the Bosch's cam, but on a Lucas K2F it is ~2.4-deg. of engine rotation per 0.002" of lift (1.2-deg. of cam rotation). That means that even if the cam is made perfectly, and you set the points to open on one cylinder precisely where you want them to, but if the cam is held in the housing such that its centerline is offset from the centerline of the armature by only 0.002", the other cylinder will fire 2.4-deg. too soon or too late.

Magnetos are electro-mechanical devices, so even if the electrics are in perfect condition, that only takes care of the 'electro-' half of the problem. As a preview of a future installment, one of the upcoming tests will be of the restored magneto run on my modified distributor tester at 1250 rpm (2500 rpm engine) to determine precisely where both cylinders fire during actual operation, and to see how much variation there might be during operation (due to, say, sloppy tolerances).

Posted By: johnm Re: Restoring a Rotating Armature Magneto - 10/24/12 10:23 am
Can I ask your opinion about using a strobe light to check the accuracy of timing on a mag ? At least one manufacturer's website does not recomend it but they do not explain why.

I have used my strobe powered by a separate bike battery to check the magneto timing on both cylinders for years. I have scribed a line on the belt drive plate and marked up the cover in degrees. I find this is a very quick way to check timing after and during a meeting.

I use a vernier on the sprocket to adjust the timing on my Norton twin but confess that I have also used points gap to swap the bike quickly from petrol to methanol timing in the middle of the meeting when trying to run one bike in two classes.

If you set the timing at one extreme of recomended points gap range for petrol you can then get the 3.5 degree extra advance necessary for my bike to go to methanol. I can then check very quickly with the strobe that I have got it right. I know this means the flux peak (not the right words but you know what I mean) might not be optimal but the best I can do in a few minutes between races. This is on a Fairbanks Morse/Hunt/Morris type rotating magnet mag.

This is all usually done in a 20 minute organised panic of three guys swapping carbs, front brakes and ignition timing to move up from Clubmans to Open class. It has proved successful with a couple of seconds in the NZ Classic Senior TT on a upgraded Clubmans machine!

Any other suggestions gratefully considered.
Originally Posted by johnm
Can I ask your opinion about using a strobe light to check the accuracy of timing on a mag ? At least one manufacturer's website does not recomend it but they do not explain why.
There is no reason whatever not to use a timing light with a magneto, although there are two significant practical issues that get in the way: 1) an appropriate, accurately located, timing mark, and 2) adjusting the timing.

1) Later non-magneto bikes had timing marks on the rotor, and even later ones still provided an access plate that didn't necessitate removing the entire primary case. The problem on an earlier bike is to find a component on which to make suitable marks in a location where oil won't spew out when it is uncovered. It sounds like you've found a solution to this on your racing bike.

2) Later bikes swapped the positions of the cam and the points, rotating the former rather than the latter. This makes it much easier to provide an adjustment mechanism to move the points relative to the cam. Again, you've solved this problem with a vernier on the magneto sprocket. However, it does require removing a cover to get access to that sprocket.

I've never done this myself, but it wouldn't be too hard to design a screw mechanism on the body of the mag that would basically replace the timing cable (I realize yours is likely locked at full advance, so you don't have a cable). You would still use the vernier to get as close as you could to the proper timing. But, during a race meeting if you found with your strobe that the timing was off by a fairly small amount, tweaking the cam angle slightly with the screw mechanism (securely located as part of the body of the mag) could quickly dial it in to being spot on.

Finally, this Bosch ZEV magneto is ready to go back together. After cleaning off the unknown grease that the rebuilder had used on the bearings, I packed them with Sta-Lube high temperature disc brake bearing grease, applied a small amount of Lubricam to the cam and its pivot, and reassembled the magneto. I also put a yellow tag with a note on one of the oil cups telling my friend not to use oil, since it would only wash the grease away.

End Float

BTH and Lucas armatures have shims for both the armature shaft and for the housing to adjust end float, so just the fact these shims exist indicates it is something that needs to be checked. Especially with magnetos rebuilt by someone else it almost always needs to be adjusted. Since this magneto has a new bearings it means both races were removed and reinstalled by the previous restorer, and the position of the races determines the end float. Anyway, given how many problems I had found with this magneto, checking the end float was especially important. Inserting 0.005" shim stock between the end cap and the main housing and then measuring the end float of the armature let me determine that the end float would be 0.000" with no shim at all. I don't know if Bosch ever published specifications for the end float for this magneto but, even if they did, I don't have them. However, the only reason to use shims to get a positive end float would be if the thermal expansion of the armature was significantly larger than that of the housing, resulting in undue pressure on the bearings at operating temperature. Several years ago I made careful measurement of the thermal expansion of a Lucas armature and housing constructed of similar materials as used by this Bosch. Those measurements, plus the fact Lucas called for zero end float (0.005" max.) for their magnetos, makes me confident in using zero for this Bosch ZEV.

Contact Spring Pressure

An important measurement is of the spring pressure on the contacts. If it is too small the points will float at high rpm, and if it is too large the rubbing block will wear too rapidly. Because removing the cam leaves the rubbing block exposed on this magneto it is particularly easy to measure, as is shown in the next photograph taken using my ZE1.

[Linked Image]

Lacking specifications for this pressure from Bosch required deciding on a reasonable value for comparison. Literature from various magneto manufacturers has at the low end of the recommended range for tungsten points 10-18 oz, and at the high end 15-30 oz. Consistent with this, I measured my Vincent's Lucas KVF to be 24 oz, although its spring needs to keep the rubbing block in contact at nearly twice the rpm as does the 1923 Harley-Davidson's magneto. Complicating this further is that when I removed the ZEV's moving point arm from the assembly I found its balance point was slightly towards the contact end. Because of this, centripetal force will tend to open the points against the action of the spring, and this force increases as the square of the rpm.

With the above as background, I measured both the Bosch ZEV and the ZE1 at 13-14 oz. Although it is encouraging that they are the same, unfortunately, it may only mean their springs have weakened with age by the same amount. However, since 14 oz. is a reasonable value, although toward the low end, I decided to proceed with testing. I made a note that cannibalizing the spring from my ZE1 to use to double the pressure on the ZEV is an option, depending on what I find from my dynamic tests of the magneto (although this would move the pressure from near the low end of recommendations to near the high end).


It's impossible to properly restore a magneto without being able to remagnetize it, so several years ago I made an appropriate electromagnet. I based its design on requirements given in the 1953 Lucas Workshop Instructions booklet Remagnetisation of Magnetos, after also checking specifications for magnetizing Alnico in several texts. The Lucas booklet calls for an electromagnet with a core winding value of 65,000-70,000 A-turns in order to magnetize their post-WWII Alnico-based magnetos.

Incorporating construction principles detailed in Laboratory Magnets by D.J. Kroon (Philips Technical Library, 1968), the electromagnet I built weighs several hundred pounds and consists of ~4500 turns of 14 AWG wire of total resistance 12.9 Ohms wound on a yoke made of Armco magnet iron. Since its inductance stores a serious amount of energy at full current, and such a DC current is difficult to interrupt without arcing, I use a 20 Amp Variac to ramp the current up and back down over a few seconds. Also, having an electromagnet whose field can be varied continuously, rather than only operate on/off, has other advantages for experimentation on magnetos. In any case, applying the full 240 V of rectified AC from the wall results in 18.6 Amps, and therefore 83,721 A-turns, which is comfortably above the values Lucas recommended for remagnetizing their Alnico-based magnetos. I use a clamp-on ammeter during operation to verify the applied current, and also used this ammeter along with a Bell digital gaussmeter to determine the full field vs. current curve of the electromagnet.

Interchangeable Armco iron pole pieces have faces that are shaped to closely conform to a variety of Lucas and BTH rotating armature and rotating magnet magnetos to minimize flux leakage. Further, I have pole pieces to remagnetize Lucas rotors from later motorcycles. Actually, the operating fundamentals of magnetos don't leave a lot of room for unique designs, so these pole pieces also work on Fairbanks-Morse (and A.R.D., Joe Hunt, and Morris), Splitdorf, Wico, etc. If I ever needed, I also have blanks to machine into whatever shape is required. However, simple flat pole pieces are all that are needed for this Bosch magneto.

Because the magnets on the Bosch ZEV are exposed they can be placed in direct contact with the pole pieces, so the amount of flux lost to "leakage" is significantly less than with a post-WWII magneto where the Alnico is encased beneath a shell of aluminum. Even if this were not the case, the ~84,000 A-turns of my electromagnetic is over 2x higher than that needed to fully magnetize the tungsten steel used for the ZEV's magnets (as well as higher than that needed for the later cobalt steel that preceded the Alnicos), so there is no question this electromagnet is able to fully magnetize this magneto. The next photograph shows the magneto ready to be magnetized. I already have attached the aluminum pulley that I normally use to drive magnetos at 2000 rpm on my long-term tester (more about this in the next installment).

[Linked Image]

For the magnet to be left with the maximum remnance the armature must be oriented correctly when in the electromagnet. However, the precise orientation isn't too critical, so this is easy to do by rotating the armature "backwards" by ~90-deg. from the position where the points are about to open (i.e. turn forward until maximum resistance is felt, then back by ~90-deg.). Although it only requires one cycle up to the full magnetic field to magnetize a magneto, I ran the magnet up a second time for good luck. After magnetizing the Bosch ZEV I attached the two HT cables and found that just a gentle flick of the armature gave an impressive spark.

Note, though, that while it is comforting to see a spark, such a "flick test" is often incorrectly used to claim a magneto is functioning properly. First, if a small gap is used for this test (e.g. the ~0.02" of a spark plug), a much smaller voltage is required to create a spark than will be necessary at the ~150 psi cylinder pressure during actual operation. Second, the instantaneous rpm when the armature is flicked through the position where the points open and the magnetic flux reverses typically is higher than it will experience at tickover speeds, again deceptively making it appear that the magneto is performing better than it actually is. Only a test using an appropriate gap (~0.2") and under steady-state operation can properly determine if the magneto is functioning as it should.

----------- Sidebar About Magneto "Chargers" -----------
Since it seems to be a common misunderstanding by many people who restore magnetos, it is worthwhile explaining why an electromagnet designed for earlier magnetos will not fully magnetize a post-WWII Alnico-based magneto. Just the fact the magnetic energy stored in Alnico is over twice that of the previous generation of Co steel magnets indicates a higher field electromagnet is required. Also, post-WWII magnetos have their Alnico magnets encased within an Al housing so the pole faces of the electromagnet cannot be brought into direct contact with them as they previously could be with the older horseshoe magnets. Because of this, some of the magnetic flux from the electromagnet "leaks" away through the Al housing without reaching the Alnico and as a result an even larger electromagnet is required for these "modern" magnetos than otherwise would be needed.

As an aside, the field produced is determined by the current and number of windings, which in turn determines the wire diameter, operating voltage, and overall size of an electromagnet. Because of this, someone who is familiar with the design of electromagnets can tell just by looking at one if it is capable of magnetizing Alnico. Just as someone who is familiar with engines can tell just by looking at a BSA Bantam and a Vincent Black Shadow that one is capable of 120 mph and one is not.

An electromagnet must be capable of driving the magnet into full saturation in order that it be left with the maximum remnant field once the magneto is removed from the electromagnet. The 2672 Oe of my electromagnet is ~20% more than that needed to fully magnetize a magneto containing anything from the Alnico family (Alnico, Ticonal, Alcomax, etc.), and it is 2.3x more than needed for older steel magnets. Stated differently, the field from an old magnet charger is a factor of ~2x too low for Alnico. This leaves an Alnico-based magneto less than fully magnetized (however, magnetic properties are nonlinear, so the performance is degraded by less than 2x). Although the field from earlier chargers is enough to magnetize an Alnico-based magneto sufficiently to spark an engine when it is kicked over at a higher speed, the magneto will have significantly degraded performance because the Alnico is only partially magnetized. The effects of this will be most pronounced in the form of harder starting and missing under load at low speeds.

Despite these scientific facts, some restorers insist their old chargers work "just fine" with Alnico, and that their customers are happy with the results. If you would accept as "just fine" the performance of your car if returned from servicing with a plug wire disconnected, you can accept the use of an old magneto charger on your post-WWII magneto. Otherwise, not.

Another important point is that a magneto has to be magnetized after it is fully assembled. The negative consequences on performance of magnetizing it with the armature removed (or removing and replacing the armature after magnetizing it) are amply documented in a various authoritative texts on electromagnetic devices. For example, in their chapter 'Magnetising and Timing a Magneto' in 'Automobile Electrical Equipment' (Iliffe, 1958), A.P. Young and L. Griffiths write that if the armature is removed and replaced "The flux density instead of being of the order of 10,000 lines per sq. cm., would be more nearly approximate to 7,000 lines per sq. cm…," i.e. the output of the magneto would be reduced by approximately 30%. In 'Permanent Magnets' (Pitman, 1949), F.G. Spreadbury shows that the output from a magneto with a Ticonol ("Alnico") magnet is reduced by 23% in actual operation if the armature is withdrawn and then replaced after magnetization. Even the Lucas shop manual on 'Remagnetisation of Magnetos' says "... it is necessary to remagnetise them, particularly after an armature or rotor has been removed from a magneto for repair or examination." My own measurements are consistent with the 20-30% reduction documented by these authors.

Despite the information in the previous paragraph, one company offers a service that magnetizes magnetos with the armatures removed from them. Further, they don't merely say that it is "just fine" to do it this way, but actually claim their measurements show it is just as good as magnetizing them when fully assembled. To paraphrase a line from a Marx brothers movie, "Who are you going to believe, this company, or all those lying books?"

Like the use of older magnetizers, people who have their magnetos remagnetized with the armatures outside them are not getting what they pay for. As the references cited above show, these magnetos have 20-30% degraded performance, depending on which member of the Alnico family is inside. By the early 1950s Lucas had used Alnico, Alcomax, Ticonal E, Ticonal G, and Alcomax 2, and my guess is that by the end of the decade they also had used Alcomax 3 and Alcomax 4. Although there is no reasonable way to determine just which member of the Alnico family is inside a given magneto, the B-H loops of all are similar enough that the figure of 20-30%" degradation should cover all possibilities.

Once again paraphrasing Bruce Springsteen's 'Magic', as far as magnetizing a magneto is concerned, "Trust none of what you hear, and less of what is claimed." Only a large, 65,000+ Ampere-turn electromagnet used on a completely assembled post-WWII magneto will fully magnetize it. Further, if you remove the armature for any reason from one of these magnetos (such as to replace a faulty condenser), it will have to be remagnetized after it is reassembled or it will have 20-30% lower output than it should have. It is even worse for a pre-WWII magneto, which won't effectively function at all until it is remagnetized.

A few final comments about the magnets: An often repeated piece of advice when restoring magnetos is to immediately place a steel "keeper" across the poles of the magnet as soon as the armature is removed to keep the magnet from losing its strength. Unfortunately, this advice is wrong. No matter how fast you are, a keeper will do you no good at all because the magnetic domains rearrange themselves nearly instantaneously (less than a millisecond). However, enough strength will be left that the magneto will still spark across a spark plug at atmospheric pressure, so you might think things are fine. They are not. A few other wrong, but harmless, pieces of advice you might run across for achieving full magnetization include charging the magneto 4-5 times (once is enough), holding the electromagnet at full field for a number of seconds (a fraction of a second is plenty), and tapping the magneto with a brass hammer while the field is applied (harmless, but pointless).
----------- End Sidebar About Magneto "Chargers" -----------

Send questions or comments to [email protected]
In his last post, Magnetoman said something very important, albeit in parentheses:
Originally Posted by Magnetoman
... a magneto needs to be magnetized when fully assembled (or with proper keepers).

The Lucas N1, KN1, K1F, K2F and KVF magnetos and MO1 and MN2 magdynos include internal keepers (the extensions of the pole laminations that extend underneath the armature and almost meet opposite the magnet). That is presumably why Lucas explain in their 1953 workshop instructions specific to these models of magneto that it is not necessary to apply a keeper when removing the armature. Equally, an extra keeper is not required when removing an internal remagnetising core from and inserting the armature into these particular models.

However, some other models of Lucas magneto (such as the GJ4 I am currently working on) and many other makes of magneto (such as the Bosch discussed in this series) do not include an internal keeper. Their magnets will lose a significant amount of strength the first time they are made keeper-less. That is presumably why the Lucas workshop instructions on remagnetisation of magnetos generally, which cover a vast range of Lucas models including those without an internal keeper, make the point that sometimes it is necessary to remagnetise after an armature has been removed.

It is therefore important, when remagnetising magnetos without an internal keeper using an internal remagnetiser, to apply a keeper before the magnetising core is removed, and to keep the keeper in place until after the armature has been replaced. That is what we do, and the result is as good as can be achieved using an external remagnetiser.

Of course, there are huge benefits of an internal remagnetiser, which is far less wasteful of copper, iron, money and bench space. Going by the wire gauge and resistance of Magnetoman's rig, and if my calculations are correct, it includes about 30 kg of copper. By comparison, an internal remagnetiser core takes about 300 g. Magnetoman's rig weighs 'several hundred pounds', whereas an internal remagnetising core weighs less than 2 pounds. And yet both can drive a magneto's magnet fully into saturation.

Brightspark Magnetos

Originally Posted by Ken Tee
but, in the debate, Magnetoman likes to quote from authoritative sources to support the points he's making.
Yes, I provide facts that can be independently checked, not unsubstantiated claims. My only interest in writing these posts is to provide what I believe to be correct information on a subject where much mystery and misinformation exists. It takes me a lot of time to write these posts, and I make no money as a result of their content, so my only "gain" is knowing I've provided useful information.

Magnetos are quite complex electromagnetic devices, so it would be impractical here to provide all possible background information. For example, the 7th ed. of 'Automobile Electrical Equipment' alone has four chapters devoted just to magnetos, plus relevant material in various other places in this 450 page book. I've included citations to authoritative sources in this thread where appropriate to supplement what I've written, so anyone with the interest can look further into the matter. Again, the reason for the cited references is they contain facts that people can check for themselves, not unsubstantiated claims.

Originally Posted by Ken Tee
were you telling the truth or an untruth? Please don't come back with more hundreds of words of bluster. A simple one-word answer please, "truth" or "untruth".
To phrase an accusation in the form of a question makes it no less an accusation. It is acceptable in any discussion to point out what you believe, correctly or incorrectly, to be an error. It is never appropriate to call someone a liar. To do so is bad enough by itself, but to do so without even pretending to provide a fact to support your false accusation is grammar school behavior. Unless you delete your post and issue an apology, you will get no answer from me to any question you ask.

Note to everyone else: I see that the post Ken Tee wrote after accusing me of lying about magnetos is another thinly-veiled advertisement in the guise of information. Even though it contains substantive errors, I don't plan to take the time to respond to anything from him.


I normally run long-term magneto tests at 2000 rpm (4000 rpm engine) because I want to generate a high internal voltage to reproduce what the coil will experience in later operation, as well as to put the largest number of "miles" on the magneto as fast as possible to reveal any problems. However, it's unlikely a 1920s Harley-Davidson V-twin will operate at 4000 rpm, so this magneto might not be designed to spin at 2000 rpm without the points bouncing. The reason this is a concern is shown in the next photograph.

[Linked Image]

On the left is the Bosch ZEV magneto and on the right is the Lucas KNC from my BSA Gold Star (note: I flopped the photograph of the KNC by 180-deg. to give it the same rotation sense as the ZEV, which is CCW when viewed from this end of the magneto). The KNC magneto spins to at least 3000 rpm (6000 rpm engine) and, at first glance, it might appear to have a more massive moving point assembly than the ZEV, so perhaps 2000 rpm will be fine. However, closer inspection shows that this initial impression is false. Most of the massive-looking KNC assembly is made of lightweight phenolic, while the entire ZEV assembly is solid steel. I didn't weigh them, but since steel has a density 7x higher than phenolic, a simple estimate is the moving point in the ZEV weighs 3-4x more than that in the KNC. This means that even 2000 rpm might be a problem for these points.

To see if this was the case, I installed the magneto on my long-term tester, removed the cover from the cam assembly (holding it in place with my hand, since the cover spring no longer was doing that), and used a General Radio Strobotac to watch the operation to see if everything was moving as it should.

[Linked Image]

Setting the strobe to precisely the rotation frequency of the magneto "freezes" the motion. Setting the strobe to a slightly different frequency makes the points plate appear as if it is rotating forward or backward in slow motion. This slow motion effect makes it easy to closely examine the operation everywhere throughout the entire 360-deg. of rotation, or to freeze it at any point, in order to spot any problems with the mechanical motion. This is why I always do this test. The next photograph shows that indeed there is a problem trying to make this magneto run at 2000 rpm.

[Linked Image]

For this photograph I had set the strobe to freeze the motion just after the points have been pushed open by the cam (rotation is CCW viewed from this end of the magneto). The reason it appears the points gap is quite a bit larger than it should be is because it is. The inset shows why this is the case. The rubbing block is "floating" above the cam because the inertia from its heavy mass has overwhelmed the ability of the spring to keep it in contact after encountering the ramp at this speed. This is the same phenomenon as valve float, except there is no piston nearby to wreak havoc. Unfortunately, this means I either will have to double the pressure by adding the spring from my ZE1 (as discussed in the previous post), or take the time fabricate a larger pulley to reduce the operation speed on my long-term tester. I decided to do the latter.

Another problem the strobe helped me spot, that I had not noticed earlier, is the spring comes within only a few thou. of touching the cam twice per revolution. Although I could slip a piece of paper between the spring and the cam at the distance of closest approach, which means they weren't actually touching, they were too close. I looked at the assembly in my ZE1 and saw the short "helper spring" that is on the inside of the main spring and attached at the 5:00 end in the above photograph instead is at the 11:00 end in my ZE1. Although I had no way of knowing if its location in the ZE1 is the correct one, after I moved it in the ZEV the clearance improved significantly. This seems to be yet another mistake to add to the long list of mistakes the restorer made when rebuilding this magneto. Send questions or comments to [email protected]
Originally Posted by Magnetoman
Another problem the strobe helped me spot, that I had not noticed earlier, is the spring comes within only a few thou. of touching the cam twice per revolution. Although I could slip a piece of paper between the spring and the cam at the distance of closest approach, which means they weren't actually touching (which would have shorted the armature), they were too close. I looked at the assembly in my ZE1 and saw the short "helper spring" that is on the inside of the main spring and attached at the 5:00 end in the above photograph instead is at the 11:00 end in my ZE1. Although I had no way of knowing if its location in the ZE1 is the correct one, after I moved it in the ZEV the clearance improved significantly. This seems to be yet another mistake to add to the long list of mistakes the restorer made when rebuilding this magneto.

Sorry to interrupt, but in the interests of correcting misinformation, I think you'll find that, in that Bosch set up, the cam ring is at housing earth, and the contact-breaker back-plate, spring and moving point are all at armature earth. So if they touched, the only thing which they would short out would be the earth brush. They would not short the armature.

But of course the spring touching the cam would be a bad thing because of the resultant wear.

The Lucas 'low-inertia' CB assembly in the other photo is a different kettle of fish. There, the spring touching the cam would indeed short the armature.

Magneto man, this has been a very educational thread, thank you for posting the beautiful pics and explanations. The strobe shots of the points bounce is very illuminating, it does look like there is a fair bit of extra material on the points moving arm with a lot of scope for judicious weight reduction especially compared with the Goldie points set up.

It is not clear to me how the Bosch points arm pivots, is the fulcrum point beneath the rounded tapering plate secured by the brass top hat washer, is it possible that the pivot bearing could introduce stiction and exacerbate the points bounce?
Originally Posted by gavin eisler
Magneto man, this has been a very educational thread, thank you for posting the beautiful pics and explanations.
Thanks very much for your comment. I've spent a lot of time trying to make sure the information is correct and verifiable, as well as understandable by people who may never have seen the inside of a magneto before. I've never seen information at this level of detail on the web or in print on the restoration of a magneto. Of course, this hasn't made everyone who sells magneto products or services happy. I'm reminded that John Wycliffe was burned at the stake for the heresy of having translated the Bible into vernacular English, taking the information out of the sole control of the priesthood to interpret for them. Although some responses have been irritating, it hasn't reached the point of a bonfire being started in my yard.

Originally Posted by gavin eisler
It is not clear to me how the Bosch points arm pivots, is the fulcrum point beneath the rounded tapering plate secured by the brass top hat washer, is it possible that the pivot bearing could introduce stiction and exacerbate the points bounce?
The pivot is a pin located at the ~10:30 position on the photograph, underneath the almost-vertical clip that has the elongated depression (elevation) in it. The points assembly has a ~1/16"-dia. pin that extends above and below it. At the bottom the pin is a slip fit in a hole in the brass plate. At the top the pin is retained by that elongated depression. That depression could have been a simple hole, but Bosch must have made it elongated to allow for the build up of tolerance (if it were a hole that was not located precisely over the hole in the plate below, the pin would be forced out of alignment and would bind).

Anyway, the points assembly pivots on the pin between the hole in the plate and the clip on the top. I lubed both locations with Lubricam, and at least by feel there was no stiction. That is, I could rock the points open and closed without feeling any resistance. Stiction would provide damping, and thus would help reduce overshoot, not exacerbate it.

The strobe is a very powerful tool for studying magnetos under actual operational conditions. Basically, the strobe lets me study everything in ultra-slow motion. Aside from floating points, if there is any jitter in what I see, that means there is variation due to some sloppy tolerance (which I then can look for and fix). If I were rebuilding a more modern magneto for a racing bike that needed to go to, say, 7000 rpm (3500 rpm magneto), the strobe -- plus different pulleys -- would let me make sure there were no mechanical issues up to that speed. I can't think of another instrument that would provide the same information the strobe does. This is a case where you need to see it, because that's what you're going to get.

Long-Term Tester

The reason for conducting an extended test is that, assuming a motorcycle is geared to go ~40 mph at the engine speed corresponding to my test speed for the magneto, a 12-hour test is equivalent to it covering 500 miles. Any teething problems should be revealed in this length of time, but it is not so long that it would cause significant wear on the "new" magneto. In the case of this Bosch ZEV magneto for the Harley-Davidson, I planned to extend the test to 24 hours.

My lathe has continuously variable speeds from 40-2000 rpm so I could use it to spin the magneto at whatever speed I want to within this range for further tests. However, since I run magnetos for at least 12 hours after rebuilding them I don't want to unnecessarily subject my lathe to that much use. So, some time ago I built a dedicated magneto tester using a reversible 1/2 h.p. motor and a universal mounting base that lets me test every type of platform- and flange-mount magneto I've yet come across. With my usual pulley the motor spins the magneto at 2000 rpm (4000 rpm engine). However, after finding with the Strobotac that the points on this magneto float at that speed, I had to fabricate a larger pulley in order to slow it down. Doing this was straightforward, but it ate up valuable time as the deadline for shipping it back approached.

I wanted the new pulley to be ~4" dia. to cut the speed by a third, but didn't have any Al bar that large at hand. So, I bought a 4" pulley from the hardware store, machined an Al rod a thou. oversize to press fit in the pulley's bore (and also held with the set screw), and then used my lathe to bore the necessary 1:10 taper for mounting it on the magneto. With this new pulley the magneto now spins at 1400 rpm (2800 rpm engine), which should be close to the upper limit it will experience on this motorcycle. Checking with the Strobotac showed that the points no longer bounced at this lower speed.

[Linked Image]

The first thing I found when I started the actual test was that the spark from this magneto was so hot that within a couple of minutes it began melting the plastic insulator on the 6-gap board at the left side of the tester. I got this Merc-o-tronic board on eBay recently and installed it in place of the one I had made myself (the fact I connected it to leads 5&6 instead of 1&2 is irrelevant).

[Linked Image]

Rather than take the time to reinstall my old gap board, instead I attached the two HT leads to nylon screws that can be seen at the right of the housing in the above photograph of the tester. I then used 0.032"-dia. stainless wires to make 5 mm gaps, corresponding to 6 kV at atmospheric pressure, and continued the test. This voltage is about 50% higher than what will be required to jump the spark plug gap in the operating engine.

[Linked Image]

At the left of the above composite photograph you can see that ~1 mm of the wire nearest the tip is glowing red hot from the heating caused by the spark current flowing through it (the bluish sheath around the tip is from the ionized plasma created by the high electric field, and the out-of-focus wire from the other lead is in the background). Since each spark lasts only a msec, it's easy to capture an image when it is not sparking, and that is what is at the right. Here it is even easier to see that the tip of the wire is red hot. What this shows is that, even though each current pulse lasts only ~1 msec. and is separated from the next by 35 msec. (i.e. a duty cycle of less than 3%), enough current flows to maintain the tip of the stainless steel wire at ~1000 oF (i.e. the temperature where the steel glows red). Note, though, that a spark plug electrode has both a larger diameter and a higher thermal conductivity than the stainless steel wire, so it would not get nearly as hot as the wire I used for these tests.

Even more dramatically than the glowing wire, if I slip a piece of paper between the electrodes it immediately bursts into flame (it did not touch the hot electrode; it is the spark itself that ignited the paper). Since the sparks from this magneto so easily set fire to paper, I can be reasonably confident they will ignite the mist of gasoline in the combustion chamber.

[Linked Image]

Elevated Temperature Test

After running the magneto on this tester for 18 hours, I wrapped it in heating tape and heated it to ~50 oC (122 oF) using a Variac, with a thermocouple to monitor the temperature. I ran it another 6 hours at that elevated temperature and it continued to spark reliably. Earlier, in the interest of time, I had skipped testing the armature by itself at elevated temperature, and this 6-hour test of the full magneto vindicated having taken that shortcut. However, had the magneto failed this elevated temperature test due to a faulty armature I would have had no choice but to disassemble it and find the time to wind a new coil myself. I took the next photograph after completing this test and already starting to remove the heating tape. Send questions or comments to [email protected]

[Linked Image]
Posted By: Peter R Re: Restoring a Rotating Armature Magneto - 11/11/12 3:42 pm
Very interesting material MM, thanks for posting.
You must have invested a lot in all this equipment.
Did you ever consider rebuilding magnetos on a professional basis ?
Originally Posted by Peter R
You must have invested a lot in all this equipment.
Did you ever consider rebuilding magnetos on a professional basis ?
I try not to think of how much money I've spent on instruments, or time on designing and fabricating magneto-specific equipment, fixtures, etc. These posts haven't even shown all of it, because not everything was needed for this particular restoration.

Although I have no way of knowing for sure, I very seriously doubt even the most heavily equipped professional restorer has the equipment and facilities I do for diagnosing and repairing magnetos. As I wrote in a previous post, well over a decade ago I became obsessed with understanding at the most detailed level the operation of magnetos, and cost is no object when feeding an obsession.

As for making money from this accumulated knowledge and equipment, the fact is, I doubt very many motorcyclists would be interested in paying me what I would have to charge in order to make restoring magnetos worth taking the time away from other things that I do.

There will be two more installments in this thread, the last of which I've drafted to help people identify restorers for repairing their magnetos. I won't name names, but will describe what to look for to separate restorers who won't be able to do a good job from those who might be able to (in the sense that you don't necessarily need to know the guy's name to understand that if you're in a bank and you see someone holding a gun that he's probably not someone you should offer to buy a gun from).
Posted By: edunham Re: Restoring a Rotating Armature Magneto - 11/12/12 2:05 am
I have been very impressed by this thread and have found it helpful, although some has been a little beyond my level. I would like to get your opinion on something. I have 2 bikes with Lucas magnetos, a k1F and a k2f. For years I have been setting the timing by removing the center bolt that holds the points plate, and using a continuity light. i don't remember where I first came upon this method, but it is not in the manuals, and I have not seen it referenced anywhere. Typically, I see references to using cigarette papers or expensive boxes. While the method has always worked for me, the fact that I do not see it referenced anywhere makes me wonder if there is some reason, theoretical or otherwise, why I should not be using it. From your posts, you seem like you might have some thoughts on this method.

Ed from NJ
Originally Posted by edunham
I have been very impressed by this thread and have found it helpful,

a k1F and a k2f. For years I have been setting the timing by removing the center bolt that holds the points plate, and using a continuity light. ... While the method has always worked for me, the fact that I do not see it referenced anywhere makes me wonder if there is some reason, theoretical or otherwise, why I should not be using it.
Thanks very much for your comment. I'm happy to hear you're finding it helpful.

The center bolt performs two functions. One function is as an electrical conductor to connect the points to one side of the primary. That's why removing it lets you use a continuity light. Otherwise the ~0.5 Ohms of the primary would effectively "short out" the light, so it would be lit whether or not the points were open. The other function is to mechanically lock to points plate to the armature. What this means is that if the points plate doesn't twist or move sideways by even 0.001" when the tension on the bolt is removed, the orientation of the armature when your light tells you the points open will be the same as when the bolt is retightened. Otherwise, it won't be, and the timing will be off.
Posted By: edunham Re: Restoring a Rotating Armature Magneto - 11/12/12 1:04 pm
Thanks for your thoughts. I think I will add a step to my procedure and make absolutely sure that the points plate is still tight on the taper after removal of the centerbolt and prior to setting timing simply by giving it a wiggle with my fingers.

Ed from NJ

Finally, we're in the home stretch. This Bosch ZEV magneto has passed all my electrical and mechanical tests so far, giving me good reason to be confident it will be function without problem for some thousands of miles. But, there are still a few more things to check before I am ready to ship it back to the engine builder for installation on the bike.

Low Speed Test

It's great that this magneto works so well at high rpm, but first the bike has to start. So, I next moved it to the lathe, whose continuously variable speed would allow me to run it as low as 40 rpm (80 rpm engine). The point of doing this is to determine the lowest speed the magneto will reliably produce 6 kV sparks with it fully retarded (where it will be for starting).

[Linked Image]

The photograph shows the magneto mounted on a bracket that I substitute for the lathe's compound and that sits 45 mm below the center line of the lathe. I also have a 10 mm spacer for mounting magnetos having a 35 mm spindle height, as well as a vertical plate with appropriate holes for holding flange-mounted magnetos. Although I don't expect anything in the magneto to seize, the lathe's 1-1/2 h.p. motor could cause quite a bit of damage if it did, which is why if you look closely you will see a short length of plastic tubing is part of the drive train for the magneto. I ran this test for several minutes and the magneto continued to spark reliably across a 5 mm gap down to 135-145 rpm (270-290 rpm engine) with the cam at all positions between fully advanced and fully retarded. A Lucas manual says 300 rpm is a the low end of kick starting speeds, with 500 rpm normal, so the magneto passed this test.

Distributor Tester

My final test used a modified distributor tester to see if during operation the sparks are precisely 157.5/202.5 degrees apart as they need to be for this Harley-Davidson engine, and as my static measurement of the cam profile indicated they should be. However, there are several reasons why under dynamic conditions the firing could be off, or even fluctuate around the correct values, and the only way to know for sure is to measure it. To some extent the Strobotac already addressed part of this issue, since I would have seen fluctuations in the positions of the points when they opened if larger than it a degree or so.

Ten years ago I made several modifications to an Allen distributor tester, including adding an adjustable platform that accommodates both platform- and flange-mounted magnetos. However, when the Bosch arrived the tester had been partially disassembled for a few months to make upgrades to it (actually, most of that time it had been just sitting there waiting for me to find the time to finish), so the temporary configuration I used for this test had the sparks strike just outside the markings on the large protractor. This tester spins the magneto either CW or CCW, whichever is appropriate, at up to 2500 rpm (5000 rpm engine). In addition to testing magnetos with manual advances, like this Bosch ZEV, a digital tachometer allows me to determine the advance curve of a magneto's auto-advance unit, to make sure it is operating properly. I also have adapters that let me check the advance curves of auto-advance units from newer motorcycles that don't have magnetos.

Ideally, the sparks always would happen at the same angles. Since their positions can be easily read to a fraction of a degree on this tester, any variation in spark timing larger than this is immediately apparent. The quantitative results from this tester allow me to decide what further work on a given magneto might be required (e.g. stoning the cam to alter the timing by a specific amount).

The first photograph shows what this tester looks like, although with a ZE1 magneto sitting on the base. The second photograph shows the sparks from cylinder #2 of the ZEV, with the protractor adjusted so cylinder #1 was at 0 deg.

[Linked Image]

[Linked Image]

The magneto was running at 1250 rpm (2500 rpm engine) for the above photograph and I used a 1/4-sec. exposure to capture 6 sparks. The protractor is slightly blurred because of vibration of the tester during the long exposure. Although it might appear that the timing was wandering by a degree, the apparent variation is largely an illusion due to the spark finding a different route to earth each time. Note that all the sparks radiate from the same point (to within ~0.2-deg.), which is the tip of the spark wire passing by much too quickly to be photographed with this long exposure (some of the ~0.2-deg. variation could be due to vibration of the tip of this wire in the temporary configuration I used, since the last ~3/8" was unsupported). This test shows that the timing of the magneto wanders by no more than ~0.2-deg. from one cycle to the next. Although this photograph only captured 6 sparks, I watched it closely for several minutes, seeing no sign of problems.

As I wrote in an earlier post, the points for cylinder #2 should open at 157.5 deg. on the magneto (315 deg. engine) for a 45-deg. V-twin engine, but this test shows that the spark is 1.3-deg. early (2.6-deg. engine), at 156.2. Again, this is with the protractor adjusted so #1 is at 0 deg. In terms of engine timing, what this means is #1 would spark that cylinder 2.6-deg. late if #2 were set to fire at the perfect spot. Since the spacing between #1 and #2 on the cam is 1.3-deg. too close (and between #2 and #1 that much too far), it might seem I should improve the timing by stoning a slight amount from the #2 ramp (if they were too far apart, it would require a new cam). However, the static measurement I made on just the cam, described in an earlier post, found it to be good to better than 0.1-degree. This indicates the source of the 1.3-degree problem is a buildup of tolerances of the several components, so grinding one of the ramps would be attacking a symptom rather than the actual problem. And, once ground away, metal can't be put back on the cam.

It bothers me to accept this 1.3-degree error in magneto timing (2.6 deg. engine), and if I had more time I would develop a proper solution, but sometimes perfection is the enemy of perfectly acceptable. The magneto has a manual advance, so if the rider hears pinging from one of the cylinders and retards the ignition until the pinging stops, the other cylinder will be 2.6 degrees further retarded from the optimum timing. However, given the low compression of the engine this magneto will be used on, this will not have a significant effect on performance, especially for its intended use in the cross-country Cannonball Run. Still, even though it won't matter much in practice for this particular engine, it bothers me not to have the time to resolve this.

Disassembly, Inspection, Remagnetization, and Reassembly

After having run the magneto for ~24 hours (~"1000 miles"), I disassembled it to inspect the bearings, measure the brushes for wear, and look for any signs of distress. Everything was fine, so I relubricated the bearings with Sta-Lube high temperature disk bearing grease and the rubbing block with Lubricam, reassembled it, checked that the gap was still 0.012", and then remagnetized it.

Although I previously wrote that running it on the modified distributer tester was the final test, the actual final-final test was to run it for another 15 min. on the long-term tester before packaging it up and sending it back to the engine builder.

Timing a Magneto Using an Inductance Meter

Timing this magneto to the engine wasn't part of the restoration, because that would be done 1500 miles from me after being delivered to the person rebuilding the bike. However, I wanted to at least briefly address how to do this using something quite a bit better than cigarette paper.

There are two aspects of timing a magneto to fire at the right moment. First, the engine has to be rotated to the correct position before top dead center (BTDC) where you want the magneto to fire. Most commonly, this is done at the fully advanced (high rpm) position, rather than retarded (low rpm). Finding the correct position -- most engines are between 30 and 40-deg. BTDC -- can be done to varying degrees of precision with a dial indicator and protractor, a ruler stuck down the spark plug hole, a factory mark on the crankshaft, etc. For the purposes of this post, assume the engine is now at the correct angle BTDC where you want the magneto to fire.

Next, the taper on the magneto's armature is loosely inserted in the gear or sprocket in the engine's timing chest. What you now need to do is to rotate the armature until the points have just opened by the slightest amount, and then tighten the armature to the gear to lock in that timing. Assuming for the purposes of this post that nothing slips when you do this and that there is no backlash in the gear train, the magneto is now correctly timed to have the points open when the engine is at the correct angle BTDC. But, how to find the position where the points have just opened? Forget using cigarette paper.

The resistance across the points of a magneto when they are closed is 0 Ohms, and when open is only ~0.5 Ohms, so a standard ohmmeter would barely register the difference. However, the inductance of a magneto's primary when the points are closed is some number of milliHenries, and when open is some number of Henries, i.e. ~1000x greater. So, an inductance meter across the points will register a huge change the instant the points have separated. Although measuring the inductance might sound difficult, it isn't.

If you search eBay for 'LCR meter' (without the quotes; also try 'LRC meter' for more choices) you will find precision ones go for over $1000. Luckily, though, you don't need precision, so Chinese-made ones that sell for ~$18 (delivered price) are just fine. Attach the leads across the points and set the scale to whatever mH value gives a reading when the points are closed. The actual value is irrelevant, and you don't even have to know the difference between a milliHenry and a megaOhm because all you care about is seeing the meter abruptly go over-range when you slowly rotate the armature. When that happens, tighten the nut to lock the armature to the gear, and the magneto is now properly timed to fire when the engine is at the correct angle BTDC.

Performance of the Magneto on the Road

A month after I shipped this restored Bosch ZEV back to the Harley-Davidson's rebuilder the bike was used in the 2012 cross-country Cannonball Motorcycle Run. Unfortunately, the builder was able to finish the bike only a few days before it had to be transported to the start in New York so there was no time for my friend to give it a proper shakedown. The bike suffered a variety of problems that would have been easy to fix under different circumstances, but that kept it from covering more than ~800 miles over the course of the Run. However, I'm happy to say that the magneto was trouble free. Added to the ~1000 simulated miles I subjected it to on the long term tester before shipping it back, this restored 90-year old magneto has "travelled" nearly 2000 miles without problem thus far. I have every reason to expect it to be good for many thousands more.

This post is the last on the actual restoration of this Bosch ZEV magneto, but there will be one final "Epilog" that I hope will help people identify someone who can properly restore their magneto. Send questions or comments to [email protected]

As far as I can tell, at the time of this writing (Fall 2012) this is by far the most detailed description of the restoration of a magneto on the web or in print. Because of this, some additional observations might be useful for people on this Forum as well as those who find their way here thanks to Google.

Basically, there are three audiences for the information in the previous posts in this thread: people who plan to rebuild their own magnetos; people who want their magnetos rebuilt for them; and people who rebuild magnetos for profit. This Epilog is primarily for the second group: people who want help identifying someone who can properly rebuild their magneto.

The Major Problem Areas in Restored Magnetos

Briefly, when magnetos are improperly rebuilt, the subsequent problem(s) they develop most likely result from one or more of the following factors, all of which I have addressed in my previous posts in this thread:

-- Inappropriate condenser
-- Improperly rewound coil
-- Improper (or no) remagnetization
-- Aftermarket brushes that are either too hard or too soft

There can be issues other than these, but these four account for most of the failures I have seen in magnetos that were professionally rebuilt. Because of this, my first recommendation is that you make sure whoever you hire to restore your magneto at least has the tools required to correctly deal with these four issues at a minimum.


I wrote in the initial installment that my goal in restoring this Bosch ZEV rotating armature magneto was simply to return it to the condition it had when it left the factory. In my experience, to do less than I described in these posts would have resulted in a magneto that was not as reliable, had a shorter life, and/or produced a lower output than it did when it left the factory nearly 90 years ago. The major tools and test instruments shown in this thread that were required to accomplish this were:

Mill with digital readout
Lathe with continuously variable speed
1/2 h.p. long-term tester
Modified distributor tester
General Radio Strobotac
Talyrond roundness tester
Bench center with four dial test indicators
Surface roughness tester
Contact breaker pressure gauge
Precision scale

84,000 A-turn electromagnet
500/1000/2500 V megohmmeter
LRC meter
Merc-o-tronic magneto tester
Eisemann magneto tester
[coil winder & vacuum pump-- not used, but would have been had there been more time before the deadline]

Centering microscope
Stereo microscope
Traveling microscope
Metallurgical microscope

Although I have no way of knowing for sure, I seriously doubt even the most heavily-equipped professional magneto rebuilder has the range of equipment and facilities I do for diagnosing and restoring magnetos (not all of which were needed for this Bosch ZEV). But, not all of this equipment would be needed for many rebuilds. So, which of the above tools would someone not need if they only planned to "restore" a magneto as "efficiently" as possible? That is, what is the minimum set of major tools that a rebuilder would need to return the majority of faulty magnetos to customers in a condition where they appeared to function properly when removed from the shipping box?

If the screws in the armature of this Bosch ZEV had not been broken, microscopes and a mill wouldn't have been needed. Although I used the lathe and mill to help remove the epoxy, if someone just wanted to get the stuff out they could have used an acetylene torch instead (I'm definitely not recommending this, but I have seen armatures where the restorer had done this). Anyway, under these conditions, the only tool from the above list actually required would have been an electromagnet. Further, if someone only worked on post-1930s magnetos containing Alnico magnets, even that wouldn't be essential in order for the "restored" magneto to spark when turned with an electric drill (albeit, with a significantly weaker spark than it should have). The point being, there is a huge range of equipment, expertise, and time required between being able to claim to "restore" magnetos, and actually being able to restore them to the reliability and performance they had when they left the factory.

As a recommendation, since even the most straightforward rebuild will require the following, you should not consider sending your magneto to anyone who does not, at the very minimum, have a:

-- 65,000+ A-turn electromagnet (can be less powerful for older, pre-Alnico, magnetos)
-- 2500 V megohmmeter
-- Merc-o-tronic, Eisemann, or equivalent magneto coil tester

I can think of five people who posted to Britbike Forum over the last six months who said they do magneto repairs as part of their business. There was enough information in the posts of four of them for me to see that they do not meet even these minimal requirements, illustrating that by no means is everyone who claims to be able to repair magnetos actually able to do a proper job of it. It also illustrates why such a large number of "professionally repaired" magnetos fail.

Magneto Repairers

There is no accreditation board for magneto repairers, so anyone can claim to be an expert. One YouTube video shows a magneto being turned with an electric drill and the "expert" declaring -- quite incorrectly -- that the intermittent spark it develops at higher rpm indicates it needs to be remagnetized (the problem almost certainly is due to a bad condenser). Neither does a Consumer Protection Agency evaluate advertising claims for veracity. For a number of years one well-known supplier sold replacement condensers with the claim "Modern substitute, very high specification, zero failure." Despite this claim, many failed in service. Ignoring self-proclaimed statements of expertise or reliability, if you are looking for someone to restore your magneto, at a minimum you should determine if they have the necessary equipment to do a proper job (described in the previous section). That alone will eliminate a significant number of possible rebuilders from consideration.

Unfortunately, even if someone owns equipment more advanced than an electric drill, it still may not be obvious whether or not that equipment is appropriate. For example, a well-known magneto rebuilder has a web page showing the equipment he used to rebuild a post-WWII magneto, with one photograph showing it being remagnetized. The commercial magnetizer shown being used for this does not have the necessary field strength to fully magnetize Alnico, which means that magneto was returned to the customer with sub-standard performance. The same is the case for a small "internal magnetizer" another firm inserts in place of the armature. Irrespective of what field that magnetizer is able to produce, the moment it is removed from the magneto the reluctance of the circuit changes significantly. For reasons explained in references cited in earlier posts, this change in reluctance forces the working point on the Alnico's B-H curve to shift, resulting in permanent partial demagnetization, in turn resulting in sub-standard performance of the magneto (i.e. 20-30% reduced output).

The component that is responsible for most failures of rebuilt magnetos is the condenser. Given the countless magnetos that have failed because rebuilders used inappropriate condensers, coupled with the dismal history of false claims from suppliers like "Modern substitute, very high specification, zero failure," I strongly suggest you determine what condenser the rebuilder uses. As I wrote in an earlier post, the condenser I used in this Bosch ZEV was a pair of Panasonic polypropylene film/foil capacitors. I used these capacitors because the manufacturer rates them for high pulsed currents as they will experience in a magneto, they passed extensive environmental and electrical stress tests described in a two-part article in the Fall and Winter 2011 issues of 'The Antique Motorcycle', and they are the only replacement capacitors I am aware of that have passed such stress tests. However, if it were not possible for the rebuilder to get these particular ones, other film/foil capacitors (retail cost ~80 cents) have similar electrical specifications, so alternatives exist for installation in the original location in the condenser cavity. In any case, I recommend that you do not send your magneto to any rebuilder who uses a ceramic chip capacitor (retail cost less than 10 cents) that is packaged by one supplier for use in the points housing.

Armature Winders

If your magneto needs to have its armature rewound, chances are the person doing the restoration will arrange to have that work done by someone else. This is perfectly fine, except…

Although various people offer this rewinding service for ~$150, I spent considerably more money than that to buy my own coil winder, pump, etc. in order to wind and vacuum impregnate armatures myself. As just one of many examples for why I did this, a rewound armature supplied by a well known rebuilder repeatedly seized in the 2010 Cannonball Run due to him having used improper resin to encapsulate it, which continued to ooze for several days. No doubt some rewound coils are made to proper standards, but many are not, and there is no way to look inside the coil of a completed armature to see if it was wound with appropriate insulation and has correct encapsulating resin in the right locations.

Despite the rewound armature that came in this Bosch ZEV having passed all my tests, there is no way to test for possible slow abrasion of the insulation due to relative motion of the wires that could happen if the coil had not been properly vacuum impregnated. Although I judged the chances of failure of the coil as not large, this is the only aspect of this rebuild where I had any uncertainty. As a result of it, I breathed a sigh of relief when my friend called me from the finish line in San Francisco.

Coil winding is such a fiddly job that I would gladly pay someone $250 to do it for me rather than doing it myself. Unfortunately, after having seen a number of rewound coils, I concluded that winding them myself was the only way I could be sure the magnetos I restore will be at least as reliable as they were the day they left the factory. It takes me at least one long day without interruption to set up the equipment, remove the old windings from an armature, wind new ones, vacuum impregnate the coil, and put everything away again. However, if winding armatures were my only job, I can imagine the continual practice would allow me to wind them in maybe two hours (or even less) rather than a full day.

Magneto armatures have been around for more than a century so you would think that by now it should be very well established how to wind them correctly. As late as the 1950s magnetos were produced by the thousands every year just to supply British factories, so techniques of mass production resulted in excellent reproducibility and reliability. Today, though, the winding of magneto armatures is a cottage industry, done one at a time by people working alone and without any independent quality control. To repeat something I wrote in an earlier post, "there is a difference between hand made, and home made." This alone explains why many rewound armatures fail in service. Also, it is clear from what I have seen that many armature winders simply don't understand that there is a lot more to winding a reliable armature than putting many turns of thin wire on top of fewer turns of heavier wire.

The relative number of turns in the primary and secondary determine the output voltage, but the windings ratio is just one aspect of a proper armature. Other essential aspects include the total number of turns of each coil, not just the relative number; thickness of the wires; type and thickness of insulation on them (the insulation on "magnet wire" sold today is available in four thicknesses and at least seven different classes of material, not all of which are appropriate for use in a magneto, with breakdown voltages differing by over a factor of 4 -- and there's no guarantee someone isn't winding their coils using 50-year old spools that has obsolete and age-degraded insulation they bought cheaply on eBay); type and thickness of insulation wrapped over the core and between the layers; type of resin used to encapsulate it (i.e. not just that it has enough viscosity not to ooze out, but that it completely fills all the voids and fully hardens to eliminate all movement of the wires); and whether it was properly vacuum impregnated (which itself requires more than just applying a vacuum to the armature).

Despite my experience with the poor quality of some of the rewound coils being produced, in trying to prepare for possible eventualities I might face I contacted one well-known firm to ask them their turn-around time for rewinding a Bosch ZEV armature. Since I knew it would be difficult to find the time to wind a new one myself if the magneto arrived with a bad coil, I wanted to know if there was another option. When I contacted the firm I also wrote that I would test their rewound coil for a number of hours at 120 oF on my Wiedenhoff magneto tester before installing it and, since I would be working to a deadline, I wanted to know their policy on refunding money if the armature failed (rather than offering to rewind it again, since there wouldn't be time for that). I don't know if it was because I mentioned having my own coil tester, but I never received a response.

Unfortunately, the possible need for a rewound armature makes the restoration problem two levels deep. Not only do you have to find someone with the tools and expertise to properly rebuild your magneto, that person has to have the expertise to identify someone else who properly rewinds armatures. Of course, both good and bad magneto restorers will assure you that the person who rewinds the armatures for him does an excellent job. Maybe they do, but odds are they do not. I realize few people reading this will have an interest in winding their own coils, and I wish I could offer a constructive suggestion here, but all I can do is offer this information on coil winders as an observation.

How Much Should it Cost to Have Your Magneto Rebuilt?

So, how much should it cost to have someone properly rebuild a magneto? The good news is your magneto again can be as reliable as it was when it left the factory. The bad news is, it can't be done for $150.

I didn't keep track of the hours I spent on this Bosch ZEV because I was doing it for a good friend and had no intention of charging him for anything. Also, I had never worked on a ZEV before, so I had to spend additional time researching obscure screw threads, and machining a fixture and pulley specifically for it which, if I were doing this as a business, already would be in hand. Also, if I could have worked on it full time from start to finish it certainly would have been more efficient than piecing together a few hours at a time between trips. With this in mind, how long would it take for a fairly straightforward rebuild of an ailing rotating armature magneto that had been "professionally" rebuilt before, but whose armature hadn't suffered at the hands of the rebuilder as much as this one had?

No matter what, count on having to spend time removing the globs of epoxy that almost certainly would be holding in place the inappropriate condenser the rebuilder had used, in order to make room for a proper replacement. Doing that, skimming and preparing the surface for the earth brush, truing the slip ring, adjusting the end float, etc. all take time beyond just replacing brushes, greasing bearings, and bead blasting the body (even if what's inside is a bodge, magneto restorers always make sure the outside looks pretty, because that's how 95% of their customers will judge whether or not it has been properly restored). Assuming the coil did not need rewinding, my guess is a magneto in reasonable condition would take me the better part of a full day to rebuild to the same "as-new" standards as this Bosch ZEV. This includes conducting the necessary measurements and tests described in these posts (but not counting the hours chugging along by itself on my long term tester).

It should be clear from the posts that doing such work properly is skilled labor, and it requires specialized tools and instruments as well as expertise. One yardstick for cost might be the $85/hour that a machine shop in my town charges. At that rate, the labor alone to properly rebuild a magneto would be ~$700. Although this may seem high, I would have to charge at least that much if I were willing to take on the work. Other than perhaps a few Brough-Superior and Vincent owners, I doubt there would be many motorcyclists ready to pay that (owners of expensive classic automobiles might be more plentiful). Even if you think I am wildly off on my time estimate, and that it actually only would take four hours, that still would be $350 plus parts. But, having said that, I don't believe anyone could restore a magneto to as-new operation in only four hours, no matter how efficiently they were able to work. Even if someone who was skilled enough to command higher wages were willing to work for only $40/hour (which also has to cover health and business insurance, retirement savings, rental of space, and repair and replacement of equipment), and even if they could do a professional job in only six hours, that's ~$250 for labor. Again, these figures all assume a magneto that was in reasonably decent condition to begin with.


I restored this Bosch ZEV for use in a motorcycle rally, so I spent no time on its external appearance. Had cosmetics been important, just soda blasting the alloy and painting the magnets would have added at least an hour. However, a full concours restoration would have required sending fasteners out for replating (after spending time polishing them), polishing brass and alloy, and finding better-looking HT pickups (or spending time making the current ones look much better). To do this would take more than a few hours and could double the estimates given in the previous paragraph. Again, this is just for the labor.

Final Comments

Even with the proper equipment a rebuilder still needs expertise, but that's harder to verify, and recommendations mean almost nothing in this area. There are a lot of unqualified people doing poor jobs rebuilding magnetos, but who get good recommendations despite magnetos that routinely fail. As an aside, I'm amazed at the number of times I've heard people recommend the person who rebuilt a magneto for them that subsequently failed. I'm not a psychologist, but this seems to be some form of the Stockholm Syndrome. To paraphrase Bruce Springsteen's 'Magic' one final time, as far as magneto rebuilders are concerned "trust none of what you hear, and less of what they claim."

Although I am sure qualified people do exist, I would have to personally know someone's work before I could recommend them as being able to properly restore your magneto. However, in your search for that person keep in mind that it is not that you get what you pay for, it's that you very seldom get more than you pay for. Even though a $150 quote for labor is highly unlikely to get you a $750-level rebuild to as-new performance, paying $750 still may only get you a bead-blasted housing that disguises a $150 repair of dubious quality. To end this thread on a positive note, when looking for a magneto rebuilder, Trust, but verify. Send questions or comments to [email protected]
Posted By: Magnetoman Re: Restoring a Rotating Armature Magneto - 12/03/12 10:23 pm
If you go to the top of the first post in this thread you will see I've just added a Table of Contents to make it easier to find the information in the individual posts. However, I couldn't figure out how (or if) this Forum encodes hyperlinks to individual posts. If someone can tell me how to do that, I will be happy to update it to make it easier to jump to any post of interest.
Posted By: TR6Ray Re: How to link to individual posts - 12/04/12 7:31 pm
Originally Posted by Magnetoman
If you go to the top of the first post in this thread you will see I've just added a Table of Contents to make it easier to find the information in the individual posts. However, I couldn't figure out how (or if) this Forum encodes hyperlinks to individual posts. If someone can tell me how to do that, I will be happy to update it to make it easier to jump to any post of interest.

I'll make no claim that this is the best method to link a specific post, but it does work. I'll use blue font (like this) for the steps to be followed, so they stand out amongst the rest:

{Edit: Due to the havoc wrought by Photobucket in destroying all their picture links and due to the updated format of this BritBike Forum, all the instructions that I once placed here to help MagnetoMan have gone obsolete or missing. In case someone might find it useful, I updated the info on 9/01/2017. I used a different example thread, because MagnetoMan's pictures fell prey to the evils of Photobucket and disappeared.}

Find the existing post that you want to reference. (It will be more convenient to open a second browser window to do this.) With that post on the screen, point your mouse arrow to the Post Number at the upper RH corner of the box. For example, suppose you want to add a link to a particular post within the thread, "1964 TR6/R Resto" as shown here:

[Linked Image]

Now, RIGHT click the Post Number. When the dialogue box opens, LEFT click the "Copy Link Location" command to copy the desired link onto your windows clipboard, like this:

[Linked Image]

Now, in your "New Reply" dialogue box (using the Full Editor), LEFT click at the spot in your text where you want to place the link. With the cursor flashing at the correct spot, LEFT click the link icon at the top of the dialogue box (see below):

[Linked Image]

When the dialogue box opens, RIGHT click in the text field (there's only one, so you can't go wrong):

[Linked Image]

Another dialogue box will open.

[Linked Image]

LEFT click on the "Paste" command. This will paste the link from your windows clipboard into the field in the dialogue box. Immediately, another dialogue box will open and prompt you to type in whatever info you want to actually appear on the screen as the label for your new link. In this case, I'll type in, "1964TR6/R Resto".

[Linked Image]

After you name your link, LEFT click the "OK" button. That's it, your link will appear in your Posting form dialogue box. Because of the format now used in BritBike Forum, your link will not be underlined like my example one below. I find it easier to recognize a link when it is underlined, so I edit mine to appear that way. If anybody is reading all of this, can't figure out how to underline their links but would like to, send me a PM and I will add that info here.

1964 TR6/R Resto


Originally Posted by TR6Ray
I'll make no claim that this is the best method to link a specific post, but it does work...
Thank you very much. With three windows open, and only a few screwups along the way, the thread now has a "live" Table of Contents. It should make it a lot easier for people to find whatever they might be looking for without having to scroll through all sorts of nonsense to try to locate it. Again, thanks very much for the detailed instructions (which I kept on screen the entire time in one of those three windows).

p.s. I tested about half the links without finding any issues. But, of course, if anyone finds a problem, please let me know so I can fix it.
Posted By: Magnetoman Re: Restoring a Rotating Armature Magneto - 12/05/12 12:34 pm
Originally Posted by Ken Tee
Anyway, Magnetoman, what you said about Young and Griffiths is untrue (they were talking about a horseshoe magnet and keeper, not a magneto and its armature), and what you said about Spreadbury now also turns out to be untrue (he was talking about a generator, not a magneto). Very sad.
The physics of electrical generation by a generator is identical to that of a magneto. Maxwell's Equations apply to both in an identical fashion. Spreadbury describes how a generator/magneto that is to be disassembled in the field without the possibility of remagnetizing it has to be overdesigned if it is to still function, albeit at reduced output, to compensate for the loss of magnetism when it is disassembled. As I said more than once, a post-War magneto will still function after it has been disassembled, albeit at reduced output because of the loss of magnetism.

The quantitative loss of magnetism Spreadbury reports for the type of "generator" he addresses is consistent with what my measurements show for a magneto if it is disassembled and then reassembled. The power of an understanding of physics is that the same general principles (e.g. Maxwell's Equations) describe a wealth of phenomena, so understanding the commonality of a "generator" and a "magneto" lets data from one be correctly used to understand the other. What you wrote is as silly as saying measurements on a BTH magneto tell us nothing about a Lucas, because different names are stamped on their housings. For both books I correctly summarized for this thread the relevant conclusions that would have taken many pages to describe in complete detail. What I said about Spreadbury is completely true, as is what I said about Young and Griffiths.

For the reasons I explained earlier in this thread, the way your "internal magnetizer" functions leaves the magneto with significantly reduced output over that which it would have if properly magnetized. This is what an understanding of the content of these books shows, and this is what my measurements show. However, your lack of understanding of physics is no excuse whatever for the despicable way you have repeatedly called me a liar. Even if there weren't issues with your goods and services, I personally would never knowingly do business with someone who behaves like you.

p.s. I just noticed that Ken altered the text of mine that he quoted in his post, making it appear as if I had emphasized something that I had not, and that I had not emphasized something that I had. What I wrote was "F.G. Spreadbury shows that the output from a magneto with a Ticonol ("Alnico") magnet is reduced by 23% in actual operation if the armature is withdrawn and then replaced after magnetization." Taking the time to alter someone's text to change the emphasis and then presenting it as if it were a direct quote is unacceptable, and begs the question of whether he has altered anything else.
Seeing that magnetoman has been outed by someone else I think people who are reading this tome should know who he is:

The good Dr. has one of the most extensive annotated motorcycle and related mechanical engineering libraries in the World. Not only has he read them all, but collected a huge database allowing him to research subjects quickly.

While his approach follows a strict academic model that some would consider overkill, he is not satisfied to have the skill to do something, but to also understand the underlying physics.
Now, if I could get Dr. Falco, Kevin Cameron and Rob Tuluie together with a tape recorder or better yet write for Vintage Bike...
John Healy
Note: A version of the material in Appendix I and II was part of a series of two articles I wrote for the Fall and Winter 2011 issues of 'The Antique Motorcycle,' the journal of the Antique Motorcycle Club of America.

Appendix I: Post-WWII Magneto Condensers

The Condenser

For the reasons I explain below, if you have a post-WWII British bike, either your magneto's condenser already has failed, or it soon will fail. When the condenser does fail, there are at least a dozen different categories of replacements (tantalum, metalized polyester, ceramic,…) made by dozens of manufacturers that have the necessary capacitance and will fit in the available space in the armature. Unfortunately, despite anything you might have been told before, essentially all of these will fail in operation, because most lack the ability to handle high current pulses. However, as I will describe in a subsequent post, I have conducted a series of tests that identify the type of replacement condenser that will provide many years of service.

Symptoms of a Bad Condenser

Often people who have a malfunctioning magneto say "I need to send it in for a rewind." In my experience, once other potential sources of problems have been eliminated (most commonly, a fouled plug, cracked high tension lead, short in the cutout circuit, or a worn or seized high tension pickup), in nearly all cases the problem is a bad condenser, not a faulty coil (unless it is a replacement rewound coil, in which case the coil could have developed an internal short). If your engine runs fine when cold, but misses and backfires heavily as it warms up, odds are high it is due to a bad condenser. Obvious sparking at the contact breaker points is the "smoking gun" of a faulty condenser.

Why all post-WWII Lucas and BTH Condensers Have Failed, or Are About to Fail

In 1915 B. H. Davies wrote "Riders need not worry about the action of the condenser, which never gives any trouble." In fact, this was largely true at the time he wrote it. Unfortunately for us, pre-WWII mica condensers gave way to paper condensers that were developed in no small part because of the disruption in the supply of mica from Zimbabwe (Rhodesia) and India caused by WWII. Also, while mica condensers don't suffer the specific degradation problem described below, they can fail from mechanical delamination or corrosion of the leads.

It turns out condensers (capacitors) of identical materials and construction as the ones Lucas and BTH used in their post-WWII magnetos also were of much interest to the telecommunications industry. Because of this, the greatest solid state physics laboratory of the time, Bell Telephone Laboratories, researched these "impregnated paper capacitors" in great detail. A 1946 book by a Bell Laboratories scientist, along with eleven research papers on paper capacitors published between 1942 and 1961, contain information that fully explains the cause of the problems post-WWII British magnetos are facing today (the book and papers are listed in the References section at the end of this post).

The reason these particular capacitors merited so much research was they filled an important niche. Each of the millions of phones Bell Telephone supplied to its customers contained one of these impregnated paper capacitors as part of the circuit that made the phone ring, so cost and longevity of individual components was a major concern. Paper capacitors, impregnated with wax, were much cheaper to produce than any alternative, including mica, and they worked reasonably well for as long as they worked. They weren't needed to last forever, but only long enough to span the gap until the next generation of telecommunications equipment was deployed. Because of this, much effort went into finding chemicals to add to the wax to ensure that essentially none of the capacitors would fail for at least a decade. While it was essential that no capacitor fail for at least 10 years, achieving longevity beyond 20 years for a significant fraction of them was not of much concern for manufacturers of these paper/wax capacitors, since that was beyond the planned service life of the equipment that made use of them.

A perfect paper/wax capacitor would consist of thin sheets of metal foil separated by thin sheets of paper soaked with some appropriate wax whose dielectric constant is as large as possible and whose electrical resistance is infinite. When it is fresh, chlorinated naphthalene -- trade names included Halowax, Seekay wax, and Nibren -- works quite well as that wax. As an aside, this substance is a PCB, which is now internationally banned because it is carcinogenic. Although the electrical resistance of this wax is not infinite at room temperature, and decreases rapidly with increasing temperature, it still remains high enough when the wax is fresh not to result in unacceptable capacitor performance. However, even with the best chemical stabilizers, the wax still degrades with time, although accelerated tests showed it would be good enough for the required decade of service.

The following photograph and magnified inset shows the paper and wax layers in a Lucas condenser.

[Linked Image]

There are ~125 layer pairs of area ~1"x1-1/2", with a separation between metal foils of ~0.001". Waxes have dielectic constants in the range 2.1-3.1. Using a value of 3 for an estimate, capacitance = dielectric constant x permittivity of vacuum x Area/separation x 125 pairs = 0.11 uF. The actual capacitance from Lucas literature is 0.15-0.18 uF, agreeing very well with this estimate based on my measurements of the internal structure of the condenser.

Even with stabilizers, research showed it also was essential to hermetically seal the capacitors because the wax is somewhat hydroscopic, and moisture accelerates the breakdown. When the wax breaks down it releases hydrochloric acid which then attacks the aluminum sheets of the capacitor, releasing aluminum chloride. Unfortunately, aluminum chloride accelerates the breakdown of the wax further, in turn releasing even more HCl. While breakdown of the wax happens no matter what, the process rapidly accelerates in the presence of moisture. The slight statistical variation in the permeability to moisture of the plastic seals is why Lucas and BTH magneto condensers fail over a range of ages. However, as the Bell Labs aging tests showed, even if you found a perfectly sealed new old stock Lucas or BTH condenser from the 1960s. it now would be approaching its maximum lifetime due to the chemical breakdown of the chlorinated naphthalene wax that happens even without moisture, and even if the condenser has never been used.

A few years ago I was given a truly new old stock Lucas condenser. The condenser was sealed in thick wax paper, in a cardboard box, in turn sealed in thick wax paper, and finally wrapped in paper on which was printed the part number and manufacture date of September 1956. All of the layers, including the outermost paper, were in fine condition. However, when I measured the electrical properties after extracting it, the capacitance was 0.601 uF (~4x larger than when new), and the dissipation factor was 15x larger than when new. This condenser would not have functioned if installed in a magneto, so paying a lot of money to buy a new old stock one on eBay is a very bad idea.

For a magneto, the relevant electrical consequence of the breakdown of the wax is an increase in the Equivalent Series Resistance (ESR) of the condenser. The condenser is connected in parallel with the contact breaker points specifically to provide a low AC resistance bypass for the current, i.e. to suppress arcing. While a perfect condenser would have ESR = 0 Ohms, as the ESR increases due to the growing electrical losses in the wax, the condenser's effectiveness as a bypass decreases, and the arcing increases. Typically, the first sign of problems is a magneto that functions acceptably when cold, but fails when warm. The reason for this is the electrical conductivity of the deteriorated wax changes exponentially with temperature. As a result, at this point in the life of the condenser, when cool the ESR is still low enough for the magneto to function, but when warm it becomes too high to suppress arcing. Whether the condenser is used or not, the wax will continue to deteriorate with time, and the ESR soon will be too high for it to suppress arcing even on the chilliest night. Confirming this behavior, I have measured over 50 Lucas paper/wax condensers, and their ESR values neatly follow a smooth curve that allows me to calculate how much longer any particular still-functional condenser will continue to suppress sparks.

The results of accelerated testing were known in 1946, so it is quite likely that Lucas and BTH were aware at the time these paper/wax condensers would begin failing in significant numbers starting in a decade or two. But, they also knew only a fraction of vehicles would remain in use after that many years anyway. And, when the condensers did fail, they could be replaced with no more cost and effort than, say, fitting a worn engine with a new set of rings. Also, although mica was again readily available, the cost to make a condenser with it was 20x higher than that to make one of paper/wax. In an industry driven by customers who made their purchases based on "value" (i.e. cheapness), and where warrantees expired after one year, their choice to use paper/wax was quite reasonable.


B.H. Davies, The Modern Motorcycle: How to Run, Ride, and Repair It (C. Arthur Pearson, London, 1915).

M. Brotherton, Capacitors: Their Use in Electronic Circuits (D. van Nostrand, New York, 1946).

D.A. McLean, L. Egerton, G.T. Kohman, and M. Brotherton, Paper Dielectrics Containing Chlorinated Impregnant: Deterioration in D.C. Fields. Industrial and Engineering Chemistry vol. 34, p. 101 (1942).

D.A. McLean and L. Egerton, Paper Capacitors Containing Chlorinated Impregnants: Stabilization by Anthraquinone. Industrial and Engineering Chemistry vol. 37, p. 73 (1945).

L.J. Berberich, C.V. Fields, and R.E. Marbury, Characteristics of Chlorinated Impregnants in Direct-Current Paper Capacitors. Proceedings of the I.R.E., p. 389 (June 1945).

L. Egerton and D.A. McLean, Paper Capacitors Containing Chlorinated Impregnants: Mechanism of Stabilization. Industrial and Engineering Chemistry vol. 38, p. 512 (1946).

D.A. McLean, Paper Capacitors Containing Chlorinated Impregnants: Benefits of Controlled Oxidation of the Paper. Industrial and Engineering Chemistry vol. 39, p. 1457 (1947).

L.J. Berberich and Raymond Friedman, Stabilization of Chlorinated Diphenyl in Paper Capacitors. Industrial and Engineering Chemistry vol. 40, p. 117 (1948).

J.R. Weeks, Metallized Paper Capacitors. Proceedings of the I.R.E., p. 1015 (September 1950).

H.A. Sauer, D.A. McLean, and L. Egerton, Stabilization of Dielectrics Operating under Direct Current Potential. Industrial and Engineering Chemistry vol. 44, p. 135 (1952).

D.A. McLean, H.A. Birdsall, and C.J. Calbick, Microstructure of Capacitor Paper. Industrial and Engineering Chemistry 45, 1509 (1953).

L. Borsody, New Impregnation for Paper Capacitors. IRE Transactions on Component Parts, 15 (March 1960).

Paul D. Garn, Stabilization of Capacitors. Industrial and Engineering Chemistry 53, 311 (1961).
Appendix II: Replacement Condensers for Post-WWII Magnetos
Note: A version of the material in Appendix I and II was part of a series of two articles I wrote for the Fall and Winter 2011 issues of 'The Antique Motorcycle,' the journal of the Antique Motorcycle Club of America.

Tests to Find a Suitable Replacement Condenser

The full range of parameters that affect a magneto's condenser are current, voltage, temperature, and frequency (i.e. for a condenser, it's not just the magnitude of the voltage across it, but how fast it is applied). Some of the less-common instruments that have allowed me to make my studies of condensers include a variable frequency impedance bridge for determining AC losses to within 1%, a resistance bridge with maximum range 50,000x higher than a standard "megger" insulation tester, and a 200 Watt pulse generator for high current pulses of rise time 0.01 us and repetition rate up to 1 MHz. I also have a four-channel 400 MHz oscilloscope, a 40 kV probe for directly measuring the time dependence of the output of the coil up to 75 MHz, a 2 kV probe for directly measuring the voltage across the condenser up to 100 MHz, and specialized current probes for directly measuring the flow in/out of the points, condenser, and high tension leads up to 60 MHz. Stated differently, this oscilloscope and probes have allowed me to simultaneously measure all electrical parameters of operating magnetos to within less than 0.02 us of the onset of ignition. Even on an engine at 6000 rpm, in 0.02 us the contact breaker points have opened by just 36 uinch (0.92 um), which is only twice the wavelength of light.

As for how I conducted my studies, after using manufacturers' specifications to select the most promising replacement condensers (described in the next section), I tested them when attached to an armature and contact breaker points, as well as with other appropriate laboratory tests made directly on the condensers. Using a modified distributor tester and two commercial magneto testers I was able to simulate actual field conditions in my laboratory, including with a condenser and coil in an oven at temperatures up to 150 oF (65 oC). Where appropriate, I then used those measurements to accurately simulate certain parameters under accelerated and/or overstressed conditions. For example, I used a 200 Watt high frequency pulse generator to subject the replacement condensers to current pulses 60x higher than the ones I had measured using a magneto tester, doing so at the equivalent of >100,000 rpm so that I was able to simulate "150,000 miles" of operation in minutes instead of months.

Some of my tests were a combination of simulations and "field conditions." For example, one long-term, test involved submerging condensers in 181 oF (83 oC) beakers of 30W Castrol and hot water for a number of months, taking them out periodically to measure using a low voltage capacitance meter (which measures them at only a few volts; much less than the several hundred volts they are subjected to during operation in a magneto) and a General Radio 500 V Teraohmmeter (which measures their resistance at an appropriately high voltage, but at DC rather than the ~100 Hz repetition rate of a magneto). Although neither of these electrical measurements tested performance under operating conditions, I designed this aggressive "environmental simulation" to test their ability to survive heat and solvents -- it would have been no good if a possible replacement condenser had the necessary electrical performance, but if it degraded in the presence of oil vapor or humidity.

The water test was particularly harsh since, even if a magneto fills with water, it will quickly dry once it is operating again, so seldom will the condenser of a functional motorcycle be in contact with liquid water for more than a short period. While the ones in oil had no detectable change in their capacitance or resistance, beginning at 1680 hours— the equivalent of them having spent 42,000 miles at an average speed of 25 mph submersed in hot oil and water — the resistances of the ones in water started dropping with time, from over 2 TOhm when new to ~100 MOhm at 3624 hours. However, 100 MOhm is still much higher than required to function in a magneto. Further, the ESR at room temperature of even the most degraded of them was still 3x lower than the lowest ESR I have measured of a Lucas condenser removed from a functioning magneto. Plus, the ESR remained unchanged at elevated temperatures, while by 120 oF (49 oC) that of the still-functional Lucas had increased a further 3x, to a value 10x worse than that of the most degraded replacement condenser.

Since my tests had established that prolonged immersion in hot water degrades the electrical properties of the condensers, albeit very slowly, at the 3364 hour point ("90,600 miles") I removed the water. After a further 120 hours ("3000 miles") in air at 181 oF (83 oC), the resistance of even the most degraded condenser had recovered to above 1 GOhm, and its ESR had improved by 58%. After an additional 336 hours ("11,400 miles" total since removing the water) the resistance was over 1 TOhm and the ESR more than 90% of its as-new value. The fact these properties were easily reversible indicates the measured degradation was due to the electrical conductivity of tap water that had slowly permeated the protective coating, rather than a permanent water-induced chemical breakdown of the dielectric. This means that in the actual operating environment of a magneto these replacement condensers would operate significantly longer than the at-least "90,600 miles" they survived immersed in hot water.

Periodically during my long-term "environmental" test of their resistance I also made a full set of measurements on these replacement condensers. After "90,600 miles" in hot oil and water, these replacement condensers then survived the equivalent of an additional 150,000 miles subjected to current pulses 60x higher in power than are generated by a magneto. I also connected the condensers and an armature coil to one of my magneto testers and measured all of their electrical properties at 150 oF (65 oC), comparing these measurements with ones I had done on them when they were new to see if I could detect any degradation. Judged from the lack of any apparent increased sparking at the contact breaker points, and unchanged oscilloscope patterns, all of these replacement condensers performed as well in a magneto after "90,600 miles" in hot oil and water as they had when fresh out of the box. Continuing on with hot oil only, my final complete set of measurements was at 7080 hours ("177,000 miles"), with the condensers again passing all the tests.

One typical accelerated lifetime test of electrical components is based on the observation that most chemical reactions approximately double in rate for every 10 oC increase in temperature. This "doubling rule" makes possible another kind of lifetime estimate. Assuming it applies to the chemical processes at work breaking down the dielectric material of the replacement condensers, surviving 7080 hours at 181 oF (83 oC) predicts they would survive at least 51 years parked in a storage shed at 73 oF (23 oC).

What Replacement Condenser Should Be Used?

The replacement condensers I recommend are a pair of Panasonic 0.082 uF polypropylene film capacitors (part no. ECQ-P4823JU). When soldered in parallel they produce a 0.16 uF condenser that fits into the available space in the end caps of Bosch, Lucas and BTH single and twin rotating armature magnetos. I hasten to add that it is possible to damage them by applying too much heat during soldering so, if you are not careful, they can be inadvertently made to fail before you even start. This capacitor has the published specifications that caused me to select it for my tests, the demonstrated electrical performance to survive the high voltage, high current pulses generated by a magneto, and the ability to survive the hostile environmental conditions of heat, oil, and humidity.

Although I believe the tests I've conducted are as thorough and comprehensive as they need to be, and although none of the capacitors failed, to extract a statistically meaningful minimum expected lifetime would require subjecting a much larger number of them to these tests. However, based on my measurements on a limited number of units, my conservative estimate is there is a very high probability these Panasonic capacitors will function without failure in a magneto for at least 140,000 miles or 40 years.

Unfortunately, these Panasonic capacitors are now out of production. However, certain other polypropylene film/foil capacitors made by other manufacturers are likely to function just as well as the ones from Panasonic, although I cannot recommend them until I have an opportunity to test them.

Importantly, no electrical measurements are even needed to know that any capacitor that easily fits into the cavity of an armature, with half the space left over, definitely is not up to the electrical rigors it will face. A half-century of developments in chemistry has resulted in significantly improved capacitor lifetimes, but no amount of development can overcome fundamental laws of physics. Surviving high current pulses requires relatively thick electrodes. Surviving high voltages requires relatively thick dielectric layers. From the known electrical properties of materials, what this means is any suitable condenser for this application necessarily must be quite substantial in size. Physics, not coincidence, is why the soldered Panasonic pair is remarkably similar in total length, width, and thickness to the magneto condensers it replaces.
Appendix III: Anatomy of a Post-WWII Lucas K2F Magneto

Although most students who took high school biology dissected a frog or fetal pig, I suspect few people who own a motorcycle have dissected a magneto. Much is to be learned in both cases (but without the aroma of formaldehyde for the magneto). Since the basic thread is about a Bosch ZEV, I've included several images to illustrate the similarities between it and the Lucas despite a half-century of evolution (just as the reason for dissecting a fetal pig is because of its evolutionary similarities to human anatomy).

The Armature

The next photograph shows the armature of a post-WII Lucas K2F twin magneto on the left, and a c1915-20 Bosch twin magneto on the right. Other than the asymmetry of the steel in the Bosch armature, needed because it is for a V-twin with offset firing angles, the similarity is remarkable. I won't go into the details in this Appendix, but the asymmetry of the armature and of the magnet pole pieces in the housing (shown later in this Appendix) advances and retards the firing each revolution by half the angle of the engine's V. That is, since this Bosch KEV is configured to work with the 45-degree V of a Harley-Davidson engine, one firing pulse each revolution of the armature comes 11.25-deg. (22.5-deg. engine) earlier than it would for a vertical twin like a BSA 650, and the second comes 11.25-deg (22.5-deg. engine) later, for a total difference of 22.5-deg. (45-deg. engine).

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The next photo is repeated from an earlier post in this thread, and shows that the Bosch ZEV armature is made up of individual steel laminations. Early in the development of this type of magneto it was discovered that the eddy currents induced in solid-core armatures due to rotation through the magnetic field resulted in significant losses. Because of this, armatures are built up using laminated steel plates, each varnished to electrically isolate it from its neighbors to minimize eddy currents. The armatures of all Lucas (and Bosch, BTH, and all others) are similarly constructed using laminations.

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The next photograph shows the armature of a Lucas K2F after I sectioned it through the middle. The six rivets holding the laminations together can be seen, along with the two holes through which long screws attach the brass end caps to the central armature/coil section. Also seen are the ~200 windings of the primary (closest to the core) and the ~10,000 of the secondary, separated from the primary by a thick layer of insulation.

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The next photo shows the sectioned coil after removing it from the armature (it differs slightly from the previous one because it isn't the same coil, although they are nominally identical). Seen in this photo are that the coils of the primary are nearest the armature, and surrounded by the many turns of fine wire of the secondary. The horizontal wire exiting the coil carries the high voltage to the slip ring from the last turn of the secondary.

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On the left of the next photograph is a magnified view of the left half of the coil of the previous photograph, and on the right is the same coil under additional magnification. The largest wire is 0.045" in diameter, and the small ones making up the secondary are 0.003" in diameter (i.e. roughly the diameter of a human hair). The relatively large wire of the primary (0.030") connects to the 0.003" wire of the secondary, which in turn connects to the 0.045" wire that leads to the slip ring.

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The core of the armature is rectangular in cross section, as can be seen from the next photograph of coils cut from two armatures, one sectioned "horizontally" and one "vertically." As can be seen from the scale in the photograph, the core of the armature is ~1/2" x 3/4". However, the fact the core doesn't have a square cross section isn't particularly significant for operation.

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Earth Brush

There are a few more electrical parts to examine before turning to the magnet. An earth brush is needed to complete the primary circuit, and the Lucas K2F holds this carbon brush in a hollow "screw" inserted at the drive end of the housing. The next photograph is of the earth brush taken through a window milled into the housing (red paint outlines areas where I've milled away sections of the housing). The carbon brush extends less than 0.06" from its holder, so it can be seen from this that the clearance between the housing and the armature is quite small.

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This brush makes contact with the smooth brass endcap of the armature, completing the circuit (the brass on the armature in this photograph is rough, and would abrade the brush more rapidly than it should if used in this condition).


The voltage needs to get from the coil in order to make its way to the spark plugs, and this is done using a slip ring connected to the coil along with a carbon brush held in a nonconductive pickup. The next photograph shows the slip ring and pickup through a window milled into the housing. The output wire from the coil is inserted in the slip ring where it makes electrical contact with a brass arc molded into the base of the slip ring. This brass arc in turn makes periodic contact with the carbon brushes of the pickups located 180-deg. apart in the housing.

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The carbon brushes project ~0.08" from the end of the pickups, which is why the carbon itself has to be at least twice this long (or longer) to provide the necessary mechanical support for itself within the pickups.

Safety Gap

Under normal conditions the spark plugs fire at less than 5 kV, so the voltage experienced by the insulation in the coil doesn't exceed this. However, if the plug, plug wire, or pickup breaks the voltage would rise to a high enough value that it could break down the internal insulation. If that happens, permanent damage to the coil can occur. Because of this, a safety gap screw is located next to each pickup to keep the voltage within acceptable levels. The next photograph shows one of these screws viewed through the hole for one of the pickups (enlarged with an additional window milled into the housing).

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The brass contact in the base of the slip ring is long enough that a safety screw is always over it when the voltage is high. Normally that voltage is routed to the spark plug, but if for some reason that doesn't happen it will jump the ~0.25" gap to the safety screw if the voltage ever reaches ~20 kV. Also, it should be clear from this photograph why if the safety gap screws are not removed before attempting to withdraw the armature, the relatively thin and brittle lip of the slip ring will be broken.

The Magnet

Having finished with the electrical elements of the magneto, it's time to dissect the housing to examine the magnet. The next photograph shows the inside of a Lucas K2F housing before and after I sectioned it. I've color coded the parts as follows: the Alnico magnet is blue, the laminated steel pole pieces are red, the remnants of the brass cap that held the pole pieces together with the aid of rivets is gold, and the remainder is cast aluminum.

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The actual magnet is just the fairly small slug of Alnico at the top of the housing. However, the steel in direct contact with the Alnico is magnetized by it and "carries" the N and S poles to the right and left sides of the magneto, exactly as would be the case if everything shown in red were Alnico instead of steel. However, Alnico is such a strong (but expensive) magnetic material that inexpensive steel can be substituted for it in this way. It can be seen that the pole faces are symmetric since this is for a vertical twin engine with equal firing intervals, not for a V-twin.

From ZEV to K2F: A Half-Century of Evolution

On the left of the next photograph is the housing of the Lucas K2F and on the right is that of the Bosch ZEV (with just one of its two tungsten steel magnets in place). It can be seen that the magnets for both are in the shape of horseshoes (including the steel in the case of the Lucas), although a half-century of materials development allow that of the Lucas to be considerably smaller (the available energy from Alnico is ~9x greater than that of tungsten steel). The fact the Lucas directly evolved from the Bosch is readily apparent.

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The asymmetric pole pieces of the Bosch show that it is for a V-twin engine. Together with the asymmetric armature shown above, this design fires one cylinder earlier and the other later than the even 180-deg. firing interval of the Lucas K2F. Although I haven't shown any examples here, the internals of single cylinder rotating armature magnetos (e.g. Bosch ZE1, Lucas KNC, BTH KD1, etc.) are essentially identical to their twin-cylinder siblings, differing only in some details.

From a rebuilder's point of view, it should be clear that the most time consuming aspect of a complete restoration of any magneto is winding a new coil. Removing the old coil is relatively easy if it is the original one, but can be quite troublesome if it is a replacement that has epoxy firmly bonding it to the armature (n.b. if the replacement had been properly wound it would not have failed). As can be seen from the cross sectional views, once the old coil has been removed, winding a new one requires ~10,000 turns of very fine wire in a close-packed arrangement, with insulation layered in appropriate places, connecting 0.003", 0.030" and 0.045" wires without breaking them, and then vacuum impregnating the final assembly. However, although time consuming to wind, a correctly rewound coil will outlast anyone reading this post by many decades, so it's worth doing right.
Posted By: Magnetoman Re: Restoring a Rotating Armature Magneto - 09/21/13 10:29 pm
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A few weeks ago when I was in Europe a friend gave me the aftermarket slip ring from the BTH magneto he has on his Velocette. The magneto had been professionally rebuilt in England but had failed after ~2000 miles due to shorting to earth of the slip ring and he asked me to do a post-mortem. He said these same slip rings have been supplied to other people he knows by at least four retailers in England so the information below may be of direct interest to others besides him. There are no manufacturer's markings on the slip ring so whether or not you have one of the same ones only can be judged by comparing its dimensions and appearance with the photographs below.

Aside from the issues I found with this specific slip ring the results of this autopsy again illustrate the broader problem of determining the suitability of aftermarket electrical components before using them. Unfortunately, as I've documented at several places earlier in this thread, with aftermarket magneto components one cannot take at face value unverified assertions like "Modern substitute, very high specification, zero failure" that turn out not to be true.

Preliminary Inspection

The following photograph shows a ~1 mm deep channel in this slip ring that starts at the trailing edge of the brass contact and runs for ~ 0.65". I can't know for sure, but it seems unlikely the rebuilder would have used this slip ring had this channel been visible when he was installing it, which means it almost certainly developed after the magneto was in operation.

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As can be seen from the next two micrographs, this channel starts immediately at the edge of the brass contact, which means it cannot have been caused by wear from the pressure of the brush. Aside from the channel being irregular, if it were due to wear the depression would not start abruptly at the edge of the brass segment since the channel is much narrower than the brush and doesn't extend the full circumference of the surface. The features of this channel are consistent with "spark erosion" of an inhomogeneous material rather than wear having caused it. However, there is no way to know if the channel grew steadily over the 2000 miles, or if it developed rather quickly.

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Electrical Resistance

One factor that could have contributed to the problem is if the black material used for the slip ring is electrically conductive. To check for this, I measured the resistance of the material itself by attaching the leads from a megohmmeter adjacent to each other on the flange:

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The resistance measured this way at various places on both flanges was greater than 20 GOhm at 2.5 kV (i.e. the maximum R and V of this meter). To find the actual value of resistance I repeated this measurement with a more specialized meter (General Radio Megohm Bridge) and found a resistance of 10 TOhm at 1 kV (i.e. ten million, million ohms). This is a very high value, indicating there isn't an overall issue with the composition of the material used. However, possible inhomogeneities in the material (i.e. voids and/or conductive inclusions) are something that will have to be checked.

As can be seen from the above photograph I mounted the slip ring on an expanding mandrel to make good electrical contact with its inner surface as would be the case when mounted on an armature. I next measured the resistance between the brass segment and this "armature."

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Although this resistance also should have been above the maximum of this meter (i.e. at least 20 GOhm at 2.5 kV) it was far lower as well as voltage dependent:

~1.5 MOhm at 250 Volts
~300 kOhm at 500 Volts
~150 kOhm at 1000 Volts (i.e. ~100,000x lower than 20 GOhm)

The meter couldn't go any higher than 1 kV due to the large current draw at this relatively low resistance. Also, although accuracy isn't especially important for this test, the 'overload' warning came on for the readings below 1 MOhm so those values may or may not be accurate. In any case, conductive path(s) to the armature of resistance ~150 kOhm or less is sufficient to cause failure of a magneto. For comparison, making the same measurement on a NOS Lucas slip ring using the General Radio instrument that goes to much higher resistances found 5 TOhm (5 million, million ohms) at 500 V.

The following figure from a book on magnetos shows that if there is a conduction path in parallel with that of the spark plug (i.e. as is the case here with the slip ring) of resistance lower than ~500 kOhm it begins to increase the minimum speed needed for the magneto to generate a spark. If the resistance drops below ~140 kOhm the magneto won't generate a spark at any speed.

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Adapted from 'Automobile Electrical Equipment', 6th edition, A.P. Young and L. Griffiths (Iliffe, 1958)

What these measurements have revealed so far is that a conduction path (or paths) between the brass segment and the armature had developed during the 2000 miles of operation, and that the resistance of that path(s) became low enough to cause the magneto to fail. This is consistent with the formation of a carbon track(s) due to the breakdown of the material of the slip ring in the presence of an electrical discharge.

To locate the low electrical resistance path(s), I used a piece of paper to insulate all but a short section of the inner surface, as is shown in the next photograph.

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By successively rotating the section in contact with the "armature" I found three low resistance regions: at the leading and trailing edges of the brass segment, and at the end of the channel. I used the 250 V setting on the megohmmeter for these measurements because, as above, the resistance of each of the three regions was low enough to overload the meter at higher voltage settings. In each of these three regions the resistance was below 1 MOhm. However, for all other regions of the armature the readings were above the 20 GOhm maximum of the meter.

Internal Structure of the Slip Ring

The above measurements show there is a relatively low resistance path(s) between the rubbing surface of the slip ring and the armature but it remains to be determined why this is the case. Next, I clamped the slip ring on my mill and used a 0.01" slitting saw to cut it in two at a place where I would be able to inspect the inner surface directly under the end of the channel, in the hopes of being able to see evidence of a carbon track. Note that the end of a carbon track large enough to have a resistance less than 1 MOhm would be quite small, so not being able to see where it emerged on the inner surface of this (black) armature would not mean it wasn't present. Although I was not able to see such tracks, the cross sectioning revealed the problem anyway.

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As can be seen in the micrographs below there are quite a few voids in the material. The resin should be absolutely homogenous, so this shows there was a flaw in the manufacturing process. The location of the voids is consistent with a vacuum pump having been used during the molding. Although this should have eliminated all of the voids, if the resin set too fast, or was too viscous to begin with, air pockets would have been left behind. The fact the air pockets are concentrated toward the center of the thickest region of the slip ring is consistent with this explanation.

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Cross-section of the slip ring. The highlighted region is magnified in the next photograph.

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Magnified portion of the slip ring corresponding to the highlighted region of the previous photograph.

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After inspecting the above cross sections, I then cut one of the sides in half in the other direction, revealing additional voids that are quite large (~2–3 mm).

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Note: the discoloration and "smearing" of the brass is an artifact of the slitting saw that I didn't take the time to polish away with abrasive paper.

Reason for the Failure: Voids in the Resin

Note that I sectioned this slip ring only in a few places so there is no way to know the size, number, or distribution of additional voids at other locations within it. However, that they exist in the size and numbers we can see explains why the slip ring functioned at first but "died" after 2000 miles. On every revolution the voltage developed on the brass segment would have induced a corona discharge in any voids near the brass. The ozone produced by those discharges is quite reactive so it "burned" a conductive carbon layer into the surface of those voids, steadily extending the reach of the high electric field beyond that of the immediate vicinity of the brass. As the corona relentlessly extended its reach from one void to the next as a result of the conductive carbon, eventually the conductivity of the path or paths between the brass segment and the armature became so large (i.e. the resistance decreased to less than ~150 kOhm) that the magneto could no longer be turned over fast enough to generate a spark to start the motorcycle.

As an aside, as I noted earlier in this thread, the reason why the points cavity of a magneto needs to be vented is to reduce the concentration of ozone created by the residual sparking of the points that is present even when a proper condenser is in the circuit. The failure of this reproduction slip ring is graphic evidence of how destructive ozone is.

Other Issues with this Reproduction Slip Ring

While the above has identified voids left behind during the molding process as the reason for the failure, there are additional issues with this aftermarket slip ring worth mentioning. The next photograph shows that the slip ring is thinner than a genuine BTH item. Although the restorer should have used a shim on the inside end in order to center the track under the brush, the offset wear pattern shows he did not do this.

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Genuine BTH slip ring mounted on armature at left. Reproduction slip ring at right.

Also, since both slip rings are sitting on the same surface in the above photograph, it can be seen that the flanges of the aftermarket one are ~50% thinner than on the genuine BTH. The thickness of the genuine ring's flanges taper from 0.17" at the rubbing surface down to 0.08" at the outer edge, while on the reproduction they are nearly constant at a rather thin 0.09" at the rubbing surface and 0.06" at the outer edge of the flange. Dimensions of a slip ring are not arbitrary, since they are chosen to guard against corona discharge and direct electrical shorts, so the inferior specifications of this aftermarket part are of concern.

Adding to the issues is the reproduction slip ring is 0.01" too thin (0.628" vs. 0.638"). Since the slip ring also serves as a spacer for the bearings, this means the bearings would be 0.01" too close together unless an additional spacer were added to compensate. The fact that the spacer(s) used by the rebuilder were incorrect can be seen from the wear pattern in the next photograph.

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The longer white marks are at the sides and center of the slip ring and the shorter one is 0.05" to the left, showing the wear pattern is offset ~0.03"~0.05" from the center line.

BTH and Lucas Slip Rings are Not Interchangeable

Although they may look the same on an autojumble or swap meet table, a BTH slip ring is thinner and has a smaller OD than a Lucas:

The black slip ring is the reproduction BTH and the tan one is a genuine NOS Lucas.

BTH in front; Lucas in back.

As an aside, if the 2000 miles were covered at an average of 30 mph and 2000 rpm, this magneto made ~2 million revolutions before failing. During that time the carbon brush was dragged across the surface of the slip ring some 120 miles at an average speed of 2 mph. Although the rubbing surface of this reproduction slip ring is fairly rough, on a proper one the brush will travel ~600¬¬ miles before being abraded away. I suspect the roughness of this one would wear out a brush twice as fast.

Implications for Using Aftermarket Slip Rings

The failure of this slip ring after 2000 miles has wider implications for anyone rebuilding a magneto. Although I don't have a new slip ring from the same batch to test, the fact that the resin in the flanges (where there are no bubbles) of this failed one has a resistance of ~5 TOhm means the resin itself is intrinsically fine. Thus, this slip ring very likely would not have failed if the manufacturing process had not left (hidden) air bubbles in it.

It's worth mentioning that while 5 TOhm is more than adequate for this application, the same measurement on a NOS Lucas slip ring was off scale on the highest range at over 1000 TOhm at 1000 V. Aftermarket slip rings are for sale on eBay with the claim they are made with "better materials" and are of "better quality" than NOS Lucas. Although no data of any kind is provided to support these bold assertions, I note that it would be very difficult (and completely unnecessary) to exceed the materials and quality of NOS Lucas slip rings. To be clear, my concern isn't whether an aftermarket slip ring actually is better than NOS, it's whether it might be significantly worse despite unverified advertising claims, as was the case with the one dissected in this post.

Although the voids in this slip ring would have been revealed by x-raying it, that isn't a practical test for most people to apply (unless they know someone who works in a dentist's or radiologist's office…). Unfortunately, because the reason for the failure was slow degradation due to repeated application of high electric fields near the internal voids, it is unlikely that any usual electrical test would have detected a problem when this slip ring was new.

A rule commonly used for high potential testing ("Hipot testing") of electrical components is to apply twice the operating voltage plus 1000 V for a few seconds during which time one looks for current flow above some minimum value. In the case of a slip ring, this means a 15-20 kV Hipot tester would be needed (note: Lucas rated their slip rings for 35 kV, so a good aftermarket slip ring would have no problem passing a 20 kV test). Unfortunately, even applying 20 kV to this particular slip ring for some number of minutes quite likely wouldn't have revealed a problem since it takes some time for a conductive path of carbon to form. In any case, the failure mode of this slip ring has resulted in me now investigating other ways for stress-testing slip rings that could be used by people who don't have the specialized instruments that I have available to me.
A very true & sad situation on aftermarket parts in general. Some are made poorly, while others exceed original spec.s. How is a consumer to know which is good & which is bad.
I too am very familiar with the porous slip rings (and pickups). In fact, I am the one selling the "better than Lucas" slip rings & pick-ups on ebay.
I have found that several large British parts wholesalers are selling pickups & slip rings made in both India & Taiwan. I have found them to somewhat conductive & allowing loss of spark. after much searing & testing to find a better product for my customers magnetos & located 2 companies that actually make these items in the UK to a very high standard & these have tested well & I have been very happy with both companies products. Because material that they use is less brittle & less likely to absorb moisture that the original Lucas products,I consider them to be "better than Lucas" . The Lucas made Bakelite parts are becoming very hard to find NOS & if found are very expensive & have no advantage over the English made products that I have found, in fact the Lucas ones have disadvantages.
Rather than write a few thousand words on Bakelite (or some call it Barkerlite), here is a link on the material:
Even Lucas knew of the tenancy for Bakelite to absorb moisture, in fact at least 2 Bakelite items that I am aware of were coated with Glyptol ( ) to reduce this effect. Glyptol is an red-orange paint on coating that you will find on the pickups on a Lucas K2FC competition magneto, and you will also find this coating inside the distributor cap on a Lucas SR2.
This is a similar situation to the original Lucas capacitors, people will buy them NOS, pay a ridiculous inflated price for them, even though a much better product is available. An original Lucas NOS slip ring, or pickup is just fine, but there are better materials available.

I am sure as time went on Lucas would have changed over from Bakelite as most other industries did, but as is well documented, the English motorcycle industry at that time was too cheap & wouldn't pay for any R&D unless it was less money, but that's another story for another thread smile

A side note: I also sell an Ewarts replacement plungers that use 2 o-rings, in place of a cork seal. I consider it to be "better than Ewarts" & much safer, but some will always want the original Ewarts plunger with the cork, so they can say it is original, & thats fine too.
Send questions or comments to [email protected]



Unfortunately, the Panasonic condensers I tested so extensively are now out of production and no longer easily available. Although there never was a question that other proper high-quality substitutes were available, it takes time identify and test those alternatives. There were two obstacles to overcome before this could happen. First, I have enough Panasonic capacitors on the shelf to last me into the 22nd Century so doing this work had a hard time making it to the top of my to-do list. The other obstacle -- related to the first one --is it requires mind numbing work to page through catalogs and spec sheets to identify suitable candidates.

The specifications for capacitors are in different formats in different locations, not all of which are easy to find, so it takes sitting down for quite a while to systematically comb through catalogs and manufacturers' literature. Earlier this summer someone else did this for me and sent me a short list of capacitors that seemed to be appropriate given the specs I had sent him. Thanks to his work, I only had to go through a small number of web pages to arrive at a few prime candidates. Once I did that I ordered ten of each for testing.

Basically, an appropriate capacitor has to satisfy three criteria: have the proper electrical specifications, be of a size that fits into the cavity in the armature, and not dissolve in oil. I selected several that met the first criterion, possibly met the second, and would have to be tested to see if they met the third. Although one was back ordered and wasn't shipped until early September, the others arrived in early August. The construction of magnetos, and the size of appropriate capacitors, are such that every fraction of a mm counts and one can't be completely sure whether or not a capacitor will fit from the published dimensions alone. Anyway, two of the capacitors I ordered were just a teensy bit too fat to fit in a Lucas armature so had to be rejected.

Unfortunately, a few weeks after I started my tests the hot plate lost control and heated to nearly 200 oC sometime between the temperature checks I did every few days. This forced me to throw out those capacitors and start over using a different hot plate. However, I'm happy this happened a few weeks into this month-long test instead of many months into the years-long test I did of the Panasonics. On the right of the following photograph is one of these overcooked capacitors, next to one that spend 31 days at 102 oC in the center, and a new on the left.

[Linked Image]
Vishay capacitors from left to right: new; after 1 month in 30W Castrol at 102 oC; after some unknown time up to a few days in 30W Castrol at 200 oC.

It is easy to see which capacitor in the above photography was accidentally subjected to 200 oC, but the only visual difference between the other two is the slight yellowing of the base material of the one that spent a month at 102 oC. This shows that oil does not attack the outer case of these capacitors.

Recommendation for a Replacement Capacitor

Since not everyone may want to read the details of the tests described below, I'll go straight to the conclusion. The capacitors I recommend as replacements for use in Lucas, BTH, and other rotating armature magnetos are a pair Vishay 0.082 uF "AC and Pulse Double Metallized Polypropylene Film Capacitors," manufacturer's number BFC238320823. These capacitors have pulsed current and voltage ratings of 1400 V/us and 630 VDC with a maximum operating temperature of 105 oC. These specifications comfortably exceed those needed to survive for years in the hostile electrical environment of a magneto. They are available from Digi-Key for $1.35 each ($2.70 for the pair required to be soldered in parallel) under part number BC1883-ND.

Relevant Properties of these Capacitors for use in Magnetos

At the left of the following photograph is a Lucas condenser from a K2F-type magneto, in the middle a pair of Panasonics I've already assembled into a package with bracket ready to install in a magneto, and on the right is a pair of Vishays.

[Linked Image]
Left to right: Lucas condenser; a pair of Panasonic 0.083 capacitors soldered in parallel and ready to install; two Vishay 0.083 capacitors.

For a better idea of the size, the next photograph shows a pair of the Vishay capacitors in the cavity of a Lucas armature.

[Linked Image]
Vishay capacitors in the end cap of a Lucas magneto.

Finally, as the arrows in the next photograph show, the Vishay capacitors are thin enough to fit below the lip in the end cap of the armature so they will not interfere with the coil of the armature when it is installed.

[Linked Image]
Vishay capacitor in the end cap of a Lucas magneto.

As an aside, remember that it was someone other than me who took on the task of narrowing down the list of all possible types of capacitors that are on the market. All he had to work with were the specifications I gave him based on the measurements I made on operating magnetos. It is no coincidence that the Vishays he found have the same metalized polypropylene construction as the Panasonics. Because surviving high pulsed currents for extended periods requires thick metal electrodes, which in turn makes the capacitors fat, it is also no coincidence they occupy approximately the same volume as the Lucas and BTH condensers they replace. Also, like the Panasonics, the 0.16 uF version of the capacitor from the same family is too thick to fit into the available space so a pair of the thinner 0.082 uF capacitors has to be soldered in parallel.

Why These Magneto Capacitors Could Not be Any Smaller

To illustrate an important point made in the previous paragraph, the next three photographs show the internal construction of a Vishay capacitor.

[Linked Image]
Cross section of a Vishay capacitor. The shiny flat electrodes are at the left and right sides, but the individual layers of the capacitor appear black in this micrograph.

[Linked Image]
Higher magnification view of the lower left corner of the above photograph showing the metal layers connecting to the flat electrode.

[Linked Image]
Micrograph at approximately 300x magnification showing the individual layers of the Vishay capacitor. The layers are 7 um thick, but the waviness was caused by the fairly crude cross sectioning process I used (a slitting saw on my mill).

The flat electrodes at the far left and right connect directly to the external leads, and have cross-sections of 0.0011 square inches. This is equivalent to the cross section of #19 AWG, wire which is approximately the same as the #20 wire used for the magneto's primary. This is no coincidence, since these capacitor electrodes have to survive the same high current pulses as does the primary coil. From there the current spreads to the metal layers that make up the capacitor itself (along with the polyethylene layers that separate them).

There are only two ways these magneto capacitors could be made smaller: reduce the thickness of the metals, which would reduce their ability to handle the high current pulses without burning out; or, reduce the thickness of the dielectrics, which would reduce their ability to handle the high voltage pulses without being destroyed by arcing between the layers. Actually, these particular ones could be made a tiny bit thinner without affecting their electrical properties. Vishay uses the same outer shell for a family of 630 VDC capacitors of values from 0.082 uF (i.e. this one) up to 0.11 uF. Looking again at the top micrograph it can be seen that there is a white plastic spacer between the actual capacitor layers and the outer shell. The thickness of this spacer is incrementally reduced as more layers are added to give higher capacitance values within this family until there is no more room left. At 0.12 uF the outer shell is replaced by one that is 1.5 mm thicker and the process starts over. If Vishay used a different shell for every capacitance value, this particular one could be made ~1 mm thinner.

Other Possibilities

There certainly are other capacitors with appropriate sizes and specifications. However, my goal here was to identify just one appropriate replacement capacitor that is in current production, not all possibilities. A potential replacement capacitor from a different manufacturer was out of stock when I placed my order in August and wasn't shipped to me until a few weeks ago so I won't finish testing it for a few more weeks. However, although its electrical specifications are even more robust than those of the Vishays, it is slightly thicker so would not fit in Lucas and BTH armatures. I ordered some of them anyway because if they pass my tests they will be fine for magnetos that happen to have deeper cavities. Also, sometime in the future (years? months? weeks?) these particular Vishays almost certainly will go out of production, so someone will have to go through this testing process again when that happens.

Tests on the Replacement Capacitors

For those interested in more details, the tests I ran on these capacitors were an abbreviated version of the ones I described on the Panasonics in Appendix II:

Although the tests were not as extensive as the ones I conducted on the Panasonics, taken in combination with the manufacturer's specifications they are enough for me to recommend them. If I did not already have a lifetime stock of the Panasonics of my own, and even if it were not possible to conduct any additional tests on these Vishay capacitors, I would use them myself.

As I wrote in Appendix II, one typical accelerated lifetime test of electrical components is based on the observation that most chemical reactions approximately double in rate for every 10 oC increase in temperature. This "doubling rule" makes it possible to derive a lifetime estimate without having to conduct an experiment that runs for decades. Assuming it applies to the chemical processes at work breaking down the dielectric material of the Vishay capacitors, if they still functioned after a month in 30W Castrol at 102 oC this "doubling rule" would predict they would function for at least 20 years on a motorcycle parked in a storage shed at 22 oC (72 oF).

Further, although I've never measured a temperature as high as 52 oC in an operating magneto, but if we take that as a worst-case upper limit and assume that during the next 20 years the motorcycle also covers 30,000 miles at an average speed of 30 mph, that would be 1000 hours (42 days) of operation at that temperature. That time on the road is equivalent to an additional 31 hours at 102 oC. Specifically, the test itself had two Vishay capacitors spend 745 hours immersed in 30W Castrol at 102 oC, which is equivalent to 30,000 miles at a very high operating temperature plus 20.8 years storage at 22 oC (72 oF).

One of the specifications that caused me to select these Vishay capacitors is their 630 VDC rating is significantly higher than the voltage they will experience in operation, and thus they will be understressed in a magneto. Although I could have assembled an external voltage divider to allow me to test their resistance at 630 V, I just used the nearest built-in setting of 500 V. Although lower than their rating, this is still significantly higher than they will experience in operation. Also, the Milspec requirement calls for testing magneto condensers at only 400 V, specifying s a resistance greater than 2 MOhm at that voltage.

Before starting the test my General Radio Megohm Bridge found 8 TOhm at 500 V one minute after applying the voltage (the polypropylene in the capacitors slowly polarizes so their resistance continues to slowly rise). This exceeds the Milspec resistance requirement by a factor of at least 4 million. During the month-long test it was convenient to use a less specialized megohmmeter limited to 20 MOhm to make quick checks. However, at the end of the test, after removing the capacitors from the hot oil and letting them cool to room temperature the resistance at 500 V was the same ~8 TOhm after 1 minute as it had been at the start. I found no degradation of the electrical properties whatever caused by the test, with the yellow discoloration of the white plastic base shown in a photograph above the only change I could determine.

Having passed the resistance test, I next used a Hewlett-Packard 200 Watt high frequency pulse generator to subject the capacitors to current pulses of 1000 V/usec, which is more than 10x higher than the pulses I had measured using a magneto tester. I did this test at the equivalent of >1 million rpm so that I was able to simulate "30,000 miles" of operation of a twin magneto in only a few minutes. Although I did this test at room temperature, Vishay does not derate the AC or DC voltage of these capacitors for temperatures below 90 oC so this test, in combination with their published specifications, indicates they would have been fine if I had tested them at an operating temperature of 40 or 50 oC.

After having been subjected to a month in 102 oC oil, followed by "30,000 miles" of 1000 V/us current pulses, I made one final measurement of the resistance. The resistance at 500 V was the same ~8 TOhm after 1 minute that the capacitors had when they were new. Since this exceeds the Milspec requirement for the resistance of a magneto condenser by a factor of at least 4 million, my conclusion is these Vishay capacitors, like the Panasonics, should be good for at least 30,000 miles of operation plus 20 years in storage .


Although these tests were not as extensive as the ones I conducted on the Panasonics, the Panasonics are by far the most extensively tested magneto condensers that I am aware of, which means these Visays are now the second-most extensively tested condensers available for use in a magneto. Taken in combination with the manufacturer's specifications, and the fact they have the same internal polypropylene structure as the Panasonics, I would have no hesitation using them myself if I did not already have a very large stock of the Panasonics.

If you want to install a new capacitor in your rotating armature magneto and not have to deal with it again for a very long time indeed, based on my tests I recommend either the Vishay BFC238320823 or the Panasonic ECQ-P4823JU. These metalized polypropylene capacitors have capacitance values of 0.082 uF so have to be soldered in parallel to give the 0.16 uF required total. The Vishay capacitors are currently available from Digi-Key for $1.35 each under their part number BC1883-ND. Note, however, that there are other polypropylene capacitors on the market with much lower V/us ratings because they use thinner metal electrodes so it is essential to check the specifications if you plan to use any other than these two specific ones that I have tested.

It is worth emphasizing that 30,000 miles plus 20 years is only a lower limit because, other than discoloration of the white plastic base, these Vishay capacitors showed no sign whatever of degradation when I ended the test after a month. There is no reason to expect they would not continue to last quite a bit longer.
Send questions or comments to [email protected]


Depending which end of the armature holds the condenser in a given magneto, the central bolt through the points plate may bolt directly to it. If this is the case, you will have to manufacture an adapter in order to replace the condenser with a modern one. The original condenser itself could serve this purpose if its top were machined away, but it is filled with a now-banned carcinogenic PCB so I definitely do not recommend doing this.

At the top of the next photograph are Bosch (left) and Lucas (right) condensers with the threaded "nut" on the underside to which the bolt through the center of the points plate attaches.

[Linked Image]

At the bottom of this photograph are the end caps of two armatures. Note that the cavity of the one at the left (Bosch) is much deeper than the one at the right, making it easier to deal with when adding a modern condenser. Also note that the one at the right has a "square" opening for the condenser nut, so the Lucas condenser above it does not go with it.

My approach to making an adapter "nut" for the replacement condenser starts with a 1/4"-20 brass screw as shown at the left of the next photograph. This screw came from a local hardware store and there is nothing special about its diameter or pitch (other than the diameter of the head, as discussed below), or even that it is brass. Although brass is a nice material to machine and tap, a stainless M6 screw would function as well.

[Linked Image]

I use a lathe to reduce the thickness of the head, shorten the screw, then tap if for whatever screw is used to hold the points plate in the magneto I'm working on. Depending on the magneto this could be BA, metric, or some now-obsolete American thread so a pitch gage is needed to determine the required tap. The next photograph shows a jig I made to hold the screw in the lathe while doing all the modifications to it (although everything could be done without such a jig).

[Linked Image]

The points plate mounting bolt needs to be insulated from the magneto housing, and the next photograph shows a nylon insulator in the shape of a top hat that accomplishes this. Delrin, acetyl, phenolic or some other insulating materials would work as well. The OD of this insulator fits the ID of the hole in the end cap of the armature, and the thickness and OD of the "brim" of the cap fits in the larger diameter recess. I tap the insulator to match the brass screw (i.e 1/4"-20), but I haven't listed other dimensions here because they depend on the particular magneto. Also, as noted above, the necessary insulator might not even be cylindrical, and anyway anyone who has the tools to make this piece will have calipers to make the measurements.

As shown in the next photograph the brass piece threads into this insulator. One of the condenser leads is soldered to it to make the necessary electrical connection to the points via the central mounting bolt.

[Linked Image]

Although it isn't clear from the photographs, the OD of the head of the brass screw is slightly larger than the OD of the main section of the insulator (i.e. slightly larger than the ID of the hole in the end cap of the magneto). To minimize the height of the final condenser assembly the top of the brass screw can be reduced in one dimension in order to fit between two condensers, as seen on the right of the above image as well as on the left of the next one.

Although fabricating this "nut" might sound time consuming, it actually doesn't take much time at all. Also, since it doesn't take much longer to make a half-dozen of the individual pieces than it does to make just one I have a stockpile of the components ready for assembly whenever needed.

[Linked Image]

The above photograph shows the final condenser assembly with two 0.083 uF Panasonic capacitors (left) next to an original Lucas condenser (right). In this particular assembly the capacitors are being held a little above the insulator by the springiness of the lead, but they would be in direct contact when epoxied into place in the end cap (not that direct contact is important, other than keeping the overall height low enough that interference with the armature isn't an issue). The assembly would look essentially the same if made with two of the Vishay capacitors I recommended in Appendix V.

The next photograph shows what the assembly looks like from the bottom. Although when epoxied in place in the armature the leads shouldn't be able to touch any part of the housing, I like to insulate them anyway. The black substance I use for this is "corona dope."

[Linked Image]

Although I used a 1/4"-20 screw, and mentioned an M6 would be fine as well, it can be seen from this photograph that there is plenty of room for an even larger one if desired for some reason.

As for strength, because of the design my only concern would be if the "brim" of the insulator could break because of pressure from the points plate mounting bolt. Although nylon should be strong enough to avoid this, to be sure it isn't an issue I use brass screws whose heads have an OD slightly larger than the OD of the major section of the insulator (i.e. slightly larger than the ID of the hole in the end cap). Because of this, although at very worst the "brim" might compress slightly with time, slightly reducing the clamping force of the central bolt, it cannot pull through.

If brass screws with large enough heads are not available for a given magneto it would be easy enough to build up the diameter of smaller ones with silver solder, or just to make ones entirely from scratch. Finally, it wouldn't hurt to put a few drops of glue on the brass screw before tightening it into the insulator just to be absolutely sure there never will be an issue of the brass/nylon assembly trying to separate even if the mounting bolt is screwed in and out a number of times over the years.

It can from the above photograph that the new assembly has basically the same dimensions as the old Lucas condenser it replaces. As mentioned several places in this thread, the similar size is no coincidence. For a condenser to survive high pulsed currents without burning out requires thick capacitor plates, to survive high voltages requires thick dielectric spacers between each of those plates, and to have the proper capacitance despite the thick dielectric spacers requires many layers of large area. All of these factors add to the volume, which is why these capacitors that I have tested to survive the high pulsed currents of a magneto for many years intrinsically are large.

The first photograph in this post shows that the cavity in the Bosch housing is much deeper than in the other one. In such a case two, thin 0.083 uF capacitors don't have to be connected in parallel to create the necessary capacitance, but instead a single, thicker 0.16 uF or 0.18 uF can be used. However, no matter what, the larger capacitor must be from the same "family" as the 0.083 uF ones that are rated for high pulsed currents. The blue bars in the next photograph show that one of these higher capacitance units is quite a bit thicker than the 0.083 uF ones which is why it won't fit in the cavities of many magnetos.

[Linked Image]


Finally, as a reminder, removing the armature from a magneto for even a few seconds "permanently" reduces its magnetization making the motorcycle more difficult (or impossible) to start because a higher kickover speed is required to generate sufficient voltage for a spark. The only way to restore full performance after removing the armature for any reason is to remagnetize it after it has been reassembled. To do this requires an appropriate electromagnet ("magnet charger"), and one that was designed for pre-WWII magnetos does not have sufficient strength to remagnetize post-WWII alnico magnetos.
Posted By: Magnetoman Re: Restoring a Rotating Armature Magneto - 10/06/14 12:24 am
The 2014 Cannonball Run is over and the Bosch ZEV magneto performed flawlessly for the 3000+ miles without having had any maintenance since I originally restored it for use in the 2012 Cannonball.

Bill Wood happened to mention at least four failed magnetos in his daily posts about this year's Cannonball. Although there certainly could have been others as well, given the 1936 cutoff for this year's event meant most bikes did not have magnetos, so even four is a very high percentage of failures. However, between the bench tester and actual road miles my friend's magneto has traveled over 5000 miles so far, which is consistent with my contention that there is no excuse for a properly rebuilt magneto to fail. And yet, "professionally rebuilt" ones continue to fail with remarkable regularity...

I've asked that this thread be unlocked so it is open again for questions related to magneto restoration.
Posted By: johnm Re: Restoring a Rotating Armature Magneto - 05/08/15 7:47 am

You are a scholar and a gentleman :-)

Your article is now required reading amongst the Classic Bike Owners of Wellington New Zealand and has been crtically examined by most of my friends.

You have our thanks- (and if you are ever in Wellington an open invitaion to visit several workshops and inspect several interesting projects).


Posted By: Magnetoman Re: Restoring a Rotating Armature Magneto - 05/10/15 11:19 am
Originally Posted by johnm
(and if you are ever in Wellington an open invitaion to visit several workshops and inspect several interesting projects).

Thank you very much for your nice comments. However, you should be careful with an invitation like this since there's a fair chance I could be on your doorstep any given year...

Apropos of maybe nothing in this thread, but your post reminded me that I received two requests in recent months to give hands-on magneto refreshing/restoring/rebuilding workshops at club meetings this year. Unfortunately, I already had time conflicts with both dates so had to decline.

More relevant to this thread, though, is before too much longer I hope to be posting another appendix, this one on rebuilding a Lucas KNC1. First, though, I have to find the time to rebuild it...
Posted By: johnm Re: Restoring a Rotating Armature Magneto - 05/11/15 1:05 am

Make the given year after October next year :-)

Im currently posted by my company to Eastern Europe but should be home late next year. The guy who found your posts most useful is a long time bike engineer machinist who orginally trained as a post office comunications tech. He restores a lot of mags and was able to use the information to augment his own experience.


APPENDIX VII: Rebuilding a Lucas KNC1 Competition Magneto

I just rebuilt a Lucas Competition magneto that I'm told had been professionally restored less than 100 miles ago. As can be seen from the next photograph it looks great on the outside.

[Linked Image]

Since the construction is pretty much the same as the Bosch ZEV, and since it wasn't such an internal disaster, I didn't take nearly as many photographs of the rebuild as I did of the Bosch. The next photograph shows it after disassembly:

[Linked Image]
The important point is the earth brush on this magneto is hidden under the "Competition" tag, and it has to be removed before the armature can be pulled out of the housing. Next I pulled the bearing off the race at the end of the armature with the slip ring and then used a tool shown much earlier in this thread to extract the race using it and a two-jaw puller as shown in the next photograph.

[Linked Image]
Next to the slip ring are the oil slinger, a few spacers, the race, and the bearing. A quick test with my Merc-O-Tronic tester already showed the coil was good so next the end cap has to be removed to gain access to the capacitor.

[Linked Image]
No identifying markings are on the capacitor but it appears to be a 60 Hz line filter capacitor that magneto restorers often use, and which quickly self-destruct because they can't handle the high pulsed currents. I tried to test its capacitance but it was too heavily shorted to measure. At least the "professional restorer" hadn't slathered it with an excess of epoxy to make removing it time consuming.

I replaced the blown capacitor with a pair of Panasonics as on the Bosch ZEV, although attached to an adapter like shown in Appendix VI because it is required for a single-cylinder magneto. However, when I reassembled the armature I decided to use two screws from my stock that were in better condition than the ones I had removed from it. That turned out to be a mistake. As the next composite photograph shows the replacement screws were a few threads longer than the original ones so they protruded by ~1 mm..

[Linked Image]
I didn't notice the problem until I tried to put the end cap on and there was a gap of ~1 mm. So, I switched back to the original screws.

After getting the right screws in it I reassembled the magneto and installed new points since the ones in it definitely had more than 100 miles on them. I then put it on my lathe using a bracket shown earlier in this thread. It sparked reliably down to 300 rpm (600 rpm engine) on the uncalibrated dial on the lathe. I then I magnetized it at 84,500 Amp-turns and redid the test. This time it was still sparking reliably down to the lowest speed on the dial of 250 rpm (500 rpm).

Since the speed dial on the lathe is only approximate, rather than putting the lathe in back gear to go slower I moved the magneto to my modified distributor tester. However, I haven't used it since moving to my new house ten months ago and I discovered that in the meantime the battery in the tachometer had died. So, once I get a new battery for it I'll see how low this properly remagnetized magneto will go and still continue to provide a reliable spark

Send questions or comments to [email protected]

Posted By: Magnetoman Re: Restoring a Rotating Armature Magneto - 07/28/15 12:08 pm
I was busy getting ready for the visit of a daughter this past weekend and forgot I hadn't posted the results of the running tests I made after getting a replacement battery for the tachometer on my modified distributor tester.

After rebuilding and re-magnetizing the KNC1 it sparked reliably down to 160 rpm (320 rpm engine). I then stuck a piece of reflective foil to the chuck on my lathe and used a non-contact tachometer to check its speed to know how the magneto had performed immediately after rebuilding, but before re-magnetizing. The 300 rpm on the uncalibrated lathe dial I had found for my previous post turned out to be 315 rpm (630 rpm engine).

As I wrote previously, a Lucas manual says 300 rpm (engine) is at the low end of kick starting speeds, with 500 rpm normal. So, the magneto is now performing up to its full, um, potential.

The 320 rpm vs. 630 rpm is actually a significantly bigger difference than I usually see before and after magnetizing. This means this particular magneto had become significantly demagnetized prior to ending up in my hands. However, even had the "professional restorer" not used an inappropriate capacitor that failed, these results illustrate how much performance can be lost if a magneto of unknown previous history is not properly remagnetized after rebuilding. The fact it worked at 630 rpm (engine) means a healthy guy could have gotten an engine running with it, but probably would have incorrectly blamed its lousy performance on being intrinsic to magnetos. It had been "professionally restored," after all.

Send questions or comments to [email protected]
Posted By: figgley Re: Restoring a Rotating Armature Magneto - 08/23/15 5:49 pm
Ha ha, Doc.

For me, reading "...a healthy guy could have gotten an engine running with it.." is the equivalent of hearing fingernails running down a black(chalk)board, mate.

Why not, "...a healthy person could have caused an engine to run with it..."?

This ugly, American, bastardisation of the English language really gets on my tits, son.

Ditto "snugged up" versus, the less cosy, "nipped up".

That aside, and being 99.999% confident that I'll never own a motorcycle with a magneto, this thread has been fascinating to read and really sends out a message to those who dabble in the dark art of magneto or, for that matter, any other type of *restoration*.

For the uninitiated, the world of older motorcycles is a minefield to tiptoe through, check you still have all four limbs connected, and then place your trust in the hands of the "professional" to deliver the service you require.

Here's just one example of what can happen when you entrust part of your motorcycle to a "pro":

I had a later cylinder head parked on top of the barrels of a 67 T120. It was one of those with the alloy block manifolds, rather than the screw-in stubs. It needed the plug holes coiling; new valve guides blah....

So, I took it down to L*n at The Cylind*r H*ed Sh*p, who at that time was still operating out of W*mbled*n, SW London, and explained my requirements: One, possibly two, plug holes to be sorted; new valve guides and seats cut to accept whatever Fandango valves they were flogging back then; cosmetic vapour-blast of the whole head; oh, and could you tickle these manifolds so there's no discernible step where they meet the head- in other words, just make the intake flow smooth, yeah?

L*n: No problem. Road or race?

Figgley: Er, it's a road bike, so road.

Several weeks(months?)and £400 later, I take that bastard drive from NW London to effing Wimbled*n to pick up the head that is going to make the Bonneville breathe freely and to the maximum of it's abilities.

All I had required, in terms of flowing, was a smoothing of the joint between the manifolds and the head proper and a general clean-up.

Initial impression was that the head looked pretty....very clean. As new. L*n, or one of his slaves had, as promised, sealed the joints between manifolds and head with something other than paper and locked the studs, which was part of what I desired.

Turning the head upside down revealed, obviously, there had been a shitload of alloy removal that I hadn't asked for. He'd opened up everything back from valve seats to carb-side manifold faces.

Cheers for that, you hoon.

OK for 32mm Mikunis but not what I employed you to do, Len, you C*NT.

Your, slapdash, approach to customers' needs; your general lack of interest in anything other than yourself; your hiding behind that effing SERCO and pretending to be an *engineer* rendered my expensive head unfit for the other EXPENSIVE gadgets I had to feed it, mate.

Give me back that alloy you stole. I know where you hang out, sunshine.

L*n Paters*n is a cowboy trying to ride a thoroughbred. The guy's an arsehole

So, I have a nice '72 cast head that's flowed, courtesy of L*n, to breathe lots of vapour but is useless for 1 1/8 or 1 3/16 Monoblocs/ 30mm Concentrics [email protected]

I still have the original head, which is in very poor shape, but I have to treasure it and keep reading this blog, sunshine.

Where did I start? Oh, yeah- old motorcycles are the route to insanity, geezer.

P.S. Back in 81/82, before I knew the score, Ian at Roebuck's in Rayners Lane, Middx insisted I needed to turn over my Smith's Tach in order to procure a set of Jap-made TR5 clocks.

Hopefully he died of cancer.

Dave x

Originally Posted by figgley
For me, reading "...a healthy guy could have gotten an engine running with it.." is the equivalent of hearing fingernails running down a black(chalk)board, mate.

This ugly, American, bastardisation of the English language really gets on my tits, son.
Thank you for your comment. Although grammar and spelling errors bother me as much as the next person it is time consuming enough to create a 36,000-word technical document that is accurate. Unfortunately, to make it grammatically flawless without benefit of an independent proof reader would require far more time than I could hope to devote to it. So, sadly, I'm sure it wouldn't be hard to find examples in this thread of dangling participles, split infinitives and other grammatical atrocities. I envy anyone who does better under the circumstances.
"Gotten" was good enough for Shakespeare, but not for Wiggley off the internet.

Think I'll go to the pub and not come home until I've gotten nipped up.
Although the figure below is for a Zener I'm posting it because it's interesting in its own right as well as being a preview of a magneto-related instrument I'm working on that I hope to have done in a few months.

[Linked Image]

I purchased a used Lucas "12 V" Zener on eBay to use in an instrument I'm designing and needed to test it to be sure it actually works. The curve is the current-voltage characteristic of this Zener with a greatly offset x-axis showing the important region where the Zener begins to function to regulate the charging voltage for the battery. The voltage at which the current begins to sharply increase is the "Zener voltage" and here it's seen to be ~15 Volts.

Although at the scale plotted here it looks like the current below ~14.8 Volts is zero a tiny current actually is flowing. However, at 14 Volts this Zener is passing only 16 microAmps (R = 0.88 MegOhms). Why a Zener is useful a voltage regulator is illustrated by the fact that at 15.25 Volts the differential resistance has dropped by over a million to only 1/4 of an Ohm. That is, the harder the stator tries to generate more voltage than the battery wants to see, the better the Zener works to clamp the voltage at ~15 Volts by conducting excess current straight to ground. In essence, it's a nearly perfect insulator for voltages below ~15 V (~ 1 MOhm), and switches to being nearly a dead short for voltages above that (~1/4 Ohm).

I limited the current from my power supply to a little over 1 Amp for this measurement so the maximum power dissipated in the Zener was limited to ~15 Watts (Current x Voltage = Power). However, extrapolating the curve to 2 Amps (30 Watts) shows the voltage would only increase to ~15.5 Volts at that point (~15.65 @ 45 Watts, etc.)

I should add that the Zener voltage is temperature dependent to some extent (increasing with increasing temperature) so if I wanted to thoroughly test its characteristics I would need to instrument the Zener with a thermocouple (because its internal temperature, not ambient, is what matters) and make the measurements after reaching thermal equilibrium at several current/power levels. Although I could do this, the results aren't important for how I will use this Zener.

A battery doesn't care what internal temperature the Zener has, it only cares about its own temperature. Since the optimum charging voltage for a lead-acid battery that's at 20 oC is 14.6-15.2 Volts this Zener seems like it would be an excellent voltage regulator. However, on a warm day (with the battery at, say, 30 oC) the required charging voltage required drops to 14.2-14.8 while an even warmer diode will have an higher Zener voltage.

Of course, if you're riding 120 mph with your lights off through Death Valley in summer the poor Zener will be dealing with nearly all the power your stator is generating so the voltage will rise and the battery will suffer. Still, even with its intrinsic limitations a Zener is not a bad solution at all for most riding conditions, especially when you compare what you get with it vs. an MCR1 electro-mechanical voltage regulator.

Anyway, the above figure shows this Zener passes the test and so it will take its place as one of the many components in the instrument I'm building. More details in due time.
Posted By: Mark Z Re: Restoring a Rotating Armature Magneto - 12/21/15 10:48 pm
I was going to post this in a separate thread, but when I saw your link in my thread, I thought it might be more appropriate here.

I was just reading in some literature supplied with a battery charger that the correct finish voltage for a 12V flooded battery is 14.84, while the correct finish voltage for a 12V gel or AGM battery is 14.4.

This may cast some dispersion on the use of AGM batteries in our old British machines, and may explain why my Scorpion AGM battery only lasted one year. (I was withholding this conclusion because, during that year, I had a faulty alternator lead for a couple of weeks. However, I noticed the charging problem right away, and prevented the battery from going completely flat through external charging.)

After that, I bought a flooded battery quite by coincidence - I needed a battery in a hurry and the only one I could find locally was a flooded battery. However, this battery has performed flawlessly over the last two years. Notwithstanding the dreaded acid leak potential, and having finally figured out how to make the dang hose stay on the breather spigot, I may just stick with flooded batteries (or "sealed lead acid", which has the same charging requirements as a regular lead acid battery, but without the potential for acid seepage).

But this is with an "OE zener diode". It would be interesting to see if the specs differ with a modern rectifier/regulator unit. I believe many modern bikes call for AGM batteries, so I have to believe they've taken this into account in their charging systems.
Originally Posted by Mark Z
But this is with an "OE zener diode".
Keep in mind that my test was of only one device so there no way to know from it alone how wide the tolerance in Zener Voltages was for the components Lucas supplied at the time. Although it would be easy enough to test a dozen original Lucas Zeners to determine the answer to this question about variation it would take a few hundred dollars to acquire them on eBay. Or, wait a minute, maybe my friend has a couple at his shop...

I dropped by my friend's shop on the way to work this morning and asked him if he had any old Zeners on the shelf. He thought he remembered having seen one somewhere in the back room. I left the shop a little while later carrying 9 of them so even if a few turn out to be bad there will be enough to get a reasonable idea of the variation.

For reference on what is possible 40+ years later, I went to the Digi-Key site and filtered only for voltage tolerance, leaving all other options open. They show 21,207 different Zener diodes are available from them having +/-5% tolerance, but that drops to only 1,680 if I specify +/-1%, and they have just 3 at +/-0.5%.

If Lucas's tolerance wasn't better than +/-5% (a range of 14.25-15.75 V for a nominal 15 V) it would mean some people will have batteries with reasonably long lives, but others will have ones that cook fairly rapidly. Based on what is available today, I suspect a tolerance of +/-1% (14.85-15.15 V) was beyond what Lucas achieved at the time.

To do a reasonable test I'll park my good Zener at 1.0 Amps and see how long it takes for the voltage to stabilize when internally heated by the 15 Watts. Then it should be a quick matter to repeat the measurements on the other diodes. I probably won't get to this until the weekend but will post the results as soon as I have them.

Although the most significant outcome of this will be to satisfy my own curiosity a side benefit will be to give me enough information to select a diode for my Trident having the "optimum" characteristics within the intrinsic limitations of a Zener. That is, should I ever go to the trouble of measuring the one currently in it and then switching the diodes if necessary...
Posted By: Magnetoman Re: Restoring a Rotating Armature Magneto - 12/25/15 11:36 am
All but one of the Zeners I got from my friend's shop earlier this week turned out to be good so now I have a reasonable amount of data from which to draw conclusions.

The electrical setup needed for this is quite simple. I wired a precision 1.0 Ohm resistor in series with each diode and used a 3-1/2 digit Fluke voltmeter of 0.1% accuracy to measure the voltage drop across it to directly display the current to the nearest mA. This is more than adequate because the I-V curve of a Zener rises so steeply the accuracy of the current reading isn't the limiting factor. A second 4-1/2 digit Fluke voltmeter measured the voltage drop across the diode to the nearest millivolt. In this case the 0.1% accuracy means the absolute value could be off by as much as 14 mV, although relative values between Zeners are accurate to 1 mV.

The change of Zener voltage with temperature is a real issue so I'll start with the results on the three that came bolted to their original finned heat sinks.

For one of the three heat-sinked diodes I ran the current up to 1 Amp (~15 Watts), then up to 2 Amps (~30 Watts), then cycled back and forth a few times between there and 1 Amp. Each time I was at one of those current values I quickly recorded the Zener voltage. I determined from these measurements that even without air flow the heat sink was effective in keeping the Zener at constant temperature at these power levels, at least for the minute or so all the measurements took. That is, the Zener voltage I measured at 1 Amp after decreasing from 2 Amps was within a few millivolts of the value I measured when I initially had increased the current to 1 Amp when ramped up from 0. For this diode the Zener voltage at 1 Amp was 14.195+/-0.005 Volts. At 2 Amps it was 14.275+/-0.005 Volts. Extrapolating, at 4 Amps (60 Watts) it would be ~14.355 Volts, i.e. an increase of only ~1% from the 15 Watt value. Hence, to regulate a voltage at a fixed value one could do a lot worse than a Zener.

For the other two heat-sinked diodes I only measured the voltage after the initial ramp up to 1 Amp. For one of them it was 14.357 Volts and for the other it was 14.644 Volts (uncertainties in both are ~+/-0.005). Hence, the spread in values of these three was ~0.45 Volts.

For the other six diodes I didn't take the time to bolt them into a heat sink. Instead, I ran the current up to 1 Amp (15 Watts) and measured the Zener voltage as quickly as possible after I had stabilized the current (~5 seconds). Because of the internal heating caused by the current, the voltages continued to drift upwards but the range of values, if not the absolute values, are indicative of the spread of Zener voltages that would have been measured had they been in heat sinks. That spread was a full 1 Volt, or +/-3.5%.

This is a digression from my current interests so I didn't take the time attach thermocouples to the diodes, bolt them in heat sinks etc. However, I'm confident drawing conclusions from the results. My speculation is the measured spread of +/-3.5% I found for this limited number of diodes would increase to more like +/-5% with a larger sample size. This would be reasonable given the state of semiconductor production technology in the 1960s.

Assuming Lucas aimed for precisely the 14.84 Volts that Mark Z mentioned in his post, a +/-5% spread means at the low end some Zeners they supplied to the factories were 14.10 V and as a result undercharge batteries, while at the high end others were 15.58 V and boil their insides out. If someone got lucky and theirs was the perfect 14.84 Volts at fairly low speeds when it was only needed to dissipate 15 Watts, at high speeds when dissipating 60 W it would increase to 14.99 V, which by itself isn't too bad at all. However, because the heat sink can do only so much the temperature of the diode, and hence the Zener voltage, would increase further due to heating.

A typical 14 V Zener has a 0.08%/oC positive temperature coefficient while a battery needs a ~0.1% negative. For example, even absent internal heating from the current, a 14.84 Volt Zener that was optimum at 77 oF would increase to 15.02 V at an ambient of 105 oF whereas a battery needs 14.54 V at that temperature, which is nearly 0.5 V lower.

So, even a diode having a "perfect" Zener voltage at a given temperature isn't perfect when used as a voltage regulator on a motorcycle. That said, I haven't (yet) measured any of the modern replacement voltage regulators (Boyer, Mity Max, Podtronic, Tympanium, Wassell, others?) so I don't know if any of them do any better than a Zener for either the absolute voltage, or for the temperature compensation required for correctly charging batteries whether riding in snow or riding through the Sahara. However, any such measurements will have to wait until after I finish building the instrument mentioned in my post of December 20 and that's at least a few months away.

To digress for a paragraph, it turns out the temperature coefficient depends on the Zener voltage, and diodes smaller than ~4.5 V have coefficients with the opposite sign. This means four 3.71 V Zeners connected in series would have the perfect 14.84 V at room temperature, decreasing to 14.72 V at 105 oF, which is only 0.18 V too high. Similarly, cooled to 32 oF the Zener voltage would increase by 0.2 V whereas the ideal for a battery would be only 0.1 V higher than that. That is, four of these smaller Zeners in a good heat sink would regulate the charging voltage to within no worse than 0.2 V of ideal over the entire temperature range from freezing to above 105 oF.

To conclude this departure from magnetos, because of the apparent ~+/-5% variation in the ones originally supplied by Lucas the odds are the particular Zener that came in your bike is only doing a so-so job keeping the battery in good health. However, it only would take a few pieces of electronic gear to select a "perfect-ish" one that would do much better (within a Zener's intrinsic limitations due to the physics of semiconductors). Had I seriously thought I might ever want to "optimize" the one in my Trident I would have used this opportunity to select one from the batch from my friend's shop to do just that. Maybe someday I'll regret not having done so (and maybe I still will do it...). But, there are so many projects, and so little time.
Posted By: panman Re: Restoring a Rotating Armature Magneto - 03/08/16 1:27 pm
I see this is and old thread so maybe the good Dr. won't respond. But I missed in these articles how the high tension wire is attached to the slip ring. If someone could please advise me I'd appreciate it. Regards, RL
Originally Posted by panman
I missed in these articles how the high tension wire is attached to the slip ring. If someone could please advise me I'd appreciate it.
The photograph below is of the relevant section of a slip ring I discussed in an earlier post:

[Linked Image]

As can be seen there's a hole in the brass segment. The output wire of the coil has a slightly smaller diameter so it slips into this hole from the right, at the center of the projecting section of insulator, resulting in either direct electrical contact with the brass or a very tiny gap for the voltage to bridge. The first few photos in the following post will remind you how the slip ring is oriented with respect to the output wire of the coil:
Posted By: panman Re: Restoring a Rotating Armature Magneto - 03/08/16 5:44 pm
Is it glued in with a small drop of epoxy then or does the tensional stiffness of the wire hold it in place. Thanks much for the quick response, I'm new to this form. I'm working on a Goldstar flat tracker frame that I'm putting my BB34 alloy clipper engine into with some original flat tracker parts. I've read a number of your articles in AMCA newletter? I think and a number of other places and have always found them interesting and highly informative. Thanks, RL
Posted By: Magnetoman Re: Restoring a Rotating Armature Magneto - 03/08/16 10:42 pm
Originally Posted by panman
Is it glued in with a small drop of epoxy then or does the tensional stiffness of the wire hold it in place.... I've read a number of your articles...
Thanks for your very nice comment.

Glue is neither necessary nor wanted. If you glued it in place you likely would damage the coil and/or slip ring when later trying to remove the slip ring. The coil itself is quite stiff so the hole in the slip ring is more than sufficient to keep the wire from going anywhere.
APPENDIX VIII: Rebuilding a Lucas Magdyno

Since the internals of the magneto portion of a Magdyno are very similar to the Lucas K2F of Appendix II and Lucas KNC1 of Appendix VII, as well as the Bosch ZEV, much of the information for rebuilding it is the same. Because of this I won't repeat the description of disassembly and replacement of the condenser. I'll take a different approach and instead of the usual instructions to "reassemble in reverse order of disassembly," I'll suggest you disassemble in reverse order of assembly.

Although the resistances of the primary (0.72 Ohms) and secondary (5.5 kOhms) were what they should be, there still could have been an internal short between windings. If there were a current would flow through that path likely causing the damage to increase with time until the coil ceased to function. To check for this I used a growler, which most people think is only good for checking dynamos. I forgot to photograph this test, but the following shows an armature from a Dixie magneto I recently checked as a favor for someone who will be riding a 1916 Indian in the upcoming Cannonball Rally.
[Linked Image]
A growler generates a 60 Hz AC magnetic field in the 'V'-shaped cradle. If a closed loop of wire were in the cradle that field would generate an AC magnetic field in it. Although an armature has 10,000 loops of wire, unless there is a short there is no electrical connection between any of them so no magnetic field will generated in a good armature. However, if there is a generated field (i.e. a short) it will result in an AC force on any nearby steel. A hacksaw blade serves as a convenient detector since any such force can be easily felt when it is held next to a bad armature. The armature in this Magdyno passed the growler test so there are no internal shorts in it.

After replacing the condenser the end cap of the armature was screwed on, followed by the slip ring, oil slinger, appropriate shims (more on this in a moment), race, and a retaining ring that I forgot to have in place for the following photograph of another Magdyno armature:
[Linked Image]
The shims are necessary to give the proper end float of 0.001"-0.003" for the ring cam. This sandwich is assembled with a press and tool in the shape of a deep-drive socket, although a socket would be fine. I don't recommend beating the race on using a hammer instead of a press except in an emergency.

The next photograph shows how the outer race goes into an insulating paper cup in the end cap. The same is the case in the main housing. A new paper cup is at the left.
[Linked Image]

The next photograph lays out some important items. At the front is the assembled magneto with an indicator showing how the end float of the armature is measured. At the top right is a pile of armature shims. The ones in this pile vary in thickness from 0.007" to 0.049". At the top left are shims for the end cap of 0.004", 0.005" and 0.006" thickness. These two types of shims can be used in combination to give the correct end float.
[Linked Image]

Note that the fitting for attaching the advance/retard cable, in the endcap at the left of the unit in the photograph, is on the same side of the housing as the HT pickup. In this location tightening the cable advances the spark for this magneto (which rotates counter-clockwise viewed from the end with the drive gear). Endcaps with the fitting on the other side, where tightening the cable retards the spark, also exist so you need to pay attention to both the direction of rotation as well as action of the advance/retard cable in order to set the timing correctly. In the case of this magneto the lever on the handlebar will have to tighten the wire to have it in the fully advanced position when setting the static timing.

After the bearings were lubricated with Sta-Lube 'Hi-Temp Disc Brake Bearing Grease' the armature was placed in the housing, the end cap attached, and the end float measured. If there had been negative end float, i.e. the end cap didn't sit flush with the housing, the gap would have been measured with feeler gauges and the correct 0.001"-0.003" float obtained by use of one or more shims under the end cap. Or, the race could have been removed from the armature and thinner shims installed there to remove the excess float. If there had been positive end float larger than 0.003" the only choice would have been thicker shims on the armature.

If the race is removed to change the thickness of the shims the end float has to be measured again because even if the same shims are used it is difficult to reproducibly position the race to 0.001". Because of this it is typically faster to try to achieve a negative end float with the armature shims and then make the final adjustment using end cap shims.

Don't forget to replace the earth brush:
[Linked Image]

After the magneto had been reassembled it needed to be remagnetized to recover its full strength. As the next diagram shows an iron plate with four holes in it needs to be added to the magnet pole pieces to clear the pegs in the bottom of this type of magneto.
[Linked Image]

However, while Lucas says a flat plate is sufficient when using their recommended 65,000-70,000 A-turn electromagnet, it's possible to do even better. In my case, I designed my electromagnet to produce a ~15-20% greater field of 80,000 A-turns. Further, the next photograph shows the iron plate I machined placed against one of the magnet's pole pieces. As can be seen the side of the plate that fits against the base of the magneto isn't flat.
[Linked Image]

The left side of the next composite shows the bottom of the Magdyno housing and the right side shows it with the plate in place. The shape milled into the plate significantly reduces the "air gap" between the pole face and the internal magnetic structure of the magneto which increases the field that magnetizes the Alnico. The positioning of this plate isn't arbitrary; I machined it so the pole pieces would align with the location of the Alnico magnet within the magneto body.
[Linked Image]
This difference in "air gap" might not seem significant, but it is. Think about the force needed to pull a magnet from a refrigerator versus the much smaller force needed when there are only a few pieces of paper between it and the refrigerator.

The magneto with the extra plate was placed in the electromagnet and the field ramped up to its maximum value several times to leave the largest possible remnant magnetization in the Alnico.
[Linked Image]

As an aside, while the Magdyno is vertical in the electromagnet whereas Lucas shows it horizontal, magnetism doesn't care about gravity. I designed my electromagnet to use various spacers and pole pieces to be able to magnetize every automotive magneto whose dimensions I could find. As a result some magnetos have to be vertical (e.g. Lucas K2F and Magdyno), while others are horizontal (Bosch ZEV and Lucas KNC1).

After magnetizing the magneto, but before attaching the dynamo, I wanted to make sure the magneto was working well so I clamped it to my long-term tester and installed a pulley that spins it at 2000 rpm (4000 rpm engine). Assuming that corresponds to about 60 mph my 6-hour test was equivalent to ~350 miles.
[Linked Image]
The reason for this extended test is that if there were any loose parts or other issues there's a good chance they would have revealed themselves in this 6-hour, 2000-rpm test. Also, since all the electrons and magnetic monopoles were now comfortably in their final state another test would reveal how hard my friend was going to have to jump on the kick starter in order for the magneto to start the engine.

After running the magneto on the long-term tester I put it on my modified distributor tester to make sure the spark occurred at the same point each revolution (i.e. didn't wander over a range of firing angles). Unfortunately, I was rushing because I still had to get ready to go out for dinner so I didn't take the time to set up to take photos and when I came back to it the next day already had removed it from the tester before I remembered this. Anyway, the firing wandered by less than a degree.

After the distributor tester I put it on the lathe to check the lowest speed at which it would provide a reliable spark. I should have been able to make this measurement in the distributor tester but I'm in the process of installing a different tachometer on it and don't have it wired in place yet. Lucas gives 500 rpm (engine) as the normal kick starting speed and 300 rpm as the lower limit. This magneto reliably sparked down to 276 rpm (engine; 138 rpm magneto). I didn't photograph this since the setup is identical to the one I used earlier in this thread for the low speed test of the Bosch ZEV.

I already had refurbished the dynamo so it was time to install it on the magneto. As the next diagram shows attaching the coupling gears is straightforward.
[Linked Image]

First the fiber gear and clutch are bolted to the armature. To tighten the nut requires a special tool, which is simply a 1/4"-dia. rod of the right length bent into a 'U' shape.
[Linked Image]

After the nut is fully tightened the lock washer holds it in place. To make sure the clutch slips within the right torque range Lucas, having not yet discovered the existence of torque wrenches, suggests the following setup.
[Linked Image]

However, rather than raid a fishmonger's stall to get the necessary scale, I used an actual torque wrench. This test still requires a way to lock the magneto armature in place, for which Lucas suggests another jig shown in the above photograph that places all the force on a single tooth of the fabric gear.

In another document Lucas suggests a diamond-shape tool for this task, as shown at the left of the next composite photograph jammed between the fiber gear and a dynamo gear placed in position for the photograph. Clearly, this also puts a lot of force on a small area of the fiber gear. Because of this, also shown next to the magneto is the jig I made for this which engages 10 teeth of the fiber gear, reducing the force on any one of them by a factor of 10. This jig is shown in place at the right of the composite photograph.
[Linked Image]

With the fiber gear locked in place and a deep drive socket on the torque wrench to clear the end of the armature I found the clutch slipped at 7 ft-lb. Lucas calls for it to be anywhere in the range 4-10 ft-lb. so all is well with the clutch and the magneto is now ready for the dynamo.
[Linked Image]

The dynamo is pulled into position by a nut at the front but the actual holding force is provided by a strap. I first pulled it into position with the nut and then, as can be seen from the next composite photograph, tightened the strap bolts on both sides in a way that left approximately equal gaps. This allows the greatest flexibility if the strap needs to be further tightened in the future.

After the dynamo was firmly in place I removed the top nut and replaced it after installing the cover. At this point the Magdyno was now finished and awaiting installation on the bike.

p.s. I don't know why the last image won't appear, and shows up as "does not exist" in Photobucket if the code is clicked on. I've tried uploading it again to receive a new URL, and slightly changing the image in Photoshop and then uploading that one. Nothing I've tried has worked. Anyway, the image you're not seeing is a composite that shows the Magdyno without the cover beside it with the cover.
As I wrote in the previous post, earlier this year I stress-tested coils for a Dixie magneto and a spare as a favor for Kevin Naser who was rebuilding a 1916 Indian to ride in the next Cannonball cross-country rally. As well as testing his coils I also gave him two pairs of "my" condensers to use in these magnetos.

The 2016 Cannonball is now finished and Kevin was one of only 16 riders out of a starting field of 92 who achieved a perfect score in the 3306-mile, 16-day rally. He covered every mile, and did so within the allotted time every day. As this shows, a magneto -- even one 100 years old -- that is rebuilt using the proper components will take a licking and keep on ticking.
Posted By: kevin Re: Restoring a Rotating Armature Magneto - 07/03/17 12:08 am
magnetoman, photobucket has screwed your thread.

good as it is was, it's now text-only . . .
Posted By: Peter R Re: Restoring a Rotating Armature Magneto - 07/03/17 11:26 am
Shame that this valuable and instructive thread has gone down the drain indeed.
Guys, thanks for the condolences. Also trashed were my other 'projects' threads, and seven of the top ten 'projects' threads in terms of views are history. However, perhaps they're not gone forever.

Three possibilities come to mind: In the weeks to come Photobucket could cave to the bad PR, alter their policy, and grandfather in old links. A different hosting service along with an automatic search/replace as outlined by Shane could get things running again just like they were. Or, at least for my material, the people behind the software that runs BritBike (not Morgan, but the people who maintain and upgrade the code) could finally bring its image-handling capability into the 21st Century and I could upload all of my trashed threads from my computer that have the images directly embedded in them.

In any case, there are potential solutions so there's good reason not to panic (well, except for Morgan, who probably should be panicked...).

I'm reminded that sometime a year or so ago someone dismissively commented on a concern I expressed about the reliance on Photobucket that he could "personally guarantee" the service was large enough that it never would go away. I replied that he couldn't guarantee that because he had no control over it. Technically, it hasn't "gone away," it's just locked all of us out, but the effect is the same.

There's no guarantee Britbike won't go away sometime in the future, either, which is another reason to keep your own backups of any material you care about. By the way, there's no difference between relying on "the Cloud" to do this for you and relying on Photobucket itself, since "The Cloud" is just a variety of services like Photobucket.
Posted By: johnm Re: Restoring a Rotating Armature Magneto - 07/04/17 7:59 am
Indeed very disappointing. This thread was used by several of my friends. I hope it can be restored somehow and then I shall ensure I print it out - or make a PDF for my files.


Posted By: GROG Re: Restoring a Rotating Armature Magneto - 08/11/17 12:58 am
Hi Magnetoman. It is some years since I dropped back on this thread and I see you've had problems with photos being lost.
I am surprised you would not have a backup.... fortunately years ago I saved it all for my own reference and have put it up on Dropbox if you wish to make use of it.
Posted By: GROG Re: Restoring a Rotating Armature Magneto - 08/11/17 12:59 am
Here is a Dropbox link to the doco
Cheers mate, see you in NZ!!!!
Originally Posted by GROG
I am surprised you would not have a backup....
I would be even more surprised than you...

However, I wrote it in an "active" format for this forum rather than as a "static" document, which I would have edited differently. That's why I want to bring it back to life by re-linking to photos when I have the time to do it.
Posted By: johnm Re: Restoring a Rotating Armature Magneto - 08/11/17 5:25 am
Hi Greg

Good to see you back here. Yup back home to NZ for good come Christmas although there is a move here to get me to stay even longer. But 7 years in Central Asia and Eastern Europe is enough and I need to work a period in NZ before turning 65 for super and tax reasons anyway. I have been assigned Eldee 2 to investigate while Nick works on the new 350. Bill Swallow is still keen to ride it in the 2018 Manx. Perhaps you should make the trip? And by the way you are likely to get the job of assessing the Eldee head and porting when you visit next year. They have at least one more blank which could be machined up. (Get the management style - too many years of having a team working for me - I'm supposed to do the work but just assign it out to everyone else :-) )

I cant access the Dropbox here but I'm sure I will manage it somehow.

I printed out a PDF which I have stored in NZ. My young staff laugh at me but as I point out I have never had a piece of paper vanish overnight. But more than one or two computer drives have blown themselves out of existence over the years !!! It's good to be a luddite !!
Posted By: kevin Re: Restoring a Rotating Armature Magneto - 11/25/17 2:34 pm
if you're using vivaldi, you can restore the images to your screen using an extension from the chrome web store. go to this link and click ADD EXTENSION, then clear your cache and reload.
Posted By: Ponce Re: Restoring a Rotating Armature Magneto - 12/09/18 7:18 am
Best ever article, thank you. I have several BTH magnetos from Velocette MSS engines and am in the process of replacing the capacitors.
I have also removed the pin/screw in the end cap to have the end cap chromed.
I have turned new pins/screws for the cam ring adjustment. What is the process of realigning these pins ie. position of adjustment of the cam ring.
Regards Peter.
Thanks for your nice comment. I need to look at a housing to remind myself of the construction, but my box of BTH magnetos and parts is on a shelf behind a mass of Ariel-related items that are "temporarily" there. Unfortunately, it won't be anytime soon before that situation changes.
Posted By: Ponce Re: Restoring a Rotating Armature Magneto - 12/11/18 7:12 am
Thanks for the response. I will try to get a picture up which may refresh you. I know when one packs parts away on shelves it is often difficult to move them and sometimes very heavy as well!!
Regards Peter.
Posted By: Ponce Re: Restoring a Rotating Armature Magneto - 12/18/18 7:34 am
I have found out it is the screw to adjust the position of the cam ring in relation to internal timing of the magneto, called the flip point I believe.
Would you know how to determine this point electrically, please?
Regards Peter
Originally Posted by Ponce
Would you know how to determine this point electrically, please?
The inductance of the primary and secondary abruptly change at the point where the magnetic flux reverses in the armature. As you approach that point the flux lines get forced closer together so to torque required to turn the armature increases. At the point where it reverses the torque also abruptly decreases, making it difficult to stop turning the armature when that point is reached. It's like pushing on something that is stuck and trying to stop pushing at the moment it becomes unstuck so that the object remains in the same position without having moved.

Anyway, a protractor attached to the armature and an inductance meter attached to the coil lets you plot L vs. angle. Although you have the same problem of torque abruptly decreasing at the flux reversal point, fitting the curves before and after and extrapolating to where they cross lets you determine the point to better than 1-deg. without too much effort.
Posted By: Ponce Re: Restoring a Rotating Armature Magneto - 12/23/18 7:41 am
Magneto Man,
Thanks once again for your help. It is such a busy time of the year so I will wait till it quietens a little before I attempt to find the correct point.
Thanks once again I do appreciate your knowledge and am learning very slowly.
Regards Peter.
I saw a recent thread on the VMCC forum and I know MM isn't on there any more so thought I would report some details on here as it is a good example of lots of things on this thread about poor mag rebuilds.

A guy was having trouble with his magneto so sent it to a re-builder to fix. The re-builder reported back that:

"There were 2 wires, seemingly from a wire brush, inside the armature, the wire from the winding was not in good shape, the screws which hold the armature together were the wrong thread and although they seemed tight, they would very quickly become loose.The condenser was wrong and did not fit into the space available, so it had been shoved in at an angle and the wires coming from it were bare. All of these thing would cause a problem so to have them all.... no wonder the bike would not start."

I know there are reports of poor mag re-builders but whoever did this must have had zero sense of shame.

Posted By: Magnetoman Re: Restoring a Rotating Armature Magneto - 06/22/19 11:53 pm
Originally Posted by George Kaplan
I saw a recent thread on the VMCC forum and I know MM isn't on there any more....
I let my VMCC membership lapse earlier this year but someone doesn't have to be a member to take part in the Forum. However, I only check it from time to time and coincidentally had done so about ten hours ago. The 12 new posts in 4 threads since then illustrate why I don't bother checking very often.

Originally Posted by George Kaplan
I know there are reports of poor mag re-builders but whoever did this must have had zero sense of shame.
But, I'll bet the cosmetics of the outside of that magneto were excellent since it seems that's where all the restoration effort is spent by many rebuilders. After all, the customer pays for what they see, not what they can't see...

Attached picture VMCC.jpg
My VMCC membership gets me put into the official Insurers demographic of "Old Fart" which co-incidentally gives me an insurance discount of a similar amount to the subscription. If I didn't get the discount I am not certain if I would be still a member too.

I agree about the lack of traffic on the forum. I have been a bit under the weather for the last few days so probably have been on there more than usual.

Unfortunately, lots of so called Specialists who fix all sorts of stuff turn out to be Charlatans who only make stuff shiny rather than actually work properly. Electrical and electronic items are also probably worse for this as so many punters see it as a Black Art.

Posted By: NYBSAGUY Re: Restoring a Rotating Armature Magneto - 06/23/19 1:04 pm

I love the word Charlatan. The lady who delivered our vegetables from her Commer van in Dublin when I was a boy was a great malapropist. She described a neighbor of ours as 'a right bloody Charlamagne'.

Lots of Charlamagnes rebuild mags, too.
I love Charlamagnes too. Mrs Malaprop regularly gives me great amusement.

A word I don't think I've ever heard used in American English, only in old British movies, is 'mountebank'. It's close, but not quite identical in meaning, to 'charlatan'.

Charlatan. A person who pretends to special knowledge or skill that he or she does not posses.

Mountebank. A person who deceives others, especially in order to trick them out of their money

'Mountebank' would be better for describing those who possibly possess the knowledge to repair a magneto but who don't waste the time to do so because they know their customers won't look any further than the polished outside surface. However, it wouldn't convey the same feeling of outrage toward a rebuilder as 'charlatan', at least in American English ("Gadzooks, the mountebank miswired my condenser!")
Posted By: NYBSAGUY Re: Restoring a Rotating Armature Magneto - 06/24/19 1:25 am

Mountebank was originally a word used to describe an actor.. Hmm.. beware those lowlife cads.
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