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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.
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TESTS AND REPAIRS OF THE ELECTRICAL COMPONENTS:

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.
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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]

Last edited by Magnetoman; 10/06/12 6:08 pm. Reason: added sentence about elevated temp. testing