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]