Classic Harley Tech – Rebuilding a Magneto & Removing The End Cap

This article is preprinted with the permission of the author, who wishes to remain anonymous.

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.

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

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.

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 classic.vehicle.electrics@gmail.com.

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