August 29, 2012

Classic Motorcycle Tech – Magneto Rebuild, Part 6

Part 6 of a series on rebuilding an old Bosch motorcycle magneto. This is reposted here with the permission of the author, who has asked me to keep anonymous.

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.

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.

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.

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

Filed under Restoration, Tech, Wiring & Electrics by Buzz Kanter

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