Classic Motorcycle Magneto Tech Continued – Mechanical Components

This series of tech updates on rebuilding and/or restoring a classic motorcycle magneto is shared with permission of the author who has asked to remain anonymous.

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.

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.

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.

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.

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.

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

This entry was posted in Harley Tech. Bookmark the permalink.