August 24, 2012

Classic Motorcycle Tech – Rebuild Magneto Part 5

This isĀ installment #5 of a multi-part series by an anonymous friend (not me) who gave us permission to repost here.

7) In the next photograph the protrusion at 7:00 in the taper locates the points block with respect to the armature to ensure the magnetic flux reversal coincides with the opening of the points. The missing chunk at 11:00 in the taper appears to be damage caused by some heavy-handed restorer. The black cylinder at 4:00 is a carbon brush that completes the primary circuit back to earth when the points close. To do this, the carbon brush has to make good contact with the surface it rubs against.

Unfortunately, as can be seen in the next photograph, the surface the earth brush rides against is in pretty poor condition. Although the brush can make a good electrical connection to such a surface, its roughness will wear away the brush more rapidly than it should, eventually resulting in failure of the magneto. I will have to turn it on my lathe to make it smooth.

Since this surface needs to be parallel to the back surface of the plate holding it I tapped four holes in a piece of scrap Al, faced it in the lathe, then mounted this plate on it in order to face the surface for the earth brush.

After turning any rubbing surface on the lathe I use my surface roughness meter to verify it has the correct smoothness. I didn’t take a photograph after turning the ZEV surface, but the following one shows the roughness meter being used on the ring used for the earthing brush on a Lucas armature from my box of spares. I had the meter set to display the roughness in micro-in. (59 micro-in. is 1.5 um). In fact, a “perfectly smooth” surface is not desirable since some abrasion of the carbon during operation is required to ensure good electrical contact. Industry practice for the roughness of commutators calls for values 1.5-2 um, so the earthing ring on this Lucas armature happens to be fine as-is.

I measured the earthing ring on a BTH armature several years ago when I restored the magneto for another friend. It had failed shortly after he paid quite a bit to have it professionally restored by someone whose name would be familiar to many riders. The roughness measured 190-210 micro-in., which is nearly 3x too high. The grooves the restorer left in it by using a too-sharp lathe tool would have abraded the earth brush too rapidly had the inappropriate condenser he installed not failed. In fact, the grooves were so pronounced that it was easily possible to determine the feed rate he used on his lathe. I still have this armature because it was easier for me to rebuild an unrestored armature I had than it would have been to spend the time to first undo what the restorer had done to it.

8) The condenser in the Bosch armature is completely potted in the end cap using some red epoxy that I’ll have to remove. If you look closely you can see that the clear epoxy used to pot the coil also overlaps with the armature, firmly attaching it. This excess epoxy is sloppy workmanship and would significantly add to the effort required to prepare this armature if I had to wind a new coil on it.

9) A problematic aspect of the armature itself is that some of its laminations have suffered at the hands of the restorer (or an even earlier restorer — there is no way to know when over the past 90 years some of the damage was done). Armatures are built up from laminated steel plates that are insulated from each other to minimize eddy current losses that otherwise would severely reduce the output from the magneto, especially at low rpm. Someone beat quite a bit on one end of the plates on one side, and there are two holes drilled 180-deg. apart in them as well. I don’t have any prior experience with Bosch ZEV magnetos to know whether or not these holes were drilled at the factory for some reason, but neither of my spares have them. Also, although the hole in the above photograph appears to be round, the one on the opposite side is fairly crude.

I have two Bosch ZEV armatures of my own, both with their original, rotten coils, that I could press into service for winding new coils if needed. But, no matter which armature I end up using, there are issues to consider. I’m going to think about this for a day before proceeding. The armature I am repairing is in the first photograph, and one of my own is in the second.

In contrast to the armature I am working on, the photograph of mine shows what an unmolested one looks like (complete with 100 years of patina). However, the coil in mine is original, looking about as fresh as the wrappings on a mummy in the British Museum.

The dents in the armature will have shorted the outer edges of those laminations together. Also, the holes someone drilled through the laminations in the center will have shorted those laminations that are in contact with the holes. Although these are electrical shorts, each increases the eddy current losses rather than stopping the output entirely, so it’s a judgment call at this point whether their cumulative effect on the magneto output at low rpm will be unacceptable. All things considered, my experience tells me it is worthwhile proceeding with this armature despite the damage to the laminations.

Despite the issues noted in the past several posts, detailed inspection has not revealed any problems that would keep me from restoring this magneto to full operation. Unfortunately, its poor condition means it will take longer than the couple of days I had hoped would be the case had it been in good shape, since first I will have to undo all the damage done by the person who restored it. But, now have to decide how to proceed (e.g. use the present armature if tests show it is good, or wind my own replacement coil using one of my old armatures). Send questions or comments to

Filed under Classic Motorcycle Maintenance, Tech, Wiring & Electrics by Buzz Kanter

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