This is the latest installment of the on-going series of articles on overhauling a classic motorcycle magneto like a Bosch ZEV. As requested by the author this is published anonymously.
Reassembling the Armature
Although I have every reason to expect this magneto to function for at least a few decades with only minor maintenance (e.g. new points and brushes at ~10,000 miles), it is good practice to avoid doing anything during any restoration that could not be easily undone in the future. Although I could use Loctite on the armature screws without technically violating this, instead I use a torque wrench followed by pinning the heads of the screws using a small punch on the surrounding brass, as was originally done to secure them. This is illustrated in the next two photographs, using as an example the head of a screw in one of my spare Bosch ZEV armatures, with the original pinning that was done by the factory.The torque specifications for fasteners depend not only on the quality of the material used to make them, but also on the shapes of the heads. Because of this there is a fair amount of variation in the values quoted in handbooks for 3.5 mm steel screws, and in the end I decided on 10 in.-lb. as reasonable for these flathead screws of unknown quality steel. This value falls nicely in the range of one of my smaller torque wrenches.

Testing the Reassembled Armature

After reassembling the armature with the new capacitors I checked the output using one of my Merc-o-Tronic 9800 testers. The blue arc in the window of the tester shows the operation of the coil is reliable at a test current of just over 1 Amp supplied to the primary. This is as it should be, so the completed armature passes this test.


As I mentioned in an earlier post, many aftermarket brushes in circulation are too hard, and many are too soft. Since I don’t know the origin of the brushes that already were in this magneto, I made a note during my initial inspection to replace them with NOS Lucas brushes if possible. Luckily, the Lucas brushes (0.193″-0.194″ diameter) fit without problem in place of the 0.185″-dia. brushes that were in it, easily taking care of this issue.

Pursuing this a little further, I subsequently conducted a test to see how soft the brushes were. Pressing down with the pressure I would use to write with a pencil, I made a half-dozen lines side-by-side using a pencil and four brushes, as shown in the next photograph. Since I don’t know how well the subtleties will reproduce in image, following it are my observations:

— No. 2 pencil (approximately the same as an HB pencil): This serves as a convenient hardness standard others could use for comparison with their own measurements.
— “Bosch ZEV”: Aftermarket brushes that were in the ZEV. These lines are perhaps a tiny bit lighter than the pencil, but comparable.
— Lucas 451260 HT pickup brush: Significantly lighter than the aftermarket “ZEV” (note: the lines were so light that I pressed harder when making the middle line, which is why it is darker than the ones on either side of it).
— Lucas 200737 dyno/generator brush: Same as aftermarket “ZEV.”
— Lucas 451260 magneto earthing brush: Same as aftermarket “ZEV.”

The differences might be more easily seen in the following composite micrograph, where from the left the three lines are the No. 2 pencil, the aftermarket “ZEV” brush, and the Lucas HT brush. Although it may not be apparent in the photograph, in a stereomicroscope it even can be seen that the Lucas brush was hard enough to tear small fibers from the paper that are standing above the surface.

Although the above test isn’t quantitative, it definitely shows that the brushes the “professional restorer” had installed were significantly softer than the ones Lucas supplied for this application. Also, although the other two Lucas brushes I tested are about the same hardness as those that came in the ZEV, they are meant to run on metal surfaces, not phenolic. My speculation is the reason many aftermarket HT brushes are soft is they were manufactured using the (incorrect) specifications for brushes intended to be used on copper or brass. Although phenolic is softer than these metals, it is more abrasive (because of this, carbide rather than high speed steel is commonly recommended for machining it). Send questions or comments to classic.vehicle.electrics@gmail.com.

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