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 firstname.lastname@example.org.