Although pitch gauges were indispensible for identifying the threads used in this magneto, so were three types of microscopes. Because of the broken screws I had to spend a few hours researching threads before I could repair them, and what I found was that this magneto is filled with bastard threads, some of which are badly made. The interesting thing is that, notwithstanding the BA screw, this seems to be how it came from the factory.

Although the magneto was designed in Germany, it was made in the U.S. and was worked on in the UK so there is a mix of fasteners on it. There are components on the magneto that are definitely of German ancestry, but others that are American. For example, the diameters of both ends of three armatures are 2.000" +/-0.003". I would have to get my hands on a ZEV that I knew was made in Germany to be sure but, presumably, it would be 50.0mm = 1.969". Also, since it was made nearly 100 years ago, I can't assume the fasteners were manufactured perfectly in the first place, so both the diameters and pitches could vary slightly from what they should be.

There are at least 13 different functions performed by screws on this magneto (not 13 different thread forms, though). I've analyzed 7 of them, requiring three microscopes and several precision thread gauges to come to the following conclusions. It wasn't straightforward getting there, and I doubt many, if any, other people know what I'm about to tell you. First, though, a tiny bit of history.

What we now know as "metric" threads is an invention of c1965, with various national metric standards used before then. In some cases the fasteners made to the old standards are a good fit with modern ISO metric, but it some cases they are not. It happens that c1890 the Germans adopted the Loewenherz standard for small screws for use in instruments. This is the same function numbered screws serve for the U.S. (e.g. 6-32) and BA for the British, the latter of which was a standard adopted straight from the Swiss Thury system with one trivial modification.

Anyway, the smallest screw I measured was one that holds the earth/ground lead for the condenser in one of my spare ZEV armatures (an unknown screw was in this hole in my friend's armature). Although it is 2.50 mm in diameter, it has a pitch of exactly 0.45 mm, making it a slightly inaccurate Loewenherz 2.6x0.45 (the next size smaller in the series is an L2.3x0.40, whose pitch makes it an impossible match).

Next up in size are the screws for the HT pickups. I started with these, and they caused me no end of grief before I came to a conclusion. Their diameter is 0.159", whereas 4.0 mm would be trivially smaller at 0.1575". However, if they are L4 they should have a pitch of 0.75 mm. Unfortunately, as I finally concluded, pre-War factory standards must have slowly degraded by the time this magneto was made nearly a decade after Bosch's U.S. assets had been seized, resulting in a bastard version of the L4. The pitch is slightly finer than 0.75 mm (which would be 33.86 tpi), and it is also finer than 34 tpi, but coarser than 36 tpi. I could find no reference in an old edition of Machinery's Handbook to any standard U.S. screw ever being made with 35 tpi. But, since less than 1/4" of thread engages in the housing, even if the holes they go into were made with a perfect L4x0.75 tap, and the pitch of the screws were as far off as far as being 35 tpi, the total mismatch would be only ~1/4 thread, so they would go in with only a little difficulty at the end of their travel. However the fact the factory didn't just adopt the nearest American screw for the armatures indicates that after WWI they instead continued working with ever-more-worn German tooling and using ever-more-out-of-spec replacements.

Next up in size are the armature screws, with diameter 0.1345". Here both 3.5 mm (0.1378") and the old U.S. 9/64" (0.1406") are potential candidates. An L3.5 mm screw has a pitch of 0.6 mm (42.33 tpi), but also should have flat roots and crests each of width 3/8 of the pitch, and a 53-deg. 8 min. included angle (vs. 60-deg. for the American thread). Here, the (in)accuracy of pitch gauges were not up to the task of distinguishing between threads differing by less than 1%. Because these screws were broken and I needed to find a suitable tap to repair the armature, I had to make careful measurements using three types of microscopes. The illumination is from below in the following micrograph taken in a metallurgical microscope (i.e. the screw at the bottom, appearing as black).

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Although the 55-deg. angle of these screws is close to that of the Loewenherz thread form, the roots and crests are not flat as they should be if properly made. To accurately determine the pitch I used a traveling microscope as shown in the next photograph. This instrument uses a precision micrometer thread to translate a 21x microscope over the specimen, allowing lengths up to 10 cm to be measured to a precision of 0.01 mm (0.0004"). This is shown in the next two photographs using a 6-32 screw for scale.

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My measurements determined these screws are 42.0+/-0.1 tpi, not the almost identical 0.6 mm (42.33 tpi). However, the engagement in the armature is only 1/2" so the mismatch in this length would only be 0.16 of a thread even if the factory had used a perfect L3.5 tap. So, like the previous example, my conclusion is that this screw is a poor tolerance imitation of the German Loewenherz 3.5x0.6 thread, as opposed to being an American substitute.

Next, larger in size are the screws holding the covers. These are definitely 7/32"-24, which was an American standard at the time. The nearest Loewenherz screw has a diameter close enough, but the pitch differs by too much (28.22 tpi).

Finally, the four holes in the base that are used to mount it to the motorcycle are American 5/16"-18.

What the above shows is that when this magneto was made in the 1920s it left the factory with a mix of American and metric Loewenherz fasteners of dubious quality. Because of this, I couldn't make any assumptions about any thread without carefully measuring it. Send questions or comments to [email protected]