Ever wonder what those letters and numbers mean on the side of your Magneto? Believe it or not, they do actually mean something! Here's a little bit of info the next time you happen to see a Mag laying on a swap meet table for sale with numbers & letters stamped on it. Fairbanks/Morse always coded their Mags to the application their Mag was designed for. Let's take the popular FMJ4B3 Magneto found on quite a few Allis-Chalmers machines up to around 1950 or so.

The first two letters "FM" obviously mean "Fairbanks/Morse". That's the easy part. The next letter "J" or "X" is the model "series" of the Mag. The next digit "4" indicates the number of cylinders the  Magneto was designed for. Moving to the next digit "B" means its a flange mount Magneto. (there were also base mount designed Mags) The last digit "3" was Fairbanks' customer code, which in this case was Allis-Chalmers. On some model Mags, you may see another digit at the end after the "3", such as an "A".  (FMJ4B3-A) This means that particular Magneto has a specific modification for a certain AC engine, which again in this case was relocating the grounding lug to the opposite side of the Magneto's housing. Some early AC model Tractors like the WC as an example, had a rod connected to the steering column, which ran down to the Mag to ground the Mag and shut down the engine.