I started cutting my own helical gears for my small engines a number of years ago. 2 gears of the same diametral pitch and 45 degree helix angle produce 2 gears of the same outside diameter. I realized when making gears for my Ford 302 V8 engine that the cam gear and distributor gear are different diameters, like the model A engine that I'm working on,but rotate at the same speed, 1:1. This is accomplished by making the mating gears with different helix angles but the same tooth count. Cam 60 degrees, distributor 30 degrees. As long as the total angle is 90 degrees. I use a spread sheet provided by The Engineers Edge for my calculations. To get close to the required numbers for the model A I'm up to 14-16 Diametral Pitch which I'm guessing is much finer than what they probably are. Anyone have any numbers for the D.P. or helix angle?