Why are you pounding on the late "UNCA FRANK"? Neither Frank or You understood your own drawings. I never looked at Franks stuff, saw him monthly at the points races no need to read about what I already seen. The drawing helps people understand or see how the reduced diameter of the commutator would bring the brush in contact with the comm slot sooner, not sure why you "ENGINEER" type guys still don't get it? If it comes in contact sooner it is "ADVANCING" the timing. I "ONLY" have a high school education but do understand this. If com slots are cut every 120 degrees they will always be 120 degrees apart. If they are cut 45 degrees off the center of the stack, they will always be 45 degrees no matter what the com diameter. The timing only changes when the diameter is reduced due to the increasing wrap around of the motor brush, it has nothing to do with the construction of the arm. Yes Frank had a great drawing, but did not understand it, "JUST LIKE YOU".