Don't beat me up. Just wanting some information. In doing some research I see that one can purchase a 3 rotor eshaft and a 4 shaft, and it seams that with the 12A if going 3 rortor the thick center plate is not used or required. Here is my question if the journal sizes are the same which seams to be. The the shaft maybe used to build a 3 rotor using 13b parts in the same way a 12A can, can't it?

 

The difference between 12A and 13B is length.

What you are talking about is a "short 20B" which is a thing people used to do. Not sure if they still do. They have cooling issues and unless it is a peripheral port the center rotor gets less air than the outers (which given the cooling issues, may not be a bad thing).

I suspect the only reason they do it that way with 12A parts, besides 12As not being able to rev like a 13B, is that 12A rotor housings have the coolant seals in the wrong place to use a 20B wide intermediate. Although at that point in the fabricobble journey, one may as well narrow 13B rotor housings and simply fail to cut them for coolant seals.

If one wanted to prove a point, anyway. The hardest thing about building anything 12A related is finding good rotors.