what is the difference in bolt patter for the transmission points.. i am thinking about using a 12a carb 1st then swithcing to 13b fi after i get my car sorted out for the radiator and such.

914 rotary progect underway.

 

no difference in that pattern, the tranny will bolt to either one.

 

Assuming it's a '74-up 12A, of course. The '74-85 13Bs are just stretched '74-85 12As, just as the '73-earlier 12As are just stretched 10As.

The key is "stretched". Each displacement increase was achieved by making the rotors 10mm wider, so a 13B is 20mm longer than a 12A. (and a 12A is 20mm longer than a 10A, for all that matters) So replacing a 12A with a 13B means you have to rejuggle the motor mounts.

 

I put a 12B in a R100 and remounted the radiator and drilled offset holes in the motor and trany mounts to allow for the length difference. Worked great.