[Aussie Mike]

I spoke with Darren Difillippo when I decided to build mine and he recommended 1 7/8" based on what I planned to do with the motor. Back then most people on the forums were recommending 1 5/8" or 1 3/4" for stock engines. In recent years as the tuners have got the feel of these motors the primary and collector sizes have been gradually increasing. 1 7/8" is pretty popular now and I think Darren saw that coming with the requests he was starting to get from tuners.

Cheers