Not 100% sure to be honest.. vito sells a 370 and a 397 kit which uses a 66mm piston... im using wesico 66mm #pk152 on thier site states would make it a 370cc.. I also had to use a long rod setup.. as far as I knew even when you use blaster pistons which are 66mm it makes it a 370cc.. im going based on what the shop that built my motor told me as well.. jeff at F.A.S.T did the porting based on my setup as well??? Please correct if im wrong ive had this setup about 2 years now