If you are running stock cylinders with no spacer plate, you will need cut domes for the 4mil crank. (Best way to go IMO) If you run a spacer plate you can run off-the-shelf domes. Cub and Serval cylinders CAN use the cool head, but if you step up to a cheetah other other cylinder you won't be able to use the cool head you already have. The smaller the cc number on the dome, the higher compression you will have. Big-bore domes are used on 68mm and larger bore cylinders (cheetah cub, serval, bored out stock cylinders) If you are running stock cylinders up to .100 over you will be fine with the std small-bore domes.
Give kevin @ HJR a call, he can hook you up with custom cut domes for your 4mil if you go that route.
- Jared