New ZZ4 crate engine has a one-piece rear main seal. The engines with this seal have a different flywheel bolt pattern then the old 2 piece seal engines so my old flywheel will not bolt to the new engine. I have a part number for the flywheel I think I need to buy but wondered if anyone else might have done this swap already and had it figured out. I bought a centerforce clutch based on my expectation that I was going to use my old flywheel. I talked to centerforce and they said the pressure plate bolt pattern is the same for 11" and 12" clutch, but then I learned there is no such thing as a 12" clutch according to GM. Does anyone know whether the 11.85" clutch is the same thing and people just round the number up or something. This the largest clutch disc size I can find in the GM performance parts manual. Also, does anyone know where I can find specs on GM part numbers for instance, the thickness of the flywheel if I have the flywheel part number?!?!?!? thanks for any info you might have.