From memory, the flywheel is a lightweight bespoke steel affair with a Mazda ring gear fitted. It is all steel no cast iron.
I have confirmed that the stub shaft on the M3W has the same bolt pattern and hub diameter to allow a Miata flywheel to bolt up to it. I have an old Harley comp and used that stub shaft to compare to drawings of the Miata flywheel.
The only unknown is the stack height of the Miata versus the custom M3W system. Not sure if the Volvo hydraulic throw out bearing is an exact replacement dimension wise as the Miata fork and separate bearing system as in the old school days. I understand that "some" of the very early M3W's used the slave cylinder and external fork/bearing system, is that true?
Since the Centa stub shaft is an extension and represents the Miata crank hub, one could guess the Miata would work. I've found Miata racing flywheels in the 8# to 9# range, which would put it close the custom M3W flywheel weight. The Miata uses a larger diameter clutch disc and there are tons of Stage 1 and Stage 2 setups that still have a standard looking disc with the center spring pack. Unfortunately I cannot confirm any of this yet physically, but working on it as we type...
