So I just realized I have a lightweight flywheel from an fd 13b. Can I throw that into my rx8? I read that they flywheel is interchangeable for the most part, but the counter weight is of concern. Should be able to just take off the stock counterweight and swap the flywheels correct?

Or for some odd reason will the assembly need balancing?

 

Lightweight flywheels require the use of the engine's corresponding automatic rear counterweight. You'd need a Renesis automatic counterweight, but I cannot speak to the compatibility of the FD flywheel. I'm pretty sure the international 5spd for the RX-8 is the same transmission as was used in the FD, though they could call for a unique clutch/flywheel from even the 6spd cars.