I've heard from several people that it is bad for your motor to run a lighter than stock flywheel. They said that all motors come balanced from the factory and running a lighter flywheel will throw the bottom end off balance and you can ruin bearings a lot faster.

Does anyone know about this?

 

sure, if the flywheel isn't balanced right. But if it is balanced correctly, it will wear no more than the stock flywheel.