Can any help, Silvertop and Blacktop have different conrods but silvertop are chunkier at the top where you need to machine it out to 21mm for the bigger Calibra turbo gudeon pin. A guy at an engineering place says the blacktop one is too thin and may crack once machined out. Any one ever done a ZT 2.1 Blacktop if so how did you go about it? I would prefer a blacktop.