- The piston is 87mm, which happens to be 1mm greater when compared to the conventional 86mm, and the fabric is lightweight and strong A4032 forged aluminum alloy. The required clearance of 0.The RB25 crank collar challenge is really very common on many RB motors, such as the RB20DET and pre-1993 RB26DETT. The trouble Together with the crank collar