- The piston is 87mm, and that is 1mm bigger when compared to the normal 86mm, and the material is light-weight and robust A4032 solid aluminum alloy. The required clearance of 0.The RB25 crank collar problem is actually quite common on various RB motors, including the RB20DET and pre-1993 RB26DETT. The issue With all the crank collar is because th