Below is everything you need to do a top end rebuild on your MC21. Please select the correct kit for your bike:
There are different kits depending on the piston supplied and the pistons are sized to match the cylinder. The cylinders have a letter stamped on them. A cylinder = B piston B cylinder = no mark piston, C cylinder = D piston. The largest cylinder bore is A and the smallest is C. However, note that this is only for when cylinders are new or in good condition, so if the cylinder is worn, the corresponding piston will be a looser so a bigger piston might be advised. If the cross hatch honing of the cylinder is very feint then we suggest using the next size up piston. The differences in sizes are tiny (11 microns between largest and smallest) and for the most part not an issue so it is not a problem if a smaller piston is used in a larger cylinder or the other way round.
Cylinder A = 54.008mm – 54.011mm use Piston B = 53.966mm – 53.969mm
Cylinder B = 54.004mm – 54.008mm use Piston # = 53.962mm – 53.965mm
Cylinder C = 54.000mm – 54.004mm use Piston D = 53.958mm – 53.961mm
Piston to bore clearance = 0.039mm - 0.045mm