To help run in the engine you take out the PL set and put some lapping paste on the piston. Lapping paste is a fine grinding paste. You then move the piston in the liner so they are a very neat fit. The perfect fit is for the piston to be 1.7mm from the TDC position with a medium push.

After lapping the PL set has to be really cleaned out so no paste is left behind. The engine will give massive power from first use but wont last as long