Lapping the cam to the crank

When you set up the timing you also need to check if the cam is gripping the crankshaft with enough force so it will not slip.  Normally the tapers will sufficiently match, however, if the cam was previously left loose and it spun, you would need to lap the cam to the crank so it breaks up the polished finishes to the cam and crank nose.

Also, if you are replacing the cam, crank or you are retrofit installing a CDI, you would want to lap the cam (or flywheel for CDI) to the crank so it is an exact match.