4. The small center cam found on the main shaft of a points-type distributor is permanently affixed and cannot easily be removed or replaced. It’s generally stamped with a partial part number. Each number contains a unique shape that heavily affects the centrifugal-advance rpm rate. That shape can also affect centrifugal advance by restricting just how far the weights return at rest. There’s no harm swapping a Pontiac main shaft into another Pontiac distributor if the entire assembly is in good shape, so you can try to obtain the most favorable results by swapping in others if you have them available.