Mark O'Neal Mark are you kidding or trying to learn something new???
If you are looking for an answer to your question, basicly EVERYTHING.
Piston rings and walls to polish and make a good seal for compression
rod and main bearings to breakin to new parts and loosen up
heat cycles for expansion and retraction of all parts.
camshaft and roller lifter to breakin patterns on the lobes
Even if the valves are lapped the valves still beat in to the valve seats for better sealing and again better sealing of compression
Motors are living, breathing, things.
Rick L.