It's a moot point for most of your mass produced, name brand engines nowadays.....the break in is non critical because they are not sending run of the mill engines out with overly tight fits any more. It is still a good idea to pull the head and backplate to flush the engines out and look for burrs. You would be amazed at what you will find if you submaerge the engine in solvent in a closed jar, then tape the jar to an orbital sander and leave it running for awhile.