Why do my ABC engines seem to preform best well after breakin. In fact some of my best engines hardly have any pinch left. In fact you can't really tell till you pull the glo plug off. The pistons still have a shiny ring around the top, so they are not worn out. Fuelman's experiances could be the effect of too much wear during break in and I am sure some engines would be just as he described. But it is propably just as likely that one of them had a sleeve that had a port off a bit, or the top was too thin allowing the ports to sit a bit lower. My experiance is that you can have as much as 500 RPM variences between engines of the same brand, just because of manufacturing tolerances.