When it comes to determining how much piston damage is acceptable, a general rule of thumb is that any damage beyond 0.020” requires replacement or repair. Piston damage can be caused by a variety of factors, such as improper break-in procedures, incorrect valve lash settings, over-revving the engine and detonation due to poor fuel quality or mixture issues. It's important to note that even slight amounts of piston damage can cause significant performance problems and even catastrophic engine failure if not addressed quickly and properly. For example; small scratches on the crown surface may lead to increased blowby levels which will result in decreased compression and power output while larger gouges can cause oil leakage into the combustion chamber resulting in pre-ignition (detonation). Additionally, cracks around pin bore bosses could lead to complete ring land failure which would require complete engine rebuild or replacement. Therefore it's extremely important for mechanics/owners alike to thoroughly inspect pistons before installation and replace them if necessary so as to avoid potential future costly repairs down the line.