Running too hot a plug can very easily damage an engine. The hotter heat range plug will have a center electrode that runs at a higher temperature than a colder heat range plug. When that temperature gets too high it starts to cause preignition in the engine as the hot electrode ignites the fuel before the spark occurs. This has the same affect as advancing the ignition timing. If you look at the original post this is what was causing the problem with his engine with the D21 plugs. The engine could not recover the RPMs even when the load was removed because in effect the timing was way too early. Ideally ignition should occur only far enough before TDC that the burn is complete by 10° - 20° ATDC. Preignition may cause the entire fuel load to be burned before the piston reaches TDC.
This condition can cause holes to be burned through the center of the piston is some engines. In the IH engines the piston tops are thick enough, though sometimes you will see damage in the center of the piston, that the result is more likely to be scoring as the pistons overheat and expand from the early ignition. You will sometimes also find broken rings and ring lands although that condition is more apt to be caused by detonation.
Detonation is the result of combustion starting at two or more different locations in the combustion chamber. It usually is the result of too low of an octane fuel for the engine as that will allow spontaneous combustion to start elsewhere in the cylinder after the spark has begun ignition. This may also be caused by preignition because the early ignition causes increased pressure in the combustion chamber which increases the possibility of spontaneous combustion. It may also be caused by glowing carbon deposits in the cylinder. The ping you may hear is the two flame fronts colliding.
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