[HardcoreZ28]

Piston slap is when the piston doesn't fit tightly into the bore and when the engine is cold and the rings aren't expanded yet it "bounces" around off the bore walls. As it heats up and the rings expand it quiets down as the rings usually move towards the walls. This is very common with engines that have forged pistons as I believe they have a greater thermal expansion which means the cold clearances are greater. I heard that there was a problem similar to this on one of the GM truck engines in the NBS trucks...might have been the 5.3