Piston Slap

My 96 Chevy Pickup has done it since the first winter I had it (1997). While I think it is maybe a little worse, it is still running fine (130K miles later). For whatever that's worth.

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Many engines do this new. It has to do with larger tolerances. The Mitsubish V6's were notorious for this. Not an issue. Just give it a little extra time to warm before running hard.

-Jeff Deeney-

Remember when the engine is cold it's compressed. That means the piston holes get smaller due to the compression. That means the pistons and other parts might cause a little knocking here and there. Once the engine starts warming up the parts expand and everything is back to normal so it stops knocking. I think this pretty normal for most cars. Nothing to worry about unless there is some really noisy knocking and even after the car warms up.

--Viktor

Wrong.

The pistons shrink MORE than the block ( and therefore the "holes") do, so when the engine is cold the pistons are LOOSER in their bores, causing piston slap.

Severe piston slap (or running an engine with mild slap too hard while its cold) can collapse the piston skirts causing even more slap, or can even crack a skirt off causing a catastrophic failure. But mild slap, as already noted, can be there for the whole life of an engine without causing problems.