Piston slap is an issue that's occurring in many GM motors built between 1999 and 2002. It's a loud engine noise produced by pistons as they travel up and down the cylinder bores.

What causes it:
Piston slap is caused by improper fit of the piston in the cylinder bore. Typically the piston is too small or the bore is too large, making the piston wobble back and forth in the cylinder. When the engine is running this slap can become very loud and annoying. It makes a regular car engine sound very much like a diesel engine.
The noise that piston slap produces usually clears up as the engine heats up. The pistons expand and fit in the bores more snugly.

The results of piston slap usually lead to increased wear of the piston, cylinder wall, increased oil consumption, failure to pass exhaust emissions, and eventually Catastrophic engine failure.

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