[Justin Mace (Justin)]

What are the causes of piston slap? Is it serious? What is the cure?

[Dave-R (Roppa440)]

Piston slap (as I understand it) is a noise caused by the piston being a loose fit in the cylinder bore. The loose fit allows the piston to 'rock' in the bore as it is pushed up or forced down.
It is usually only heard when the engine is cold. If you can hear it when the engine is hot it would be so bad as to be burning a lot of oil and not making much compression.

It sounds a bit like a mild tappet noise.

It is considered undesirable but as long as it goes away when the engine warms up (and as long as the piston rings remain good) it is not considered a major problem. Forged pistons quite often have a bit of this as they are built with a generous amount of cold clearence.

The cure is a re-bore done by a GOOD machine shop and correct fitting pistons. You might not be able to loose it with forged pistons when the engine is cold though.