<SNIP>

  Seriously, I am wondering if anyone can come up with a description of the
sound of piston slap. I don't think I have ever heard it, and I am wondering
if it really bears any resemblance to pinging. We all know how that sound, I
just would have thought piston slap would be a bit more noisy and apparent.
Exactly what causes piston slap, and what is it 'slapping' on?

John

  The sound is similar to lifter tap, but LOTS MORE. Louder, and more massive. Cyclic, (obviously, since the piston motion is, too) and quite regular. It does not usually change with engine temprature, although heavier oils will muffle it better when cold.

   The sound is made by the skirt of the piston rocking back into the cylinder wall and striking the cylinder wall. The problem is usually caused by excessive piston-to-cylinder clearance due to wear. Some engines need lots of clearance - air-cooled especially - and the noise can be frightening. But those engines are usually worst at start-up, and get better as the engine warms, and tolerances are closed. High-speed engines also need extra clearance, to help avoid excessive drag and binding. Some racing engines I have heard have pistons that sound like they are ready to swap holes!

  The piston should rotate smoothly on the wrist pin, and if it doesn't, the normal tendency of the piston to tip in the bore is accentuated to a noticable, NOISY degree. Instead of being pushed by fluid dynamics (the combustion chamber), the tipping is driven by mechanical means, a stiff wrist pin.