I have heard this term a lot on the forum, and realise its a BAD THING! But what exactly is it? what cuases it? and how do you know if youve got it??

Thanks.

the cause of "Piston slap" is a slightly under size diameter of piston. So as the piston goes up and down the bore, instead of going up, virtualy straight, it is pushed to one side, creating a "slap" or light "knock" noise.

This scenario is normally heard when the engine is very cold,so once started up, the slightest blip of the throttle and the noise starts. On extreme cases, it can also be heard when driving, when going to overtake or climb a hil and the engine placed under load.

To identify if slap or not, simply disconnect the plug lead to number 4 piston prior top starting up, if no noise, its piston. If the noise persists, then it will be much deeper in the engine, possibly a main bearing or crankshaft bearing.

"Piston slap" itself, can be "lived with", it may get a little louder but should rarely fail totally, it just depends on how loud you can suffer the stereo !

why to piston number 4?

i thought it could be any piston

It can, I think Scooby Slut was giving an example.........

OK, thanks for that, but why should a piston be undersized in the first place? Normal wear and tear? in which case surely it should only occur at very high mileages?

Piston slap is very common on Cosworths - Scooby & Evo's rarely suffer from it (To my knowledge)

Piston slap was(is) a common problem on MY97/98 Impreza's, usually no 3 piston would develop a slap after about 30K miles.

Initial IM instucted dealers to fit a new short block, but then a kit was developed using a slightly oversized no.3 piston.

It is generally accepted that this is a design fault and it is rumoured that IM have fixed some cars out of warranty.

Andrew...

Yes, number 3 was the more popular cylinder to produce this fault, as suggested, I was only using a number for "an example".
The original machining at manufacture was deemed at fault, but dependent on driving styles, the faults appeared at various mileages on these engines, so from as little as 3500 miles to 59000
miles, the problem can occur.