I've delt with this on three sets of ROSS pistons.
They reccomend a 6k clearance to the bore so the piston will never expand and stick in the bore. (So your bore is 6k larger than the maximum diameter of the piston.)
I've found on my coated pistons, that 4k reduces the slap, and I do not think the pistons will expand as much due to the combination of coatings, and our oil squirters cooling the undersides of the pistons. Keep in mind, I've never run "bare" pistons, but only ones with thermal barrier on the crown, molydisulfied on the skirts and thermal dispersant on the undersides. The coatings in theory, and reality, limit the amount of heat the piston can absorb, and improve the amount of heat they can pass onto the oil sprayed onto the underside of them. This keeps them cooler and they do not expand as much, so they slap like a MoFo' when you put them in at 6k clearance like ROSS reccomends.
At 4k, they slap some on start up when cold, minimally on start up when hot, and it goes away as the engine warms up completely. Extreme cold weather start up's are loud. It sounds like the engine is broken... But improves when warmed up...(I'm talking starting the car after it's been sitting in a parking garage in 10f weather for a few days. Or even in sub zero weather.)
Do not rev your motor to heat it up. Just start the engine, and let it idle untill the slap decreases, or run at low engine speeds untill it warms up. (Like 3k or less IMHO.) This should help to limit any skirt wear, and keep them from cracking like IJ talked about.
The slap sound is from the pistons rocking in the bores, and rattling around in the bore when they are cold. The flip side is when the forged material heats up, it fits perfectly in the bore, not to loose, not to tight, and they are much stronger than stock pistons.
If you are going to run high boost, and go for max power, you need forged pistons. Cast ones, even coated, will not hold up to serious abuse/use, and a piston failure is death for your engine. Especially if a rod becomes loose.. it will poke holes in your block, and break stuff you did not think was possible.
I've been looking at various pistons, and ROSS is about as strong as they get. Super thick skirt walls, and crown. Plus the material around the pin is beefy.
JE's are just about as beefy.
Wiesco has longer skirts, and thinner areas, but they have been used in some very high power engines.
The Probe pistons look interesting. Look lighter than ROSS< and have longer skirts in the photos I've seen. This might help to limit slap. (The longer skirt) They are also lighter weight, and that would help to spin up the motor faster, but might limit ultimate power the piston can handle.
The perfect piston for our aplication would be beefy around the pin, short skirts on the pin sides, and longer than ROSS, but beefy skirts opposite the pins to limit piston rocking when cold. I'd gladly give up a few mm of skirt material around the sides of the pin bores to add mm's to the skirts where the pistons rock/slap when cold.
Hope this helps.
