pistons as the material the piston is made of expands at different rates than the stock pistons and the aftermarket pistons specify their own specific clearances depending on the material the piston is made of. Piston slap in a healthy stock engine is not very common as far as I know. To have piston slap in a stock engine your cylinder walls would have to have worn side to side in the cylinder, again not very common in stock engines as far as I know. Any time I diagnose knocking I try to isolate it as much as possible. A stetescope helps, put it on the oil pan, front timing cover, etc. see if you can pin down where it is. It sounds like it does go away as the engine warms up, so most likely not an oil pressure issue, but it still would be a good idea to hook up an aftermarket gauge to confirm.
