I'm not an engine reconditioner but I thought about .002" for std cast pistons and more (maybe .005") for forged pistons as they expand a bit more when they heat up???

I think we gave my motor .003" on cast pistons as it might run a bit hotter than nana's one on occasion.

Non of the pistons have their original boxes, that's part of the problem.

Who are they made by nissan ?

If there aftermarket the manufacturer should be able to tell you.

Can anyone tell me what the rule of thumb is regarding the clearance of the piston skirt, to the cylinder wall? I've got a few sets of pstons, (Both new and used) and I've got no idea which is which. (Talking oversizes here)
Obviously the slugs can't be size for size with the bore, but what is normal for clearance?
I know for a forced motor, a little extra clearance is good. But I'm only talking standard.