[gtofreek]

I'll go out on a limb by saying with 99.9% confidence that it is not piston slap.

1. It does not sound like piston slap.
2. I setup the pistons with the minimum .0045" clearance that they call for, and I use a torque plate with the same type gasket that I used to assemble with. I've setup a lot of engines with these pistons at .0045" and never had a noise from them. Plus I've honed thousands of blocks over the last 26 years so I'm pretty good at it. I hone to the tenth of a thousandth of an inch so I can say with confidence it had .0045" clearance.

My best guess at this point is possibly a piston pin trying to gall in the pistons bore, but I set those up at .0008" clearance, so they shouldn't be trying to gall. Or possibly a piston skirt collapsed for some reason.

In any case, I will be going to work tomorrow and tearing it down, so I should have an answer for you all tomorrow. My biggest fear is that I won't find anything obvious, so I think it's going to take a very close examination of everything.