[sick472]

I've had a similar problem with the hyperutectic pistons and poor machine work. Those pistons, iirc, require a tight tolerance for the bore sizes and if they are not just right...its "so long pistons". My block was overbored in two of the cylinders and could not be bored any further to jump to the next size pistons. I had to find another block.

Hypers don't swell and contract as much a forged pistons do and like a tighter bore. If all is right, they don't knock on a cold start up like forged, but if the bore is too large...they don't swell up enough to fill the bore and slap themselves to death.

Scrutinize your bores before the rebuild.