I have not gotten this motor apart so I can’t tell you the clearances yet. And yes I’ve heard about how careful you have to be with clearances on these pistons. End gaps, skirt clearance, etc. I don’t use cheap pistons. My shop won’t either. I’ve know half dozen of my friends who have tried them and they have failed in some for or another. And you are correct, some shops can’t get anything to work right. This is the fifth motor in a row that I have built that some other shop has done something wrong and the engine has failed. And they are coming from all over. I got a 68 440 in a trade deal the other day. It’s still together. I pulled the valve covers and noticed that the oiling holes on the rockers was install UP! What a shame. My shop was started by an old Super Stock racer who built race motors to keep his race cars going and ended up a really good business. To answer your question, though. The piston broke where the valve relief is cut at the rings. It is cut way to deep, in my opinion, for a hyperutectic piston. They are not meant to be as strong as forged. One good thing. It broke apart in a million tiny pieces instead of staying one big piece and punching a hole in the head. I’ll post a picture from my phone.

And before someone says detonation problems, none of the other pistons showed any signs of that. I thought that at first until I realized that the debris had bounced around through the intake plenum and blasted the pistons that on the same plane of the manifold.