lost a piece of piston...Stumped?

The clearance between the top of the piston to the cylinder wall is meaningless. It has zero to do with piston stability. Not one thing.

Piston stability comes from the piston skirt. The skirt is where piston to cylinder wall clearance is measured.

Although it looks like a lot of room between the piston top and cylinder wall, and may be, the only correct place to measure clearance is between the skirt and bore and nowhere else.

Hypereutectic pistons (specifically Keith Black) have smaller piston heads to allow for greater expansion. The reason for this is they have a higher percentage of silicon alloy. This is why ring gap is so critical and why they can also appear to have too much clearance at the top.

The Speed Pro Hypereutectics are different in that they do not require special ring gaps, because they do not have the high percentage of silicon alloy as the KB. So not all Hypereutectic pistons are created equally.

You did not specify which pistons you have, or if you did, I missed it.

My guess as to the cause of the damage is incorrect (too tight in this case) ring gap. Hypereutectic pistons have such a great rate of expansion at the top, that they will butt the ring ends together and cause the damage shown unless the gaps are within the specified specification.

Too many people blame hypereutectic pistons and call them "junk" when it is incorrect ring gaps that usually cause the problem.

If that engine was mine, It would come all the way back down and every single clearance double and triple checked. I think whoever set the ring gaps up, did it wrong. Simple as that.