[Stuckinda60s]

One thing that happened was that they didn't give enough ring gap for the KB pistons. HWYSTR455 had a good point. What happens with the hyper eutectic pistons is that they transfer more heat to the top rings and therefore need more gap. It's critical that they have it, otherwise the rings will butt when they expand and dig into the cylinder walls. This causes the piston top to break off like yours did. Check the ring gap on your remaining pistons and post what it is. If I remember right, it should be about .028 for a 389. Seems to me you may have a legitimate beef with the builder. The nut being shiny probably means the nut was loose and moving against the cap. If you don't know how to check the gap, let us know.

Rich