The stroke of the new crank removed the clearance between the rod and the edge of the cylinder wall. Once the rod tapped the wall, the metal gave causing a crack to run up the wall and cocked the piston. The rest is history.
Or, the builder failed to grind the releaf correctly causing a thermal weak spot that allowed the wqll to crack when the engine heated up.
Or, when the new lower end was installed, there was excessive passage clearance which reduced the amount of
oil to the top end causing the valve to gall and stick to which the piston struck it on the up swing and the rest is history.
Or, when the engine was reassembled, improper Valve guide clearances were allowed to be passed to the heads allowing the valve to stick and strike the head.
Then there are the several lists of part failures that could have happened.
Take it back to the builder and ask him to fix it.
Sorry to hear of your missfortune. Hope this helps.
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