For it to shatter rather than bend and snap (too much power, piston seizing) surely it must have been 'hit' so to speak? I think snapped bolts is a good shout and then the crank journal effectively 'hit' the bottom of the rod sending a nice shock wave through it, or bolts snapped and combustion pressure forced it down and it hit the crank?

Perry, this is an old engine so if anything the piston to bore clearance would be way bigger than it should be, would had to of expanded a lot!