yup....been there-done that.
sorry for your luck.i have a similar souvenir from my first rebuild.it ran for 17 minutes and cost me $2200.00 to fix. that equates to $129.41 per minute of fun.
p.s.- the piston isn't completely useless now....it can be a great conversation starter,or a new paperweight at the office
If I was guessing, and it just be a guess, the valve fell out of the retainer, or it was misadjusted very badly and the piston hit it and caused the stem to break, possibly where it protuded from the guide?
I would have guessed a timming issue but I would have thought all of the other cyls would have had a similar occurance, and the engine would not have cranked.
It may have failed with just the starter cranking it?
It kind makes the hyper piston look good as the piston was apparently ductile enough to "absorb" the valve head and shatter?