[Fot] tr 3 cam sprocket bolts sheared

[KENMUN at aol.com]

fot, at a recent race at sebring the old tr 3 went "bang" and stopped  
running. external signs were that the engine turned over like there was no  
compression, the dist. didn't turn and the push rods weren't going up/down.  
inside the cam sprocket bolts were sheared and the chain cover center bolt was  
sheared. the crank turns freely, the cam turns freely and the valves go 
up/down  with no extra effort. the bolts were the stock shoulder bolts, the 
locking tab  was in place with the ears bent up in lock position.any 
ideas/suggestions as to  why the bolts broke? thanks, ken m. tr 3 vintage  racer