we have a box of broken rollers, they really take a beating. Why a part fails has always been tough to solve unless the obvious, lash set to tight or loose, poor quality part, lack of lubrication.
I try and inspect the valve train on our roller engine as often as possible and it's still easy to miss a part that's going to fail.
I try to put these engines in perspective, I've read the engine fires a spark 50 times every second @ 7000 rpm's
that means every part in the valve train is reaching it's minimum and maximum limits thousands of times during a pass at the drag strip.
I noticad when I pulled it from the block it fit tight only when the roller section was in the bore. Upon closer inspection for gaulding I almost didn't see the cracks. Cam is custom grind .555 lift 246 in 252 exhaust @ 0.050 so it not a very aggressive cam. Is this just a bad lifter that got by in the man. process? 
I am glad I found it when I did.
