Why is there a hole in the piston?

[Jay Auskin]

I'm going through an assessment of my non-running '72 911T, and I thought all was going well. I bought it in non-running condition and was starting to figure out why. I was initially told it was a cam tensioner failure. Well, it has the upgraded pressure fed tensioners with the wider chain idler arm. And upon pulling off the front cam covers, I found...tension. Hmmm, didn't seem to be the problem.

Then, I pulled off the RH valve covers, and found the #4 cylinder rocker arm was snapped on the intake side. Upon looking into the sparkplug hole, I see what looks like a beat up piston, and...a hole in the top!! . I had to stop working on it, but I'm trying to figure out what it can be? Worn valve guide causing the valve to stick open and send itself through the piston??? Why would the rocker arm have broken?

I'm trying to get some hints as to what I should look out for as I complete the teardown to determine root cause, and be sure history doesn't repeat itself. Also, what other parts should I likely find after-the-fact damage on?

Thanks-

[Tyson Schmidt]

Worn valve guide will prevent the valve stem from transferring enough heat away from the valve haed to the cylinder head. This causes heat fatigue and eventual failure.

The valve head will actually break off and get smacked around inside the cylinder, destroying the cylinder head, piston, and sometimes the cylinder, connecting rod, case and crank all in one fell swoop. Hope you got luckier than that. The rocker breaks from the valve being pushed back up in by the piston.

[Wayne 962]

An over-rev will cause this - it will make the rocker break, and then the valve dances on top of the piston during each stroke...

-Wayne

[snowman]

I will vote for Waynes diagnosis.