After having my head gasket replaced and the head redone on my 1995 S320, the engine stopped running abruptly 30 minutes after starting it.

My mechanic turned the cylinders manually a full 360 degrees before cranking it to check for valve clearance; everything checked good. The engine subsequently started, and sounded smooth and steady when idling. He was letting the engine idle to check for leaks, (oil and exhaust) when 30 minutes after starting, the engine died.

He opened the oil cap to look around and found everything appearing to look normal. He decided to take the cam cover off to investigate further. Low and behold, he found the cam shaft, on the intake manifold side, snapped at the sprocket/gear connecting to the timing chain! The whole gear was snapped off the cam shaft, sitting tilted with the timing chain still in the teeth of the gear.

Has anyone ever experienced or heard of this happening? If not, any ideas how this could happen?

My mechanic was able to allocate another Cam shaft and will be installing this weekend. I'm hoping the valves aren't damaged...

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1995 S320
1987 560SEC
1992 300E