3rd broken cam - same place

I have a 1993 1902.3E which has had a complete motor rebuild. When I first got the car, it had a broken cam. After the motor was rebuilt, it broke the replacement cam. Now, after driving it for about 100 miles, the cam has just broken again. Each time is in the same place - just (above I think) the oil gallery. Both my mechanic and my rebuilder believe it is a bad part. It is a "genuine Mercedes" part. I'm concerned that there is something that is not right causing this to break since it always occurs in the same place. This time the valves were not bent so thank God for small favors. Does anyone have any experience which might help? Thanks to any and all who might reply.

[stevebfl]

The only times I have seen this occur it was because the chain tentioner was installed improperly. It is VERY easy to do improperly.

The tentioner is a ratcheting type and it must be disassembled for installation. If the whole thing is installed it natuarally extends during the installation process to full length. When pulled tight the chain becomes severely overtightened.

The proper method is to pull the tentioner, push the piston out (it will only go longer until it comes out), place the body in the head and shove the piston in from the back side. The spring goes in and shoves the piston til it hits the chain. When the spring is tentioned by the end cap it applies only the spring force against the chain. After the motor runs the oil pressure holds the chain even tighter.

The value to the ratchet is when the oil pressure is gone the piston can only be shoved back to the closest ratchet position (not far) keeping the chain from rattling while oil pressure is building.

I can think of around 300 V8 owners who wished they had such tentioners (bg).

Thanks for the advice very much. I'll pass this along and hope it helps!

[Benzmac]

Go with Steve on this. I have seen it too!

Thanks again so much. I really appreciate this site and the help everyone provides. I'll have a follow-up soon I hope.

Perhaps the cylinder head is out of round? A bent head would do this too...
Greg