bent valves

[rayjr01]

I have a 1972 240z I got the car not knowing what the past of it was. After getting it running, it ran really rough. Upon a compression check, 60 psi in 2 cylinders and 0 in one, and 140 psi in the remaining 3. To find out what was wrong with it, I removed the head and inspected the valves. I found 3 bent exhaust valves. Can the distributor be turned far enough one way or another to cause the valves to hit the tops of the pistons? And how did this occur?

[MikeW]

No, the distributor timing has nothing to do with the valve timing. The valves are opened and closed by the cam which should always been in sync with the crankshaft as it's driven by a chain. The only way for valves to get bent in a previously running engine is for the timing chain to slip which can happen if the tensioner breaks. The other possibility is that the engine was assembled wrong and got damaged the first time it was cranked.

[rayjr01]

ok, thanks for your post. How would I know if it was assembled wrong because I have the motor apart now? It was running before I took the motor apart, but it was running only on 3 cylinders.

[240znz]

If you have pulled it apart, there is no way of telling now.

But as Mike says, there's no way you can bend valves either without the chain jumping teeth on the cam sprocket or it was installed incorrectly. I've never heard of a chain jumping teeth on a L series before.

Have a look at the top of the pistons for marks where the valves hit.

[rayjr01]

There are marks on all 6 pistons, the one that had 0 compression has the biggest mark. I got very vague information from the guy I bought it from 3 months ago, as it was abandon on his farm 14 years ago and he thinks the guy may have done some work to the motor but he isn't sure. I personally think he may have, that's why he abandoned it knowing he wrecked something internally. But what puzzles me if the timing chain jumped, why would it still run?

[Walter Moore]

More likely the work that he did was to replace the timing chain just to see if that would fix the problem.

I have seen engines damaged in the manor you describe with belt driven cams when the belt breaks. I don't know if the timing chain can break as well, or if someone just installed it wrong, cranked it over a couple of times, then corrected the installation problem when it would not start.

[rayjr01]

Walter, I believe exactly what you just stated. Then he realized that he messed something up and then left it for me to buy and have the headache .

[MikeW]

I speak from experience. My timing chain tensioner broke once which caused the chain to slip a few teeth which in turn bent all of the exhaust valves. Generally when we hear of an engine running on only three cylinders it's due to one carb not supplying fuel. It's interesting to me that only three valves are bent and the other three are fine and the three good cylinders themselves hold pressure. If the valves were bent while the engine was running I'd truly expect all 6 to be bent. I wonder if it was assembled wrong, turned over a few times by hand, and then reassembled with proper timing but without fixing the bent valves?

[beandip]

Over revving the engine can cause the valve to ''float'' and hit the pistons. Especially if the valve springs are weak. However I would look to the tensioner as was suggested. Not knowing the background on the car though , check everything.

Gary