saraholt61

I was driving my 396 the other day doing around 1500 rpm and suddenly I heard a loud knocking noise from the engine so I stopped and got the car towed home.After removing both valve covers I noticed the exhaust valve rocker on cylinder #1 on the driver side was out of position-barely making contact with the spring,the pushrod guide was broken and the pushrod was bent!!! I removed the pushrod,rocker arm and guide and started the engine and the loud banging continued. Tomorrow I will do a compression check on the number 1 cylinder and remove the spring to see if the valve moves freely.I am wandering what in the world would cause a pushrod to bend and at the same time cause a tremendous knocking in the motor.Can I be experiencing a hydro-locking situation where the cylinder is filled with coolant from a broken head gasket or gasoline accumulating from a bad plug or wire? Or can a stuck or seized valve cause this except that a 396 is not an interference engine.A friend of mine suggested that the exhaust valve got stuck or seized in the closed position (the valve was definately in the closed position when I removed the valve cover) and the pushrod bent trying to push the valve downwards. Then the compression from the burnt mixture acted also like a hydro-lock and created a knocking when the piston tried to go upwards. I am totally bewildered and I guess the next thing to do is remove the cylinder head to check the valve and piston.If anybody has any ideas of what might have happened pls. let me know.

robert miller

Look at pulling the plug and then do what you say you are going too do. Check oil and coolant levels.

66jack

What about a siezed lifter??

jack

xiaoman

when pulling the plug check if there is fluid in the cylinder...water or oil or none etc.
water = cylider head gasket or crack in block or head allowing coolant to enter cylinder, cold also be leaking at the headbolts if no sealant was used
oil = crack in the hard stuff above
none= bad lifter
my guess/hope is a lifter as previously stated
if lifter was the problem check the valve train on the valve to see if are any pulled rocker studs or cracks at the stud aera as well as the valve itself (bent) etc..
think good thoughts maybe cost for the manifold gaskets and a lifter and pushrod.
Good luck Xiaoman

Kerrmudgeon

Any chance someone dropped a nut or something down the carb? It may have made it's way to the combustion chamber, stranger things have happened.

robert miller

Here also it has happen in the past. or even the nut off the air cleaner down in the carb.

66since71

Had the same thing happen on my brother's 64 small block. In that case we found a valve got bent, stuck and bent the pushrod. The knock you hear is probably the piston tapping the now stuck and partially open valve. Root cause in our case was an overly aggressive cam for the valve springs we were running.

Harry

robert miller

A friend did the same thing with his 34 chev. But parts came flying out of the side of the motor.

saraholt61

The lifter slides in and out perfectly-it is not seized

saraholt61

The latest theory is that a rod bearing failed and the piston went up too far and made contact with the valve bending the pushrod and the noise I hear is the loose rod.Best known builder in our area suggested this theory and told me motor must definately come out for a complete rebuid.Tomorrow I start taking off the head to see if there was any piston or valve damage.

66jack

That bites...the driving season is just now starting....good luck.

jack

Mark_Milner

This sort of thing used to happen with big blocks back in the day. Check that the valve still moves by using a lever type valve spring compressor and if it doesn't stick, then put a new pushrod in and run it.

I don't know what the cause was, as they seemed to run ok after you replaced the bend pushrod until you bent another later on.