Mystery Engine Knock

So some updates here: we took another look at changed our minds about the valves and decided to pull the engine where we found that the insert on cylinder number 3 was badly scored and damaged. Replaced the insert and sanded the crankshaft and put it back together with the same knock. It has still got us stumped. Back to the drawing board.

What I don't get is that the problen does not go away when shorting out cylinders and it is hard to start and keep running (has to have the throttle pulled all the way down along with the spark advanced more than it should).
 
Good morning...have you considered changing the distributor and taking a good look at the gear at the bottom of the distributor shaft? Ernie in Arizona
 
A real stumpper. Since you have taken the engine apart have you tried listening for the knock with the cylinder head removed and using the starter to rotate the engine? You said that the knock is more noticeable at idle so it might show up using the starter to rotate the engine. With the head off the starter should be able to turn the engine faster than with the head on. Plus you can see some of the moving parts.


Just a thought
Bob
 
I have not changed out the distributor but I was considering doing that. I rebuild distributors and I rebuilt this one so it is pretty much as new as the engine.

As for Bob's suggestion, we have previously tried to see if we could hear it hand crabking and using the starting to turn the engine and had no luck. But last night I sat there cranking it with the starter for a while and was hearing a knock and the speed slowing down. That has got me stumped! Might be worth giving your suggestion a try. We have previously ran it with a half valve cover to watch the valves.
 
1. center or front main loose.
2. Flywheel loose.
3. Crank end play.
4. Camshaft end play or loose cam bearing.
5. Why did you have a scored rod bearing?????
 
is the piston hitting the head? The engine may have been fine when it was first rebuilt but a cracked piston could be giving you trouble now. Buy the way, I hope I'm wrong.
 
The head has been off many times along with the pan. Had it on a stand to pull the rod and piston and inspected everything else. Cylinder walls and pistons looked fine. Pistons have been checked to make sure they are hitting the head and they are not. I am running a pretty thick head gasket anyways.



As for the scored bearing, the rebuilder said that it could have been a lack of oil because of a tight clearance or a possible particle of debris or something got inside there. We will never know. He made sure there was plenty of clearance and I made sure that the journal and bearing were clean and had no dust or anything.
 
Chris, I just got here and didn't read from the start, just page 3. It seems to me that just one missing cylinder would not create the start problem that you describe. It wouldn't run well, but it would start and run.

Could you have a model B timing cover? Check #1 piston to see if it is TDC when your pin drops in. I can look in the plug hole and see the top of my #1.

Shooting from the hip here.
 
I read recently where a few guys get their timing gear improperly aligned on installation. But you said the engine had 1000 miles on it. It surely ran and drove okay for that many miles? Or were they put on before this rebuild?
 
Last edited:
Chris, If you still have the pan off, turn the engine over by hand and see if one or more big end rod bearing is moving back and forth on the crank. This would be an indication of a bent rod which would cause an engine knock and is hard to diagnosis.
 
Okay:
I am using a new fiber gear from AER. Last October my timing gear went bad (must have been a bad one from a few years ago). Since then I put a lot of miles on the new one and an inspection of it tells that it is not the problem.


The timing cover is an A cover for sure. I just changed it out after the problem started because the motor mount holes had stripped out. Same problem.


I am starting to lean toward a bent rod as it seems I have covered everything else. Rob is right about the hard starting and running - that is what is the weird part.



The crank ends of the rod can move side to side a bit on the crank but I was told this is normal.
 
The hard starting and knock are probably not related.

The rod moving side to side on the crank is not normal. Have a good machine shop check the rod, or rods, that is/are moving.
 
To be clear, it looks like the crank ends of the rods are not wide enough to fit the whole journal of the crank. They are a good 1/8 inch short. I don't see how it could not knock for a long time if this was the problem - I also don't see how twisting could do this.



I will compare some original rods to a crank tomorrow just to see.
 
You must log in or register to reply here.
Back
Top