VCCA Home Forums TECHNICAL - GENERAL Mechanical - Engine, Trans, etc Engine knock

On my 31 coupe I have a little knock in the engine that seems to fade as it warms up. It also seems to fade when under a load going up a grade. I have had many car guys give me their two cents worth so I am looking for more. Some said main bearings, piston slap, push rods and lifters. That about covers the entire engine. I listened to the engine while running (not me the car) with a stethoscope and the mains seem pretty quiet. I get more noise at the side cover around the center of the engine. There is almost 400 miles on the rebuild. I haven't pulled the valve cover or the side pan yet. Any other ideas would be great. Thanks

Rod or main bearing noise fads away as the engine warms up and the oil thins out.
Piston noise is worse when engine is cold , gets less as pistons warm up and expand.
If its piston noise (when cold) retarding the timing should make it less (as a test).
At 400 miles it would be a good time to retorque the head bolts and adjust the valves.
Valve noise will be less when first started, get noiser as engine warms up, and quiet down when engine is throughly warmed up. (about 1/2 hour of driving).
When you can hear the noise you could short out each cylinder and see if noise changes.

Could be any number of things as Gene mentioned. Usually, if you short out a cylinder and the knocking goes away it is a rod. If it doesn't go away it is a main bearing. That's why the clearances on the mains and rods are critical. Also, another thing to look at would to be to make sure that a push rod is not hitting the oil line that goes through the center of the block and up to the rocker arms.

I had the same knocking problem after a complete engine rebuild. The engine only ran for about 30 minutes and then the knocking showed up. Turned out to be defective main bearings that were made by Egge Machine. The engine was removed, torn down and completely gone through again, this time using brand new main bearings. No more knocking sounds.

:o :( :cry:

Thanks Gene & Skip, I will give those both a try. Will get back to this post if I figure it out. Thanks again.

I hope that the knock turns out to be something simple. The last thing you want to do with a brand new restoration such as yours is to pull out the engine again!

Good luck!

:) :grin:

Sal as stated earlier it could be several things. The oil feed pipe slightly out of position is a fairly common problem and relatively easy to fix
Does it sound to be full engine speed or half engine speed.
Removing a spark plug lead should give an indication as to which cylinder is the culprit.
As was suggested earlier now is probably a good time to retension head studs and adjust valves which may also get rid of the noise.
Tony

a while back i had a simular noise on a 31 rebuild . iron pistons a piston pin was a little tight making he piston rock . after about fifteen hundred miles it wore in and the noise is gone . the bushings being made of brass has a expansion not the same as the pin and will cause the pin to tighen up . drive it a while and see if it changes hope this helps

Thanks everybody for the suggestions. I will keep you posted of my findings. Thanks

"Valve noise will be less when first started, get noiser as engine warms up, and quiet down when engine is throughly warmed up. (about 1/2 hour of driving)."

I can understand valve noise diminishing as the engine warms up, but what is the reason for valve noise to start out low, then increase and then decrease when the engine is fully warmed up?

Expansion.......When the engine is cold all parts are at the same temperature. As the engine heats up the block expands at a different rate then the valve train. When completely warmed up they are all at the same temperature again. Thats why the engine must be completely warmed up before the valves are adjusted.
If adjusted when not completely warm the valves will be too tight when hot, resulting in a rough idle and in a short time burnt exhaust valves.

Also valve noise wont change when individual plug leads are disconnected.
Tony

Here is an update. I took the side cover off and discovered that there were three push rods slightly rubbing the side cover. All were clear of the oil tube. After taking some measurements I found that the center stud spacer was not far enough out which caused the side cover to be pulled in to far. That is why they were rubbing. I added a couple of washers and reinstalled the cover. Fired it up and that was the end of one noise. I still have the knock. I pulled off the spark plug wires one by one and it seems to be my number one piston rod that is the culprit. I re-torqued the head and checked the valve clearance. So the next thing is to put it on the lift and pull the pan. Thanks everyone for all the help.