Knocking Sound Comes, Then Goes

Bud Man

Bud Man

Member
Odd one for the group. When I start my engine after the car has been idle for a bit, I get a dull, deep knocking/thumping sound from the engine compartment. Pretty sure it's the engine itself, but oddly, the knocking always completely disappears after the engine has run maybe 5 - 6 seconds, and doesn't reappear until it again has set a while. Advance stays retarded the whole time. What would cause the noise to disappear? Oil circulation would seem the logical reason for the end of the knocking, but would it subside that quickly?
 
Bud Man, Similar thing happened to me ~ 2 months ago. First start after sitting about a week or so and a deep knock then goes away.

And of course it does it on the morning of the Annual OC Pancake Breakfast, can't miss that! When we got home I drained the oil and got some chunks... The rear main finally decided to 'go away'.

I'm thinking with the bearing compromised, the oil drains out of the last reservoir in the valve chamber when the car sits for a few days. So the rear main (in my case) starts up 'dry'. As soon as it sees oil it quiets down.

I suggest you drop the pan and have a "look see".
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Same issue on an engine Im building for a friend,Thumps for the first few minutes then quiets right down..rods and mains good,cam thrust spring good..spring missing from oil pump,found the oil pump 'set screwed' at the plug.Best I can figure the cam endplayed hard when the pump was dry,something the spring was designed to absorb..
 
Tom nailed it, mine had a knock at startup then would go away after a few seconds took the pan off found the center main lose removed .003 of shim all is quiet now.
John
 
Try easy fix first.

Change oil, add four quarts of 20w-50 and one blue bottle of STP.

Also put MMO in gas.

If still making the same noise in a few hundred miles. Than you have to think that it could be something starting to go wrong.

Model A’s can make any number of noises at one time or another. Just like any 90 year old guy.

I know there are people that are not going to agree with what I have mentioned above, but it’s only a $20.00 outlay.
 
Bud Man said:
Odd one for the group. When I start my engine after the car has been idle for a bit, I get a dull, deep knocking/thumping sound from the engine compartment. Pretty sure it's the engine itself, but oddly, the knocking always completely disappears after the engine has run maybe 5 - 6 seconds, and doesn't reappear until it again has set a while. Advance stays retarded the whole time. What would cause the noise to disappear? Oil circulation would seem the logical reason for the end of the knocking, but would it subside that quickly?
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Had the same thing once on a 302 Ford, start it up after sitting o'night and it would knock for 1 or 2 seconds. Replaced all rod bearings with .001" undersize rod bearings and knock went away.
Disremember exactly but that engine probably had 60 or 80K miles on it at that time, about 40 years ago.
 
Many thanks to everyone for their inputs on this. WHN, I did exactly what you suggested about two weeks ago, new 20-50 and STP, but it's had no effect, so didn't get lucky on that one. Dropped the pan this afternoon, and no traces of metal debris in the oil, and nothing visually apparent, as well. Will address the center main possibility tomorrow and see what I find. Are we talking about removing the center main cap and peeling a thickness of shim away, then re-torquing the cap to 70-80 ft lbs? Thanks again!

Bud
 
You should find out what clearance you have now on the center main then remove the proper amount of shims to get it back into specs. Some say to use plastigage and others not so much for using that, I have heard of adding shim stock between the journal and the main cap to see how much it takes to lock the shaft, then remove the appropriate number of shims to reach specks. Some cut their own shim stock from an aluminum drink can. Use micrometer to measure thickness and go from there. In younger days I removed one layer of shim stock from each side and since I could still turn the engine with the crank I called it done. Sold the car so I don't know the precise outcome of that. It worked fine while I had it.
 
Pull OFF the # 2 & # 3 plug straps & start it. If the noise is GONE, it's the center MAIN! Take off the upper bell housing inspection cover, with a socket & extension on a pressure plate bolt, see if the flywheel will rock back & forth about 1/8" or so. If it does, the FLYWHEEL BOLTS ARE LOOSE!
Bill W.
 
Well, I dropped the pan and it was an identical issue to what Y-Blockhead described. Rear bearing has let go, unfortunately. :mad:The engine is tired anyway, so a planned rebuild may now be coming a bit sooner.

Thanks again to everyone who responded!
 
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