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Old 11-28-2019, 05:36 PM   #1
old31
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Default Knocking at start up?

What would cause a knock at startup? The knock is only for about 20-30 seconds, then it stops.

After startup, it does not matter where the spark lever is positioned.
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Old 11-28-2019, 05:58 PM   #2
George Miller
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Default Re: Knocking at start up?

center main is one but it depends on the sound.
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Old 11-28-2019, 06:25 PM   #3
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Default Re: Knocking at start up?

Maybe a sticking valve. Might try Marvel Mystery Oil first.
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Old 11-28-2019, 06:25 PM   #4
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Default Re: Knocking at start up?

Piston slap, until oil gets up into cylinder, and pistons warm up. If it is more noisy when cold than warm, this may be it.
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Old 11-28-2019, 06:31 PM   #5
frank mcdaniels
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Default Re: Knocking at start up?

Make a stethescope with a long rod of some kind and put it on the engine while itís running. You will be amazed at what you hear going on in there. My roadster knocks to until it warms up. Iím sure itís a main bearing Iíll have to address it sometime in the near future . Frank
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Old 11-28-2019, 07:59 PM   #6
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Default Re: Knocking at start up?

If the main bearings are a little loose, all the oil in the valve chamber will leak out thru the mains when the car sets for a period of time. When you first start it, it takes a few seconds to pump oil back up into the valve chamber and gravity feed on down to the main bearings. This will result in a knocking sound for a few seconds.


This usually only happens with babbet bearings and not insert bearings.


This usually happens after the car sets overnight or longer and not after being shut off for just an hour or so.


10/40 or 20/50 oil may help.


I ran an engine for several years like this and it did not get any worse. I was too lazy to drop the pan and take up the bearings. Just run the engine slow and do not put any load on the engine until the oil gets pumped up.


Chris W.

Last edited by CWPASADENA; 11-28-2019 at 08:01 PM. Reason: CLARIFICATION
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Old 11-28-2019, 08:28 PM   #7
Patrick L.
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Default Re: Knocking at start up?

I too would think its probably the center main. But a stethoscope could help pinpoint where the noise is coming from.

The lever should be up for starting and upon start up it should be pulled 1/2-3/4 way down.
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Old 11-28-2019, 09:33 PM   #8
J Franklin
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Default Re: Knocking at start up?

Too much slop in the thrust bearing caused just that in an engine I had. Test this theory by not pressing the clutch before starting up the car next time.
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Old 11-28-2019, 11:35 PM   #9
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Default Re: Knocking at start up?

I had something similar, only when cold, but gradually got worse over a couple of thousand miles. Ended up being a crack rod cap, Because I waited and then had the motor rebuilt, teh rebuilder had to bore the cylinders from .020 to .080 over due to wear issues caused by the cracked rod cap. So my advice is do not wait, find out what the problem is asap, am now a big believer in if there is an issue identify it sooner than later.
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Old 11-29-2019, 04:54 AM   #10
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Default Re: Knocking at start up?

Quote:
Originally Posted by Mulletwagon View Post
Maybe a sticking valve. Might try Marvel Mystery Oil first.
I always use MMO at every fill up.
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Old 11-29-2019, 04:57 AM   #11
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Default Re: Knocking at start up?

Quote:
Originally Posted by J Franklin View Post
Too much slop in the thrust bearing caused just that in an engine I had. Test this theory by not pressing the clutch before starting up the car next time.
OK, I will try this and report back.

Last edited by old31; 12-04-2019 at 09:09 AM.
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Old 11-29-2019, 09:44 AM   #12
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Default Re: Knocking at start up?

I’d say pull the oil pan and look it over. For a quick test, buy a bottle of Slick 50 and put it in. I imagine I’ve just woke up the scoffers with that comment but if you have a worn engine it DEFINITELY will help. A Teflon film on the bearings will keep the squirrel rattlers quiet.
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Old 11-30-2019, 11:03 AM   #13
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Default Re: Knocking at start up?

Quote:
Originally Posted by Mark in MT View Post
Piston slap, until oil gets up into cylinder, and pistons warm up. If it is more noisy when cold than warm, this may be it.
I had an engine like that also that had the piston slap when it was first started cold. It had been re-ringed when it first should have been re-bored.
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Old 11-30-2019, 05:18 PM   #14
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Default Re: Knocking at start up?

Quote:
Originally Posted by J Franklin View Post
Too much slop in the thrust bearing caused just that in an engine I had. Test this theory by not pressing the clutch before starting up the car next time.
i can measure my thrust with a tape measure and i have no knock
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Old 11-30-2019, 10:21 PM   #15
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Default Re: Knocking at start up?

I have a tendency to agree with some of the guys. To much main bearing clearance can cause noise until oil lubes the crank.
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Old 12-01-2019, 10:41 AM   #16
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Default Re: Knocking at start up?

Quote:
Originally Posted by old31 View Post
OK, I will try this and report back.
Follow up:

I started it up without pressing on the clutch, and there was no knocking at all.

Soooo, what does that tell you?
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Old 12-04-2019, 09:09 AM   #17
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Default Re: Knocking at start up?

J. your theory shows it does not knock when the clutch is not depressed on startup.

So, how is this fixed? If it doesn't hurt anything I won't bother.
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Old 12-04-2019, 09:25 AM   #18
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Default Re: Knocking at start up?

Check your private messages on this website.
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Old 12-04-2019, 01:42 PM   #19
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Default Re: Knocking at start up?

Too much end play can cause the crank to be pushed forward and make the rods mis-align. Things tend to find centers after it gets running and oiled. Play in the thrust area can be measured and repaired.
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Old 12-05-2019, 08:50 AM   #20
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Default Re: Knocking at start up?

On my old engine, pushing the clutch in before startup would result in a loud clunk. The thrust was very worn and the crank would move about 1/16"... knocked louder from the center main and disintegrating #4 rod bearing though.
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