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Old 02-14-2021, 10:35 AM   #1
19Fordy
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Default Two 8ba Engine Noise Questions?

1. What causes "piston slap" and what does it sound like?

2. If the retainer clip for a piston wrist pin comes loose , how can you diagnose it and what sound does it make?

My engine has a strange metallic sound that can be heard at idle after the engine warms up but goes away when you increase the RPM.

Been like that for a year and am trying to figure what it is.

Thanks, Jim
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Old 02-14-2021, 11:17 AM   #2
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Default Re: Two 8ba Engine Noise Questions?

Piston slap is caused by too much clearance between piston and cylinder and sounds sorta like a rod. Determine if the sound is engine speed ( crank or rod) or 1/2 speed (cam or valves) By chance would you have forged pistons? the extra clearance will cause a slap until warmed up.
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Old 02-14-2021, 01:23 PM   #3
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Default Re: Two 8ba Engine Noise Questions?

fuel pump, fuel pump push rod either of these could do the sound
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Old 02-14-2021, 01:34 PM   #4
19Fordy
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Default Re: Two 8ba Engine Noise Questions?

I have an electric fuel pump. Push rod removed.

Pistons are 1953 factory Ford, not forged.

Hear the noise after the engine is warmed up at idle. Then when you give it gas, noise goes away.

Valves are very quiet. Using adjustables. No tapping sound at all.

Noise doesn't cease when each spark plug wire disconnect one at a time.

More ideas welcome. Thanks.

Last edited by 19Fordy; 02-14-2021 at 01:41 PM.
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Old 02-14-2021, 01:48 PM   #5
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Default Re: Two 8ba Engine Noise Questions?

Know someone who had similar observations, turned out to be a leaking exhaust gasket.
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Old 02-15-2021, 01:42 AM   #6
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Default Re: Two 8ba Engine Noise Questions?

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Jim, I have the same noise on my 8BA and can’t figure it out. Are you using the truck oil pan ? I agree,It is a strange noise.
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Old 02-15-2021, 01:46 AM   #7
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Default Re: Two 8ba Engine Noise Questions?

Interesting, going to check that out on my car.
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Old 02-15-2021, 09:29 AM   #8
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Default Re: Two 8ba Engine Noise Questions?

dono50: I am using the 1950 Mercury rear sump oil pan. Originally, there was no noise for 30 years. Then over the last year or two it appeared.

I am wondering if the oil pan baffle may have somehow come loose, just a little.
Enough to vibrate.
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Old 02-15-2021, 10:25 AM   #9
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Default Re: Two 8ba Engine Noise Questions?

Remember, flatheads are like electricity.....funny & goofy stuff......
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Old 02-15-2021, 11:59 AM   #10
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Default Re: Two 8ba Engine Noise Questions?

Quote:
Originally Posted by 19Fordy View Post
dono50: I am using the 1950 Mercury rear sump oil pan. Originally, there was no noise for 30 years. Then over the last year or two it appeared.

I am wondering if the oil pan baffle may have somehow come loose, just a little.
Enough to vibrate.
Jim, I used the truck oil pan and will check that next for dents etc. Used a stethoscope today and my noise is loud near the fuel pump. Have an NOS AC pump, changed the diaphragm, and a repro push rod. Need to check that out as well. I know you have an electric pump.
I have never heard a sound like this on a flathead and I’m puzzled. The engine runs so nice and has decent compression. Going to add some MMO to the oil.
Bob
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Old 02-15-2021, 12:02 PM   #11
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Default Re: Two 8ba Engine Noise Questions?

Ok Bob, sounds like a good plan. Please let us know what you find.
Thanks, JIM
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Old 02-15-2021, 12:43 PM   #12
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Default Re: Two 8ba Engine Noise Questions?

Check to see that the crank pulley is on tight.
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Old 02-15-2021, 12:54 PM   #13
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Default Re: Two 8ba Engine Noise Questions?

The term "piston slap' is literal. It sounds like very fast slapping. It is heard most pronounced when decelerating in gear (engine braking). The opposite of rod knocking which is loudest on acceleration, and not heard on deceleration.

Never heard of a sound or diagnosis for wrist pin retainer clip come loose. I think the first symptom would be burning oil smoke due to a groove in the cylinder wall. If it got bad enough, it might show up on a compression test.
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Old 02-15-2021, 02:30 PM   #14
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Default Re: Two 8ba Engine Noise Questions?

Two things come to mind when you say louder near the fuel pump. First would be a weak return spring on the pump arm. Second would be weal spring on the bypass valve in your oil pump. Had this happen in a chev 235. Took me forever to find it.
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Old 02-15-2021, 04:05 PM   #15
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Default Re: Two 8ba Engine Noise Questions?

Quote:
Originally Posted by pistonbroke View Post
Two things come to mind when you say louder near the fuel pump. First would be a weak return spring on the pump arm. Second would be weal spring on the bypass valve in your oil pump. Had this happen in a chev 235. Took me forever to find it.
Hope mine is something easy like fuel pump. How did you find weak pressure valve on fuel pump ? Did you have low oil pressure ?
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Old 02-15-2021, 04:29 PM   #16
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Default Re: Two 8ba Engine Noise Questions?

Diagnosing engine noise > https://www.onallcylinders.com/2014/...engine-noises/

Piston Slap

A hollow, muffled, almost bell-like sound is usually piston slap. This condition is caused by a piston rocking back and forth within its cylinder. Continuous piston slap means the engine needs service; however, if you only notice this sound when the engine is cold, it is likely not serious.
A continuous piston slap sound is usually caused by worn pistons, excessive piston-to-wall clearance, misaligned connecting rods, worn cylinder walls, or inadequate oil.

Piston rattle or slap when an engine is cold indicates too much clearance between pistons and cylinders. This may be due to excessive assembly tolerances or worn cylinders. Some forged pistons have a higher rate of thermal expansion than cast pistons, and may require slightly greater cold clearances to compensate. This, in turn, may produce some unwanted piston noise when a cold engine is first started, but it has no effect on piston performance, durability or longevity.
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Last edited by petehoovie; 02-15-2021 at 04:38 PM.
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Old 02-15-2021, 04:57 PM   #17
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Default Re: Two 8ba Engine Noise Questions?

petehoovie: Thanks for posting.
Can that piston slap be heard if you touch a long metal rod to the area on the block near each each cylinder and the other end to your ear?
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Old 02-15-2021, 05:06 PM   #18
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Default Re: Two 8ba Engine Noise Questions?

Quote:
Originally Posted by 19Fordy View Post
petehoovie: Thanks for posting.
Can that piston slap be heard if you touch a long metal rod to the area on the block near each each cylinder and the other end to your ear?

Don't know about that but shorting each spark plug to head should tell you something....
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Old 02-16-2021, 03:38 PM   #19
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Default Re: Two 8ba Engine Noise Questions?

Quote:
Originally Posted by petehoovie View Post
Don't know about that but shorting each spark plug to head should tell you something....
Not so sure about that, not sure the actual ignition causes the piston slap vs the compression stroke transition to power stroke. Or just the same transition on the intake /exhaust stroke.
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Old 02-16-2021, 07:20 PM   #20
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Default Re: Two 8ba Engine Noise Questions?

Quote:
Originally Posted by 19Fordy View Post
dono50: I am using the 1950 Mercury rear sump oil pan. Originally, there was no noise for 30 years. Then over the last year or two it appeared.

I am wondering if the oil pan baffle may have somehow come loose, just a little.
Enough to vibrate.
Jim, today checked oil pan with stethoscope and definitely coming from there. Pulled plug wires each cylinder and no difference. I know all my rod/main clearances are good as well as oil pressure.
Must be interference. Got large clean out on truck oil pan so will try dropping that next.
Suspect the oil pump pickup tube.
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