Re: Engine knock?

Posted by Bob Carabbio on June 18, 1997 at 13:35:06

In Reply to: Engine knock?
posted by Gary Rudicil on June 17, 1997 at 15:44:07

I'm grabbing the term "Deep" that you used in your note.If the sound that you're getting is a low-pitched, rumbly sound that is felt as much as heard, then the crankshaft and mains are the likely source. Crank and main sounds typically occur most noticeably at transition when the engine is neither pushing nor coasting.Mains tend to get louder as speed increases, and quieter as the load on the motor increases. Main noise also gets louder with temperature increase.
End play can be an issue, but would have to be visible to be large enough to cause a problem.Try poking around on the motor with a listening rod made of Brass or Steel about 5/16" in diameter at your ear and see if the sound can be localized and related to motion of the crank.

My experience is that an "A" tends to be a noisey engine when warm with some Main noise as a standard feature just like the old 6-cylinder flathead Plymouth and Dodge motors.The noise seems to originate mostly in the center main.

A loose flywheel will make a rumbly sound also, but should make it cold as well as hot, and get quiet when the engine is pulling (and it wouldn't still be running that way after 17 years).

Rod bearings tend to be much lighter in pitch, not unlike banging a hardwood 2x4 with a hammer. Thay also talk loudest in transition, but when bad, rap continuously.Sometimes shorting the spark plug on the guilty cylinder will make a rod become quiet, but won't affect Main noise.

If the Piston were striking the head, there would be physical signs, primarily on the piston head of the interferance, and the sound is lighter sounding like beating a heavy aluminum pot.
Bearing sounds in general tend to have a repitition rate equal to 1/2 the engine speed, and sometimes change their sound when the cylinder they are caused by is shorted.

A stuck Camshaft loading plug will cause noise that can sometimes be described as a knock, but isn't affected by engine loading generally.

In general if the knock isn't related to engine loading, but just changes with speed, it's probably not engine bearing related.




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