Piston Rattle

Smitty

Smitty

Well-known member
Anyone experience some noise from pistons, kind of a slight knocking sound at speed.
This is a new engine except for the pistons, miked cylinders, skirts and replaced rings.
Could a high comp. head make an engine do this?

Bearings, timing gear etc. are fine.

Thanks, Steve
 
Engine noises are hard to diagnose in just words---

Piston slap (loose piston)usually is loudest when cold --as the piston heats up it expands and the clearance is less.

Loose wristpin usually gets louder when hot --the oil thins, it is a double noise because the piston changes direction at each end of the stroke.

The gasket can hang over the cylinder and the piston can hit it.

If there was a ridge at the top of the cylinder and it was not cut the new rings can be hitting it.

The timing can be off and you are having detonation.(too advanced)

A head that does not have relief for the pistons --piston hitting head, block deck has been surfaced, head has been surfaced making clearance too small.

Actually hearing the noise is worth 2000 words.
 
Kurt,

Went through this motor myself, everything is at spec.
Don't hear it when the motor is idling, only after car is going along.
This engine was rebuilt 3 years ago, expensive crappy rebuild. Is good now but the sound has been with the motor since the first rebuild..unless that is normal for a Model A?
Have we talked by phone about engine rebuilding? If so I have some updates for you. Does your brother rebuild A engines?

Steve
 
Just leaned out the mixture adjust screw and retarded the dist. a tad and seems to be much quieter.
Perhaps I am paranoid from the last rebuild and looking for things...

Steve
 
to much advance they knock/detonate every car is different most instructions say to run 3/4 down on the timing when driving. but alot of variables such as advance lever to rod to distributor plate tolerances . i run mine about 1/4 down if go half it knocks..
 
You say the big ends are within tolerance (.001 to .0015).

Do you get the clank with:

Advanced Spark
Warm Engine
Coasting
Clank goes down and/or away when spark is retarded, and/or power is added?

I just reset the tolerances on my #3 and cut out all of the above. All 4 big ends have some slight front to back movement, but #3 could be "twisted" on the journal. The other big ends had no twist.

Used plastigauge to get .0015 and no twisting- AND NO KNOCK!

This was also a rebuild, but the mechanic didn't get #3 correct.
 
Brass shims were .003 I believe. They did allow for any distortion in the plastigauge in the slightest. I used the "two layer foil" trick and could still move the crank.

Skinned 3 layers off each shim (one layer at a time, 6 rounds in all) to get .0015.
 
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