Knocking on deceleration

[700rpm]

What might cause a mild knocking on decleration? It seems to occur only above 35 mph. When standing still and revving the engine, it sounds like it's coming from the top end, not the lower end, and it isn't a deep thump. It's more loke what you might get by knocking your knuckle on a wooden table. The rebuild has about 2500 miles, and the babbitt for rods and mains was done and set by Bill Barlow, so I am confident of the work there. Bill could not offer any suggestions. I dropped the pan and checked the rod bearings yesterday and they all measured .0015, except #4, which measured .00125.
I also retorqued the timing gear last week, thinking that might be it, but that didn't solve the problem.
Per my earlier question about the oil pump, I have eliminated that. The distributor/oil pump drive gear is new and the mating cam gear is good.

Also, the knocking almost goes away if the spark is fully retarded. Timing is right, however.

Could carbon buildup cause a knock like this? If so, how can this be prevented in the future?

Or what else? I don't know what to look for next.

[BILL WILLIAMSON]

Loose flywheel??? A friends' car had the SAME SYMPTOMS!
Bill W.

[PC/SR]

Piston slap? Disconnect spark plug wires one at a time and see what changes.

[George Miller]

It could be a lot of things, with out being there it is hard to say. It could be a rod hitting the piston boss when the engine is running easy. The cause would be a bent or twisted rod, cylinder bores off location. I have seen all these. But there are many other things to check.

[H. L. Chauvin]

Hi Ray,

Even the most experienced engine rebuilders are sometimes shocked at what they did not check.

Heard an engine rebuilder tell me of his tearing an engine apart three (3) times checking Babbitt, timing gears, flywheels, piston slap, etc., etc., trying to find the source of a knock.

One day his non-mechanic idiot neighbor dropped by ...... (every Model A owner has at least one (1) non-mechanic idiot neighbor) ...... who placed a dumb wood stick on the engine with his dumb ear on the other end.

Following the advice of this notorious town dummy, he removed the fan belt and the knock just instantly departed from under the hood ....... water pump.

You may not be that lucky .... but our most simple neighbors should never be frowned upon when we are in dire need of help.

[Willie Krash]

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Originally Posted by H. L. Chauvin

Hi Ray,

Even the most experienced engine rebuilders are sometimes shocked at what they did not check.
<snip>

Following the advice of this notorious town dummy, he removed the fan belt and the knock just instantly departed from under the hood ....... water pump.

You may not be that lucky .... but our most simple neighbors should never be frowned upon when we are in dire need of help.

Amen..
I chased a ping in my T, the pin holding the pulley to the crank was loose. It slaps the pulley and makes a ring at idle. Fixed that and all was good, but for another knock. Yep the water pump.
A water pump on a T. Someone thought they'd add it.

[J Franklin]

Referencing the above story, I had a knock and it was the fan belt and nothing else!

[Brentwood Bob]

front motor mount could be on your list.
Bob

[Fullraceflathead]

Exhaust pipe rattle???

[700rpm]

Originally Posted by BILL WILLIAMSON

Loose flywheel??? A friends' car had the SAME SYMPTOMS!
Bill W.

That's the one I am really hoping against, Bill! I am pretty certain I installed it properly, but one never knows, do one?

[Synchro909]

Crook uni joint?

[Benson]

1. Does the head have a fly cut above Pistons? Maybe gasket hits a piston? That would make dull noise. Do pistons stick above deck more than .032?

2. Is bore over .080... Maybe gasket is hanging into bore and is hitting a piston? Again a dull noise. Isn't there a gasket for larger bores?

3. On flywheel: maybe dowel pin retainer is missing so a Slightly longer bolt is hitting the main cap?

4. Could a flywheel bolt (Like hardware store bolt) be too long and hits the main cap?

5. Does noise change when pushing in clutch at idle? If bolt problem seems it would change sound when pushing flywheel forward unless end play is very small.

[700rpm]

Originally Posted by Benson

1. Does the head have a fly cut above Pistons? Maybe gasket hits a piston? That would make dull noise. Do pistons stick above deck more than .032?

2. Is bore over .080... Maybe gasket is hanging into bore and is hitting a piston? Again a dull noise. Isn't there a gasket for larger bores?

3. On flywheel: maybe dowel pin retainer is missing so a Slightly longer bolt is hitting the main cap?

4. Could a flywheel bolt (Like hardware store bolt) be too long and hits the main cap?

5. Does noise change when pushing in clutch at idle? If bolt problem seems it would change sound when pushing flywheel forward unless end play is very small.

Benson, those are good suggestions, and have been checked out and checked off. It has to be something else. Thanks for your input.