Engine knock
- Thread starter bunnyc
- Start date
Mitch//pa
BANNED
Before you pull the pan check for excessive crankshaft endplay... Spec is .003-.008
Does the knock change if you put pressure on the crank with the clutch?
If it is excessive pull the engine if it has Babbitt bearings...setting the clearances will not fix the end play
Does the knock change if you put pressure on the crank with the clutch?
If it is excessive pull the engine if it has Babbitt bearings...setting the clearances will not fix the end play
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goodoldvic
Senior Member
Drop the pan and check your clearances. The usual suspects are the center main bearing then the rods. I've used the "tin foil" method, in the past with success. Make sure you have no exhaust leaks so you are truly hearing a bearing knock. Good luck
Marshall V. Daut
Well-known member
If the knocking is accompanied by rough idling as you advance the spark lever, the timing is too far advanced. Is this a new phenomenon that happened all of a sudden, or gradually? Check the timing before doing anything else. The set screw inside the distributor cam may be loose and has allowed the timing to advance itself until it has now reached the point that the spark in each cylinder is firing prematurely = a knock. Check that the special thin lock washer is beneath the set screw head.
Try a quick test first: Rev up the engine with the timing fully retarded. If no knock, incrementally advance the timing lever until the knock begins. If a bearing is bad or loose, retarding the timing will only lessen the knock, not eliminate it. By incrementally advancing the spark lever and revving up the engine, you are checking the timing.
Anyway, try this before disassembling the engine.
Marshall
Try a quick test first: Rev up the engine with the timing fully retarded. If no knock, incrementally advance the timing lever until the knock begins. If a bearing is bad or loose, retarding the timing will only lessen the knock, not eliminate it. By incrementally advancing the spark lever and revving up the engine, you are checking the timing.
Anyway, try this before disassembling the engine.
Marshall
Patrick L.
Well-known member
The engine shouldn't knock at half advance [ or full either] when idling. So I would recommend rechecking/setting the points/timing.
Just another possibility. Remove the timing pin. Once the knocking starts, insert the timing pin [ be mindful of the spinning fan] and put pressure against it and see if that helps matters. If it does, the timing gear is too worn.
Just another possibility. Remove the timing pin. Once the knocking starts, insert the timing pin [ be mindful of the spinning fan] and put pressure against it and see if that helps matters. If it does, the timing gear is too worn.
bunnyc
Senior Member
Mitch, I will have to research how to do this with the engine installed. Where to mount the dial indicator, what to push/pull on. I hear the crank pulley is fragile.
Patrick L: Here's what I have for timing: full retard - 15 deg ATDC, half advance - 9 deg BTDC, full advance - 24 deg BTDC. Measured w/timing light on an after-market timing indicator scale.
Not brave enough to try your suggestion with the timing pin.
goodoldvic: no exhaust leaks - brand new muffler and clamp. Sounds so quiet.
The noise is not new, Been there since I bought the car - about 400 miles ago. Just now decided to address it.
I am going to pull the pan anyway. The gasket, if there is one, leaks. All I see are gobs of silicone around the edges. I'll start with checking the center main.
Thanks for the replies.
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tbirdtbird
BANNED
let us know how you make out
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Far be it from me as a newish A owner... but I thought I had a "knock" when in fact I had a head gasket leak. I sprayed soapy water around the head and saw wonderful bubbles. Changed the head gasket. No more "knock". Just a thought. These old engines are tough old birds, think easy before thinking the engine blew. I'm new to A's but not to engines and these are generally bullet proof in my opinion.
Marshall V. Daut
Well-known member
Since the original poster has since added information that his engine has always made this knocking sound, may I suggest a parallel head gasket peculiarity, along the lines of what "jg61hawk" writes? This is a well-known problem that has been discussed before and specific examples cited of actual experience with it. I myself have had at least two posted experiences that were traced to this basic head gasket problem.
Some head gaskets don't have large enough cylinder holes to accommodate large bore pistons, such as .125 oversize. In such cases - and unless the piston tops have been beveled - the rising piston strikes the sharp edge of the overhanging head gasket into the cylinder. This causes a distinct knocking noise that has often been misdiagnosed as a loose rod or a shot main bearing. Usually, the overhanging edge of the head gasket eventually gets knocked far enough away from the piston top and stops knocking. This may take a while, so if the poster's engine is a fairly low mileage rebuild with large pistons and one of these 1970-1980's head gaskets, he may have to contend with some knocking noise for a while until the problem resolves itself.
Of course, other tests should also be made, such as shorting out each cylinder to check for pistons slap and loose connecting rods. But don't discount the likelihood of an ill-fitting head gasket, especially if the engine rebuild procedures and parts are unknown to the poster. With engines built 20-30 years ago, this was a fairly common complaint and one that baffled even the sharpest Model A guys.
Marshall
Some head gaskets don't have large enough cylinder holes to accommodate large bore pistons, such as .125 oversize. In such cases - and unless the piston tops have been beveled - the rising piston strikes the sharp edge of the overhanging head gasket into the cylinder. This causes a distinct knocking noise that has often been misdiagnosed as a loose rod or a shot main bearing. Usually, the overhanging edge of the head gasket eventually gets knocked far enough away from the piston top and stops knocking. This may take a while, so if the poster's engine is a fairly low mileage rebuild with large pistons and one of these 1970-1980's head gaskets, he may have to contend with some knocking noise for a while until the problem resolves itself.
Of course, other tests should also be made, such as shorting out each cylinder to check for pistons slap and loose connecting rods. But don't discount the likelihood of an ill-fitting head gasket, especially if the engine rebuild procedures and parts are unknown to the poster. With engines built 20-30 years ago, this was a fairly common complaint and one that baffled even the sharpest Model A guys.
Marshall
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To answer your question direct, quiet at idle, probably main. Knock on accelleration or load, which you do not tell us, probably main. Advance at idle, main or rod so short out pistons and see, but listen to how often you get a knock in accordance with the firing. Advance until you hear the knock and then short the pistons. If the knock goes away when a piston is shorted, rod or piston slap.
Tracing a knock is more art that science. Piston slap is a sharper, more higher pitched sound than a "knock." Checking for crank end play is to depress the clutch. That is an intermittent knock and depressing the clutch eliminates end play and the knock.
Trying to press the timing pin has been worthless to me. All I get is a clack, clack as it passes the cam gear hole. It only applies to the cam anyway and has nothing to do with timing advance.
As noted above, there can be lot of causes of "knock." Do some basic tests before dropping the pan. However, my strictly guess as this time is center main.
Bill W and Buster know this stuff. What says Buster?
Tracing a knock is more art that science. Piston slap is a sharper, more higher pitched sound than a "knock." Checking for crank end play is to depress the clutch. That is an intermittent knock and depressing the clutch eliminates end play and the knock.
Trying to press the timing pin has been worthless to me. All I get is a clack, clack as it passes the cam gear hole. It only applies to the cam anyway and has nothing to do with timing advance.
As noted above, there can be lot of causes of "knock." Do some basic tests before dropping the pan. However, my strictly guess as this time is center main.
Bill W and Buster know this stuff. What says Buster?
BILL WILLIAMSON
Well-known member
Buster T.
here,SORRY, but I CAIN'T hear it frum here
I'm NOT PSHYCIC, you know
Buster T.
BILL WILLIAMSON
Well-known member
If you suspect a center main knock, set the speed & spark to where it will knock constantly, then, short out #2  at the SAME time & it'll be SILENT.
Guru, Claude, bought a "new" engine that had a knock, I diagnosed it for him. SURPRISE, the builder DIDN'T adjust the center main at all, the shim packs were totally undisturbed!! It was WAY loose!! It ended up to be a PRIMO engine.
Bill Marvel
Guru, Claude, bought a "new" engine that had a knock, I diagnosed it for him. SURPRISE, the builder DIDN'T adjust the center main at all, the shim packs were totally undisturbed!! It was WAY loose!! It ended up to be a PRIMO engine.
Bill Marvel