MTFCA Forum

Hi all, I need help with a strange noise. I thought I had a rod knock this spring but I only hear the noise under acceleration, not coast or declining RPM. I checked #1 rod bearing and it looks perfect. I assume
The others are OK, but maybe not. I also noticed that #3 plug has a very white insulator compared to the rest which are darker and blackish. The car runs well except for the noise. I have been struggling to isolate the noise and have a video of it when turning over by hand with plugs out. The noise seems to occur once per revolution. Any and all assistance is greatly appreciated.

Also, how do I upload the video?

Jim,

I had the same kind of knock, a deep thumping sound during acceleration. Knock went away when I removed a shim from the center crankshaft cap.

Bill

As Bill suggests, that kind of knock is usually a main bearing. I had a bad one. Turned out to be the front main bearing.

Since it happens when you hand crank and the engine is not under load I would rule out a rod or main bearing. I would suspect it is cam gear/crankshaft gear back lash. I pulled my engine when I replaced my gears.

You may have an intake air leak into the # 3 cylinder with the white plug, causing a lean mixture. Another possibility is a mis-matched plug or a plug that is running too hot for some other reason. A hot-running or defective plug could cause the engine to detonate ("knock") on # 3 cylinder under load, which might sound like a bearing knock, but it would only occur with the engine warmed up and running under load. A white plug usually indicates that the plug is running very hot for some reason, and that can cause detonation or "spark knock" under load. A spark plug with a sharp center or ground electrode may overheat and cause detonation.

I upload any video onto YouTube that I want to post.

Do a compression test and a blow down test. You might have a leak in the head gasket between the cylinders. Or a leaky intake manifold gasket causing the spark plug to run too hot. A lean mixture which might only affect that one cylinder. If a compression test reads good compression on all cylinders, I don't know why be bad on just one cylinder unless the valve was burnt or sticking open.
Norm

Here is the link to the video of the noise

https://youtu.be/CbIFlaS-pEc?si=k_RdjhvtgwcQvBNb

Cam gear backlash. Move the timing gear manually to feel the slop in the gears.
This isn't the problem noise while driving. That more fits with a loose main.

When I had a loose center main, I could retard the spark to quiet it down. Eventually, retarding didnt work as the main got looser.

Well I’ll pull the main caps and take a look. Thanks for the insight. Hope I find something.

Since it does it when hand cranking, it's a mechanical knock. (MAYBE) Something must be pretty loose.

Sammy T wrote: ↑

Thu Jun 05, 2025 6:26 pm

Well I’ll pull the main caps and take a look. Thanks for the insight. Hope I find something.

Look @ the upper side for evidence of pound out Babbitt also, not just the caps.
Might put an indicator on the crank center main with the cap off. Indicator should show no deviation. Any deviation is sign of a bent crank.

Looking at the video Is the pulley on the crank out of round or is the front of the crank bent?

This wouldn't account for a white spark plug, however I had a few knocks that happened at idle One was the pin through the front of the crankshaft. This was the original type with the original type pulley. The hole in the front of the crank shaft was worn as well as the pin. At idle the pin would move up and down with every half turn. I bent a crimp in the center of the pin which fixed it for the time. I notice you have a newer type pulley, so I don't think that is your trouble on your car.
The other strange knock was on a block which had been bored very large oversize. It was so large that the top edge of the piston was hitting the head at the edge of the bore.
I still think your problem is a vacuum leak at that cylinder causing more air to enter giving you a lean mixture. The best manifold gaskets for a T are the glands and rings. These will keep the manifold centered with the port and the compression of the copper rings when you torque down the manifold will hold in place. It is also a good way to keep the rear of the exhaust manifold from sagging and warping.
Norm

You sure would think so. The cam gear has almost no backlash. You can’t move the rods on the crank and #1 rod bearing looks like new. Was wondering if it could be a piston pin bushing? Is there any to tell without pulling the pistons out? The noise does seem to come more from the front rather than the rear of the motor. Since it makes noise at the same point on each crankshaft revolution it would seem to me to be main bearing, rod bearing or piston pin bushing related vs cam or other. Am I thinking about this correctly?

The front crank pulley is loose In the video as I had to pull it off in order to remove the cam gear cover to get a good look at the backlash.

A loose piston pin or cracked piston skirt is a possibility. I'd check all the rod bearing clearances, if OK, check the mains. Look for anything like a loose crank pulley, fan issue, loose engine mount, etc that could be making the noise. Take off the valve cover and inspect the springs and keepers. Be sure the starter and generator are not loose.

Long shots: The ground electrode on one of the plugs may have broken off and got embedded in the piston such that it taps the cylinder head, or a piston ring may have broken and a fragment of it got up in the combustion chamber and stuck in the piston. If your oil pan is dented or loose, or bent back toward the flywheel by the radius rod ball, the crankshaft may be touching it and making a noise. Timing gear loose on crankshaft.... piston slap/collapsed skirt.... Rod bearing(s) too tight. (Oil pipe plugged or loose?)

I have checked the plugs, electrodes are all there no damaged plugs. All valve springs are good - none
Cracked or collapsed, guess I’ll pull the head and pull the pistons to check all rod bearings and piston pins. Should probably pull the engine. Was hoping to keep it in the car and be able to find the issue, but can’t seem to find the culprit.

Can’t really believe it could be anything other than a rod or piston pins as it makes noise on each rotation with all the plugs out - no compression load on anything. Can’t
Imagine it could be a main bearing as without any load the crank won’t flex. When running I can see how it could flex as the rods are pushing on it. But just turning it over there is no up/down load on it. So I keep circling back to a rod or piston pin.


Do you think I can see enough of the upper rod bearing from below Without removing the piston/rod assembly from the engine?

Check each rod bearing before tearing everything apart. The rods should have around .001 to .002 clearance, and they should move a few thousandths forward and backward on the crankpin with finger pressure. I'd look up into each piston with a bright light while someone cranks the engine over slowly and look for anything out of order.

I will plastigage the rods for bearing clearance. All rods move front and back with light finger pressure. Hard to look up to piston bottom, but I’ll try and report back.

"Do you think I can see enough of the upper rod bearing from below Without removing the piston/rod assembly from the engine?"

Yes. You can remove a rod cap with the piston in its lower position & push the piston up 3/4 up. Rotate the crank 20 degrees or so for a full view of the rod bearing surface. You can wiggle the rod in the piston to get a feel for wrist pin bushing wear/slop.
You can watch the piston skirt gap (still attached to the crank) for an odd side movement, looking for indications of a bent rod.

Sammy T wrote: ↑

Thu Jun 05, 2025 9:17 pm

... guess I’ll pull the head and pull the pistons to check all rod bearings and piston pins.

Do you think I can see enough of the upper rod bearing from below Without removing the piston/rod assembly from the engine?

At this point, there's no need to pull the head. I'm nearly certain that your problem is not hiding there. Remove the lower inspection cover and hove someone crank over the engine as you look inside the motor. In addition to other things, pay attention to the upper ends of the rods and look for excessive clearance at the wrist pins... or even a missing or lose wrist pin bolt. You need to play detective here and closely study everything that moves. Really, the knock is so prominent, I think once you have a look inside the motor it's going to be kind of obvious. As others have suggested, also check for excessive timing gear clearance. If you study your video carefully, you'll see that the knock is twice per engine revolution.

Didn't read all the post to see it this was posted. Worn cam gears can make a lot of clacking at low speeds and when hand cranking. The valve spring pressure can rotate the cam gear via the lifter making a click between the cam gear and the crank gear.

Mark Gregush wrote: ↑

Fri Jun 06, 2025 2:30 pm

Didn't read all the post to see it this was posted. Worn cam gears can make a lot of clacking at low speeds and when hand cranking. The valve spring pressure can rotate the cam gear via the lifter making a click between the cam gear and the crank gear.

I believe that's what we hear in the video with a slow crank. It doesn't take much wear in the gear mesh to get that clinking noise.
Refer to the service manual for the gear mesh tolerances.

It’s definitely camshaft gear backlash you are hearing. Like Mark said it’s caused by the valve springs putting pressure on the tappets when they pass over the cam lobe.

If the noise heard when cranking by hand IS the timing gear lash, which seems likely, I'd go back to the white spark plug issue. Your knock when the engine is running under load could very well be "spark knock", or detonation, caused by any of several things, such as lean mixture to that cylinder, defective spark plug, head gasket intruding into the combustion space, an overheating exhaust valve due to too little clearance or poor seating, plug wires cross-firing, etc.

Well guys mystery solved.

The noise in the video was indeed the cam gear. While the backlash was within specs (upper limit) it was definitely the culprit of THAT noise.

Now for the rod knock type noise.
While I was under the car checking rod bearings (they look like new) and pistons for cracked skirts, loose pin bolts and loose pin bushings (none found) I happened to grab and turn the center main cap bolts and both were finger tight. I tightened them up and re assembled the engine, ran it and drove it and the noise is gone. How in the world they loosened up is a mystery to me, but they did. I can’t imagine this is a maintenance item, is it?

Thanks to all who helped solve the mystery(s). I appreciate all the expertise of the forum members!

They shouldn't have come loose. Good thing you found it. I'd do some checking now and find out why the one spark plug is "white". I doubt if it is related to the loose main bolts. I'd also check the rod bolts and the other main bolts, or as many as you can reach. It's a good idea to check all the suspension bolts for tightness and go over the whole car checking for loose fasteners or missing screws, bolts, cotter pins, and safety wire.

They should have been cotter pinned. When setting the nut, torque to spec.
If the cotter pin hole doesnt line up, Tighten more until the pin can be installed.
Never back off the nut to get the holes aligned.

Glad you found the problem & that it was a simple/minor fix.