Farmall super m knocks under load

Jack Stanley · Sep 3, 2016

Hi all I am back again with another super m question. I adjusted the valves last weekend but the engine still has a tapping sound to it when it's under a load. Also when you rev the throttle up like while the engine is speeding up it makes the noise but once up to speed it smooths out and runs good. It's not a loud noise but I can hear it. It almost sounds like an exhaust manifold gasket is leaking but I can't find it. My brother said could be a wrist pin bushing? Any thoughts? Also the tractor has excellent oil pressure and starts up fast. Thanks!

Steve@Advance · Sep 3, 2016

Try setting the RPM at a speed where the tapping is easy to hear. Pull and replace one plug wire at a time, listen for the noise to change.

If the tapping stops with a certain wire pulled, chances are it is a wrist pin, cracked piston skirt, or loose piston to bore clearance.

If the sound turns to a double rap, could be a rod bearing loose.

If no change it is likely valve train noise, a cam lobe going flat, bad end on a push rod, bad rocker end, etc.

RBoots · Sep 3, 2016

I believe it was on this site where a guy had taken the head off his 450 maybe to freshen it up or replace the head gasket and found one of the pistons almost cracked fully in half, but wouldnt have known it by the way it ran until he tore it apart. I had an M a few years back that started knocking like that all of a sudden one day. This was after I had owned it for about a year. Tore it apart and found a small screw stuck in the top of the piston. A screw like you would find on a choke or throttle plate, but neither of those were missing, so who knows where it came from. I popped the screw out of the piston and put a new headgasket on it and it ran just like it did before it started knocking. It could be a wrist pin though as well.

D Slater · Sep 3, 2016

Might want to check the timing to rule that out. Over advancing could cause that under load or acceleration if no noise other times.

Bob M · Sep 3, 2016

In addition to possible causes already offered here's one more: A cracked cam gear hub. At certain RPMs a flopping cam gear can make a mechanical tap or knock every 1/2 crank revolution, yet the engine will continue to run fine - at least until the gear hub breaks apart.

It's a common failure on Super M/400/450 gassers with the live hydraulic pump. I've had to replace cam gears on both my Super M and a friend's 400.

RBoots · Sep 3, 2016

Yep, been there on our 450 as well Bob, forgot about that one

notjustair · Sep 3, 2016

In my experience with M engines, knocking on throttle up or under load has always been rod bearings. Take the pan off and plastiguage things to know for sure.

Does hot oil pressure drop a bunch at low idle?

Jack Stanley · Sep 3, 2016

Oil pressure stays pretty steady even at idle it may drop a little bit but not a whole lot. I thought maybe it could be timing related? I'm not sure how to check it on these old tractors? Do I use a timing light ?

pete 23 · Sep 3, 2016

I used to be pretty good at diagnosing connecting rod noises. Had considerable practice on IH 6 cyl truck engines. Usually hear them on the reving the engine up and when it reached set speed it would disappear again. Pistons and pins knock, (rattle actually) when no load. Have head a few main bearing pound on the M's under load. Reminds me of the time the neighbor had me and a friend listen to his Plymouth six cylinder. I said right off , connecting rod. Friend says, no way, piston. They pulled pan and later told me we were both wrong it was an insert. I didn't say anything because if he didn't know that the rod insert would be considered a rod knock, why insult him.

Janicholson · Sep 4, 2016

one more idea. If there is a carbon or deposit buildup in one cylinder it can knock on only one cylinder. a compression check can isolate a cylinder with too much compression. In reality pulling the head and wire brushing it out with a rotary brush is about all that works. Putting a valve job on it might also be a good idea with valve seals. Jim

Jack Stanley · Sep 4, 2016

Thank y'all for all of the help! I do believe I have found the problem. Was driving her around this morning with the hood off listening to it and it sounded like an exhaust leak so I got a good light and looked the manifold over good turns out right in the middle of the manifold on the inside there is several rust holes I think that is causing my problems!

Janicholson · Sep 4, 2016

To be sure, take fiberglass and pack it in the space start it and if the sound is muffled, that is it. Patching can be done, as can replacement. There are new welding materials that do really good in these circumstances. it is likely to be thin in other areas as well. Jim

Farmall super m knocks under load

Jack Stanley

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Steve@Advance

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RBoots

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D Slater

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Bob M

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RBoots

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notjustair

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Jack Stanley

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pete 23

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Janicholson

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Jack Stanley

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Janicholson

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