The Stovebolt Site Forums The Shop Area Driveline SM420 - Odd sound in neutral with clutch pedal out

The truck in question is a completely stock 1954 3600 3/4 ton with the SM420 4-speed stick transmission.

Does anyone else's SM420 make sounds with the clutch pedal out and the trans in neutral?

I rebuilt the trans recently because when it is in neutral with the clutch pedal out (released), it makes a sound like the input bearing lightly howling. Press the clutch pedal in and the sound stops.

I rebuilt the trans with all new bearings and it still makes the same noise. I'm starting to think that this is normal for this trans. My theory is that it's the natural sound of the spur cut first gears rotating when the clutch is released in neutral.

I've never been around another truck with this trans, so I don't have another truck to compare it to.

Rob

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On second thought, I guess my theory doesn't make any sense since the first/reverse gear is splined to the output shaft and has to slide fore and aft to engage - it's not a constant mesh gear like second and third.

I did have some noise, but after adjusting the pedal properly, it went away. 1" of free play is needed.

I do have the 1" of free play in the top of pedal per the shop manual.

Thanks for that though.

with my '63 with that 4 spd, I isolated the sound you are describing to my throwout bearing. Did you replace yours, check adjustment and pedal return springs?

A bad throwout bearing would be noisy in just the opposite situation- quiet with the clutch pedal out, noisy when depressed.

What lube are you using? Thin synthetics like 85-120, etc. will let the gears rattle around and possibly cause grinding when downshifting. Use 90 weight mineral oil specified for N-series Ford tractors. Tractor Supply stocks it routinely. It's the same as the OEM trans lube.
Jerry

I had a similar noise with my ‘52 SM318, and it turned out to be the bearing on the input shaft. I changed that and the pilot bearing and the noise went away.

I'm using the GL-1 90 weight mineral oil that the shop manual calls for. You can still get it at NAPA. They had it on the shelf in the back of the store though, not out front.

I tapped the input bearing onto the input shaft because it was a light interference fit. It didn't take much to put it on there, but maybe I warped the inner race. That's the only other thing that I can think of.

One of these days when the engine gives me cause to pull it for a rebuild, I guess I'll have to put in another input shaft bearing. Next time I'll heat the bearing and chill the shaft so it just drops on.