The Stovebolt Site Forums The Shop Area Driveline Troubleshooting warm transmission grind

I have a '53 3100 1/2 ton pickup. SM420 transmission with column shift.

I've posted before about this and have learned a lot and tried a lot so far. I actually have come a long way in "dialing in" my shift linkage and transmission issues.

Shifter, shift box, and linkage are all set up according to the shop manual and I'm using 90w mineral oil from NAPA in the transmission.

Shifting to reverse, 1st, and 2nd are always smooth. When the truck is cold (been sitting for a day or so) 3rd gear is smooth too. As the engine and transmission warm up, 3rd gear grinds increasingly and steadily. It never gets too bad though and if I ease it in, I can get it into 3rd with almost no grind.

I always double clutch because I learned to drive it that way and it's second nature now. It doesn't matter though, 3rd always acts this way. Would this be a bad 3rd gear synchro even thought I'm double clutching?

A 53 1/2 ton with column shift should have a 3 speed, not a 4 speed SM420. Are you sure you have a SM420?

In any event, both trannys are synchromesh, except for 1st and reverse, so I would think your synchro is going bad.

Kent

Sorry, I'm new and must have my terms confused. Yes, it is the 3 speed. What's the right name to call that transmission?

I feared it was the synchro. I dread dealing with that. Oh well, thanks for confirming my suspicion.

Try putting a heavier weight grease in it, like 85/140.

The transmission in your model has been referred to on this site as an SM318 (48-54). Closed driveline.
However, the next series trucks/cars are also referred to as an
SM318 (55-65) although they are completely different - open drive line and two different tailshaft lengths. Most of the parts don't interchange with the earlier models.
Not too confusing I hope!
Fred

Cool, thanks for setting me straight on the transmission lingo. I'll give the heavier oil a try and if that doesn't work I'll start saving to rebuild it.

Well, I tried Lucas 85w 140 oil and same issue; shifts perfectly after sitting for a while but grinds increasingly as the transmitting temperature rises. It's gotta be the synchro. Of course, I plan to do a complete rebuild while it's being replaced. Going to replace both the 2nd and 3rd gear synchros. I'm having trouble finding the right kit with the synchros. I'm seeing a lot of references to M20, M21, M22 and I don't know what that means. It looks like all the kits I find are for the later SM-318s but I can't be sure. Anyone have a good source for a rebuild kit with synchros for the earlier SM-318 transmissions?

Thanks

M20's, etc are 4 speed transmissions usually found in later Camaro's & Chevelle's. Nothing like what you have. The transmission you have uses what is basically an assembly which consists of a "clutch" sleeve with a syncro ring in each end. It is also referred to as a syncro drum. The parts are becoming increasingly harder to find. I have a source in Santa Fe Springs, CA that may have a good used assembly and other parts.
Dave Waugh 562-693-0855.
Just to put it in perspective however, you may spend way more money and time rebuilding that transmission than it is worth.
I got a good trans from another Stovebolt member several years ago to put in a 54 and it's still working fine. Many are upgrading engines and transmissions so there are some out there.
Fred

Thanks for the advice. I have the name of a guy locally who is supposed to be really knowledgeable with old transmissions and works on a lot of old cars. I'm going to call him and see what he would charge to rebuild it. From what I understand, the syncros are available and if that's all that needs to be replaced then it may not cost that much. We'll see...

Get a manual, learn to do it yourself, lots of help here.

Ed

When I replaced the 54 transmission about 5 years ago for a customer I looked into rebuilding it. I contacted a source at the time and was quoted about $700 just for the bearings and parts.
Noted it was really torn up and needed almost everything but that was over the top. Got a good used one for much less and he still has a smile on his face. I rebuild many of the later SM318 and SM319 OD's and am still able to get parts but they are also getting scarce and costly.
Fred