SM420 transmission rebuild with upgrades

[Prowbar]

A while back I rebuilt the SM420 transmission that I bought because the original transmission was pretty much shot.

The new transmission works great but, leaks some oil and has that annoying grinding noise that keeps chasing me.

I still had the original transmission in parts in the shed and figured I'd try to find out what causes this particular noise, having looked at all the other components in the driveline and pretty much ruled everything out except the transmission.

Taking a closer look at the transmission, it has to be something that is able to vibrate at a certain frequency. Everything is pretty much fixed except some parts in the shift cover, the reverse lever and the 2nd gear synchro ring.

My main suspect was the 2nd gear synchro ring. Taking a closer look, there is a step worn in the 2nd gear synchro lugs with corresponding marks on the gear, almost as it has been vibrating on the shaft. See photo.
The 2nd gear synchro is held in place by 2 wire springs riveted on the gear. See photo. When installed in the transmission, the springs grab the lugs only at the end and barely have any tension to hold it in place.

In other to prove my theory and repair the original transmission, I'd figured I'd see if I can rebuild it and fix the accompanying issues.

[Prowbar]

Went ahead and tore down the transmission. Cleaned all the grime and grease off of it. This transmission still has all the factory markings, these will be preserved.
Made a special tool to press out the bearing to remove the secondary shaft. It will be replaced anyways.

The gears have some pitting but they are not unusable.

Did a top cover rebuild. Polished the shift fork shafts and took out the pins that fixture the shifter selection plate as this could be a potential rattle.
Turns out the pins have a small washer to reduce the rattle. I pressed them back in again and secured them with Loctite 648 as a backup.

In order to get a better grip on the 2nd gear synchro I've made new rivets that are 4mm (~.150") longer with spacer bushings to relocate the springs closer to the 2nd gear synchro. This made a noticeable difference in gripping power. To allow room for the rivets, I've milled out the pockets in the 2nd gear synchro.

I am hoping that this fixes the annoying rattle.

[Prowbar]

First photo shows the final 2nd gear synchro assembly.

Cleaned out the case and re-attached the magnet with JB weld.

Next project: the main shaft needle bearing surface has some deep pitting in it.
Figured I'd see if I can repair it with a needle bearing bushing, as supplied by SKF (IR series)
These are meant for needle bearings without an inner ring in combination with a shaft that is not hardened.

Since the SM420 needle bearing surface is a very odd size, which is not available, I had to reduce the diameter of the bushing to the required size.
I first tried to turn the bushing with the CBN insert and check for through hardening. It passed the test.

Indicated the main shaft in the lathe between centers. Turned down the bearing surface with a CBN insert. Allowed for .0005" press fit and pressed on the bushing with some additional Loctite 648.

The main shaft was re-indicated between centers and turned and ground to size. The last picture shows the final product (I used the steady rest to face off the bushing.

[Prowbar]

One thing that I noted when driving with this transmission still installed was the lack of 2nd gear synchronization, even at low speeds and with hot gear oil.

Looking at the synchronizer surface, it became clear that it needed to be freshened up. Figured I could do this myself.

Indicated the first gear in the lathe (see photos) and placed the compound slide in the correct angle. The grooves are actually grooves, and not cut as threads, which makes recutting possible using this lathe.

An 45 degree V-groove tool was used to cut the grooves with the correct pitch.

Fresh grooves should allow for the oil film to be cut and easily evacuated during synchronizing.

I also polished the 2nd gear synchro for a fresh surface without burrs and damaged areas.

[Prowbar]

The bushings of the 2nd and 3rd gears on the main shaft were pretty wore out. Decided to make my own bushings from a piece of bronze I scored at the local scrapyard.

The 2nd gear bushing was made with the correct OD to press it in, but left oversize on the ID. Pressed it in and turned out the ID to spec and redrilled the oil holes. I then cut oiling grooves on the mill to help oil flow into the bushing.

The 3rd gear bushing was more of a hassle, since it is pressed on the mainshaft. I first made a blank with the correct ID. I then pressed it on the mainshaft and turned it to the correct OD.
The bushing was removed and the oiling grooves were cut. I cut in 8 grooves instead of 10, it was easier to index.

All the clearances on the shaft are now according to the manual.

[Prowbar]

Went ahead and assembled the transmission with new bearings and synchro rings. I got the rebuild kit from eBay and bought some new synchro's from Transmission Parts Distributors.
On the last transmission I did, the new synchro's definitely improved the shifting, making it quick and smooth to shift.

Last task it to pressure test for oil leaks (I hate oil leaks) and then give it a couple of coats of a clear rust preservative based on linseed oil to preserve the cast iron and all the markings.

[Jim A]

I've had my SM 420 out of my '60 1500 twice for clutch work in 54 years. Seemed even heavier the second time.
It didn't seem to need anything but a little cleanup and fresh oil either time.
It is interesting to see the innards of this super tough old masterpiece.
I hope the ones you've gone over work well for a long time.

[Prowbar]

Originally Posted by Jim A

I've had my SM 420 out of my '60 1500 twice for clutch work in 54 years. Seemed even heavier the second time.
It didn't seem to need anything but a little cleanup and fresh oil either time.
It is interesting to see the innards of this super tough old masterpiece.
I hope the ones you've gone over work well for a long time.

Glad to hear they serve you well. Indeed, they are tough and heavy beasts!
The 2 that I've owned needed to be serviced.
The original trans from the truck (this transmission) was extensively abused in its life as a farm truck. The bearings in the trans were completely shot, the magnet was full of metal and the 2nd gear synchronization was almost non existent. The gear lube looked like it had been there for the life of the truck.

The other transmission was fine and shifted great. No issues with the 305 V6. When I installed the 478 V6 the bearings started to make a little noise. Enough to convince me to rebuild the thing.
I've grown to like rebuilding transmissions.

The funny thing about that particular noise is that there are plenty of "quiet" (gear whine not included) SM420's. Both of mine have that annoying grinding noise, and I've serviced basically everything in the driveline to rule anything else out. Think I've found the issue, hopefully mine is nice and quiet from now on.