The Stovebolt Site Forums The Shop Area Driveline SM420 - Replacing the Rear Seal

I've been driving my '65 3/4 ton with the original 3-speed-with borg warner overdrive since I bought the truck in 2015. I've really enjoyed the uniqueness of the setup, but I miss the low gearing of the SM420, and I plan to swap transmissions very soon. I want to replace the front and rear seals on the SM420, but I've heard the rear takes extra care so as to not have bearings fall out or what not. Is it a straight forward thing to do? I searched on here about replacing the rear seal, and I saw some have done it, but I didn't find any info on actually changing it, just info on slingers and no-slingers on the front shaft. What is the procedure to replace the SM420 rear seal? Thanks.

Rear seal is replaced by first removing the ujoint then transmission yoke, then simply remove the 8 bolts and remove the rear bearing cover, nothing will fall out unless you have removed the top loader (shift cover)and then tipped the rear of the transmission down then the rear mainshaft bearing and shaft could slide back and could drop the needle bearing out of the clutch gear. Tap out the seal of the cover and replace. Sometimes the yoke will be grooved from the seal, try and press and position the new seal so it is on a fresh ride on the yoke. It is good to lightly polish up the yoke with some fine emery paper and wd40 too.

Steve

That's great news. Thanks. The trans is out, so the hardest part may be breaking loose the yoke nut. I'll find a way to secure the yoke, though.

Air impact will be your friend there.

This post covers replacing the rear seal on my SM420 including how to hold the yoke without damaging it and being able to accurately torque the nut.
https://www.stovebolt.com/ubbthread...ve-improve-drive-repeat.html#Post1141918

Very easy to remove seal after yoke without unbolting the rear cover (Simpler than explained in service manual).

Don't be surprised if you don't have an input shaft seal, the early SM420's did not.

Your pictures aren't showing Grigg. That seems to be the only problem with forums, go get a good how to do and the pictures disappear over time.
Steve

I saw that before coming in this morning. Good info, just no pictures.

My SM420 is a '68 or '69 if I recall correctly. I need to look at the stamp code again.

An example of a shared file/image Google Drive photo.
Does that display for you? It works for me.
It is a direct link to an image stored on a Google Drive, shareable images folder.

Grigg's links/photos are on a Google Drive.
Getting a shareable link is a Google Drive matter (not a Stovebolt problem).
It is a little odd - first, the image/file must be stored in a shareable folder on Google Drive.
Then, you have to obtain a shareable link, using the Good Drive app/Viewer.

I can see the photo in the link you shared @tclederman

The thread was started prior to Stovebolt images being enabled. By using our image hosting, the images will always be viewable in the thread itself, instead of having to go off site. It’s new and folks are still getting up to speed on it. Maybe Grigg has the images saved somewhere and can download them to this thread.

Ooops, I should have seen that.

Yes, it is far better to upload images here - now that we can easily do that.

I also have a lot of dead links to very old images - maybe one day, I'll clean them up (if I can find the images).

Thanks, Martin

Yes, the pictures in that thread are mostly not working..
I scanned the relevant post and didn't think they were necessary for this discussion, The link for the yoke holding tool is good.
Pictures here
https://goo.gl/photos/H26qPfBHb7eS15698

I use a 48" pipe wrench with a rag or piece of wood around the jaws for a yoke holding tool.

Ed

The SM 420 was discontinued in 67. If it is a 68-69 it will be a SM465. It is the same procedure for replaceing the rear seal. Like said previously, you do not have to remove the rear cover. To get to the front seal remove the front brg. retainer. Nothing will fall out but don't let the shaft & brg slide forward more then about 1/2 in. That will be a good time to check the front brg. Spray A little brake clean on the brg. to clean it & look in with a light as you slowly turn the shaft. If it has any pits on the balls or races it needs a new brg. It is common for this brg to go bad & it will cause whinning noises in the lower gears.
George

It's an SM420, I verified that years ago. I'm thinking I'll install it after replacing the front seal and checking the bearing. That way I can lock it in gear and won't need to make a yoke holding tool for a one-time job. Last night I found the shifter and put in the trans, and shifted through all the gears with it on the stand under the eave of the house. It has been a long time since I shifted that thing, it brought back some memories, really looking forward to having it in the truck.