The Stovebolt Site Forums The Shop Area Driveline SM420 Transmission Changing Input shaft Bearing

Ah, the saga continues. Purchased a "spare" SM420 to swap out while "rebuilding" the original. My son stopped by to help me switch transmissions on the transmission jack and did a quick unrequested inspection and said 'Dad, that's got too much play on the input shaft". Upon close inspection witht the top plate off there are two or three ball bearings missing from the input shaft bearing. There are no signs of damage in or around that bearing and other than the play everythin else "looks" good. I'm thinking to replace the front bearing and needle bearings in the input shaft and moving forward with the install.....or is it time to just rebuild the one that's in it and be done with it. Darned, wanted to drive the truck next week. Thoughts anyone?

I found the worst part of the job was the remove and reinstall part - the SM420 is heavy with a round bottom. Since it's out rebuild it using one of the kits.

Search for bk108 sm420 and you should find:
https://www.transmissionpartsdistri...7-gm-4-speed-with-10-5-case-1-pto-bk108/

Gord, you are correct. Using a HF transmission jack, and a fabricated bracket to stop shift/roll, along with a tie down strap it came out straight and with ease.

Can I replace just the front bearing and internal rollers without a lot of trouble? I already have the Muncie small parts kit so the rollers and retainer I have..........your thoughts??

I used a kit from Novak last time I overhauled an SM420. The supplied input shaft (clutch gear) bearing had fewer balls than the one that came out. I considered using the original (from a previous overhaul) bearing as it was in good condition and had more balls and I thought it would have greater load capacity. Anyhow I used the one in the kit and so far all is well.

The fact that the bearing appears to have missing balls may not indicate a failed bearing unless there are other symptoms, like swarf in the oil, feels rough when turned by hand, etc.

Its interesting that there is no apparent damage to the bearing inner or outer race, however, the "cage" that is supposed to encapsulate each bearing is missing from the backside (inside .case side ). That explains why it looks like balls are missing. Ok, now to go find a bearing and get on with it.....

When you take the tranny out of your truck you can check to see if you have a good bearing to use while rebuilding the transmission you want to put in .

Kevinski, now that's a good idea! The transmission I just removed , does not have hardly any wobble on the inut shaft . I may be able to swap it out and install on the "spare" transmission. I had not even considered it, but now that you bring it up - I've got a NOS input shaft ( and everything else but the bearing / gasket kit) that I can use in the "original transmission" that I'm wanting to eventually rebuild. The eventual part is "I don't enjoy working in the shop when its cold ".

So tomorrow, I'll check out the input shaft bearing on the trans I just pulled out and if it looks good I'll do the swap. I already have the small parts kit so the I have the needle bearings and assembly retainer. This just might work.

Thank you!

Ronnie, Not sure if you saw it, but you might be interested in this post in the Other Online Sources forum. The bearing listed as "907274" is what you need for your main input shaft/drive gear. It's a replacement number for the original GM# 954358 (New Departure bearing# 41209).

Maybe worthwhile to see about acquiring? The other bearings also for the SM420 could be useful somewhere down the road. Happy Holidays my friend!

Good info Dan ! Merry Christmas!

The job will test your patience. The needle bearings will fall out and have to be picked out of the bottom of the case. I don't have a press, but I was able to heat the bearing with a heat gun enough that it could slide over the shaft and be driven home with a piece of pipe. To remove the bearing, I used a puller. The puller's arms weren't long enough so I made extensions.

Thanks for the information, much appreciated.

There's a trick concerning those pocket bearing rollers. Tilt the front of the transmission down at a pretty good angle before removing the input shaft and the bearing rollers will stay in the hole in the input shaft, even if they don't stay exactly in place. Once the new bearing is installed on the input shaft, pack the pocket with stiff grease to retain the rollers. Back in the dark ages, we used "cup grease"- - - -a very stiff, waxy product that was used in threaded bearing cups like the one on the side of a stovebolt distributor.
Jerry

The needle bearings are held in by a ring that gets pushed out of the way when installed .When you take the shaft out the needle bearings will need to be gathered up and the clip or ring will need to be pulled out with a pick and you can use it again to hold in place for assembly.Kevin picture from the net.

Got it. I had read that and thanks for the reminder. I have the small parts kit which contains a new set of the needle bearings and a retainer.....but as you've suggested, I'll be sure to locate the ones that fall out and inspect them and reuse if good. Hotrods suggestion to tilt the trans is a good idea as well. I have the J-2669 Clutch Gear Remover so that will help with the removal.
Thanks everyone for your suggestions!

Here’s an idea on holding the gearbox while removing that input shaft bearing…….

That's a good bracket you got there. Here's what I made a few years ago.