The Stovebolt Site Forums The Shop Area Driveline SM420 Questions

I started disassembly of my SM420 today. It's a 1967 model - the last year GM made this transmission. I have watched a number of YouTube videos about rebuilding this transmission. I bought the BK108 kit which contains bearings, seals, gaskets and other parts. I have some questions at this point.

1. Clutch gear retaining nut. This is supposed to be removed using a spanner wrench (wrench like you can use when changing the disk on an angle grinder). I can't see any nut. There is a snap ring. Did they change the nut to a snap ring in the later models? I removed the snap ring, so I guess I can just go ahead and pull the bearing off. This bearing seems to be worn out.

2. The kit does not include syncro cones. I have taken some pictures. Can you tell from the pictures that they are OK? Or what should I look for before reusing them?

On the synchro cones, if the ring doesn't bottom out in the cone, they you can still get some service out of them. If there's no gap between the teeth on the gear and the synchro ring, then they need to be replaced.

Thanks to stovebolt member red450IH. Three years ago, he posted a link to a manual for a later model SM420. It appears that the nut was replaced with a snap ring somewhere during the transmission's evolution. I'm sure GM realised a snap ring was cheaper than a specialised nut! That answers my first question.

http://kinzers.com/don/landcruiser/transmissions/index.html

@Kevin. Thanks. I will check the gap.

Found a supplier of new syncro rings. $15 each plus $22 s+h. A that price, it would dumb to reuse the old ones. They are in the mail.

The ridges on the syncro ring on the main shaft seem pretty smooth so I'm glad I have a new one coming.

Who is this synchro supplier?

https://www.transmissionpartsdistributors.com/sm420-48-67-manual/

They are still in the mail at this time. I will provide an update when I see them.

Gord&fran Your 6th picture shows quite a bit of wear on the brass ring,some wear on the gear engagement teeth. On trannys that have been abused those teeth will be chipped away a lot worse and will be more likely to jump out of gear. Thank you for the tip on a supplier of brass rings. When you get your brass you'll see the gap will be wider as compared to the old synchronizer ring. Should work good for years to come. Also look at your shift collar for chipping and wear,the rule is if it makes noise (grinding) your hurting it .

Fox Saw your interest in sm420 synchro rings ,I think there might be a place near Indy too,have to look at their catalog. May be only car Muncie,saginaw and warner gear. Where is west of the fifth ??

The "West of the Fifth" is a link. Click on it and it will show you a map. (NW of Edmonton)

Thanks for your input. One of the rings is definitely more worn out than the other. The ridges on the inside feel much smoother. `

I'm stuck. Can't get the main shaft to fit back in the case. It seems like it's too long. I have been looking at videos and reading instructions. What trick am I missing??

I don't have SM-420 experience, but have built other tramsmissions...is the cluster gear laying on the bottom of the case? I always install the cluster gears after I got the main shaft in.

Mike B

https://www.flickr.com/photos/advance-design_parts_co/sets/72157632620943718/
Have you read / watched this ? Originally shared by Brad Allen ?

How did it come apart?

Ed

Oh, Nooooo!
If you compare the picture I took of the main shaft in my first post to the picture I posted today, it's obvious where I screwed up. I installed the second gear backwards.

Well I didn't wreck any parts. Just wasted time...

Thanks for the slides from Brad Allen. It shows how it should be assembled. Better than looking at the exploded view.

Search Denny graham sm420 he has lots of pictures on his site rebuilding his

OK. Next problem. Transmission was reassembled. I tested it on the bench and it shifted into all 4 gears and reverse - no problem - it seemed really good. I left it in neutral and removed the shifter. Now after a lot of hard frustrating work, it's back in the truck. However, during all thisfighting, it seems to have slipped into gear. I installed the shifter and I can't get it to change gears.

Is there any way to fix this without removing the top cover? Like poking around in the shifter hole and moving things with a screwdriver? I glued the top cover down with permatex and really don't want to remove it again if I don't have too.

If one opens a PTO plate, might be able the shift a gear with a screwdriver. Moving top cover would be easy, too.

Ed

Yes, should be able to move shift rails w/ small pry bar.

Yes, that worked. Thanks.

Finished the job and went for a test drive. Almost all the issues are fixed. It still jumps out of second when descending a steep hill with foot off the gas pedal. Probably going to have to live with that.

Next job is to replace the generator/regulator with an alternator.

Gord, did you replace all three main shaft bearings, shouldn’t pop out of second if you did. Spring behind ball bearing to retain gear selection might be gone or broken.

That would be on the shift rail.

Ed

I replaced the main shaft bearing, the clutch gear bearing and the main shaft pilot roller bearings. All were included in the kit. I didn't find a spring behind the ball bearings and I don't see on on the exploded view of the transmission.

Gord, what Ed is talking about is a spring that is in the cover. If it was jumping out of gear before you rebuilt it and its still jumping out of gear now, the spring on the first and second speed shifter yoke may have been the culprit. See attached file. You'll probably have to download it to view it.

Oh, OK. Now I see what's meant. Thanks.