SM 420 new synchros and bearings

Yes, I'm watching, I just have nothing to add since I have never rebuilt or "refreshed" one of these.

 

should be done tomorrow. I have never done one of these either. Its really pretty simple. i think the gears themselves are bulletproof. Its pretty easy to work on. I have put together and taken the whole thing apart at least 20 times checking things, swapping things, making sure the brass sleeve that 3rd gear or whatever rides on is good. Pressing the main shaft together, pressing it apart again to check something that was done just fine the first time. I am not satisfied with the bearing on the front of the countershaft. It might blow up, might not. I will be the first to know.

 

They are all fairly simple after you get your grubby paws on them, shift things around and see how the gear ratios change.
As you found out, the bearings can be a challenge especially on a transmission that may be 60 years old.

 

yeah, the bearings I have issue with are brand new. The old ones weren't good, but did work well enough. I don't really know how I shifted this thing into high gear at all with that wrecked synchronizer cone. It also wasn't real noisy. I am not sure what it would take to destroy one of these transmissions. Maybe all it needs is me "fixing" it?

 

Getting it back together again, then apart, adjust and back together and apart and ok enough!
In order to take out the mainshaft back half the rear being must be removed, so to put the shaft back in you need to press the bearing on the shaft while the shaft is in the case. I just used a pipe and tapped it not he shaft while holding the shaft in my left hand and tapping this pipe with a hammer in my right. Its not super close fitting, just enough to need a puller to get it off. I bet if you had a bearing heater it would slide right on. toaster oven.

tap that rear bearing in the case and shove the input shaft in. make sure to not lose the roller bearings, use a sticky transmission assembly "grease". i used to use wheel bearing grease but thats really bad. got lucky with a couple t-90s. last one failed. could have been me? This stuff is guaranteed to dissolve.

I used the missing roller bearing retainer that looks like a special washer that was missing in my transmission from the spare. It shows it on the exploded diagram, so i used it. All thats left is gaskets, front bearing retainer and the Novak adapter that is also the rear bearing retainer.


I did have to take apart the gears a couple times to find the sweet spot where they moved on the main shaft easily. There is one or 2 ways that they moved easily and several where they were sticky.

I found this to be an easy job, the transmission is pretty simple and with good pics to reference as you take it apart with a shop manual and reassemble it. Any of you guys can do this. One thing that never ceases to impress me is the guys who designed and built these over 60 years ago on non CNC machines and no computers for design. Its an incredibly impressive thing for me to think about how a 70 year old design is still very functional, robust and a pleasure to work on.

 

Nice work, Chris. I've had T-14s, Saginaw 4-spds, T-18s and an NP435 apart, but never an SM420.

...and THAT is the major difference between old stuff and new stuff.

 

Ahah! but theres the rub, if it was carp to begin with, it didn't survive this long. thats why old stuff seems to be so good, it survived. the garbage fell away and you're left with an idea that everything was good. Nope, the junk got melted. Time will tell what will last.

 

I had some real head scratching with the mechanical noise emanating from the clutch hub area. Turns out that when assembled you can push the clutch sleeve forward too far and it rubs on a part of a gear. When its located correctly by the shift fork the noise does not exist. While turning the transmission and running through the gears with just enough gear oil to coat the gear faces, it makes no noises and shifts fine. They almost all do with no power. Tell you how I feel about this in the spring. For now it sure looks pretty on the bench.



Now, go rebuild your transmissions and post em up.

 

Great job writing this all up. You had to spend a lot of time taking photos and writing it out step by step. It makes a great read.

Thank you!

 

It only takes a second to take a pic. I always have my cell phone on me so its really easy. The greatest part of all of this is that I have reference photos to look at while I am doing it. It actually made it easier to complete.

I did so many searches for any info on this transmission. There were no write ups. There is now. Hopefully this gives someone the confidence to tackle this job.

Thanks to all of you for reading this. I appreciate it.

 

I've got a couple of SM420s sitting on the bench awaiting my attention and I enjoyed this write up. Thanks. I find it distracting to take photos during a project, especially one with grease and oil involved. .

 

I have to answer my phone for work many times on days I am not actually onsite, so gloves are a must. Otherwise, my phone would be a greasy rat in my pocket.

This transmission has been worked on before. It was missing the roller bearing retainer that goes on the mainshaft inside the input shaft that looks like a thick washer and the reverse lever pivot was not a pressed in job, but a bolt, where the case was threaded to accept it and a jam nut on the outside. I also did not take out the reverse idler gear set because there is nothing I can do to repair any problems, there weren't, and there is a dowel pin that goes into the reverse idler shaft that looks like a blind hole. Super PITA for something I can't do anything with.

Happy transmission repairing.!

 

Chris i like your posts because they are very detailed and informative like above catching "the roller bearing retainer ".

 

I'm new to this forum but have been a Mechanic for a living going on 30 years now, I like how you work Chris you are very organized and neat. One of the best things you can do is take lots of pictures on disassembly , parts don't always fit or come in on time, plus life has a habit of messing with plans its amazing how you can start second guessing yourself on reassembly ! I was always under the impression that the SM in SM420 stood for Saginaw Muncie If my grandpa was alive I could ask him he worked for Hyatt bearing for GM. I really like this forum everyone is so friendly and helpful and seem mainly interested in keeping Jeeps going instead of bickering , I like all the pictures it makes it easier to wrap you brain around concepts that are new to you. Bet your trans works great. T.J.

 

T.J. Thanks for the kind words. I am glad I took so many pics and posted them because it really did help on reassembly. Hyatt bearing was GM? The bearings I took out were Hyatt IIRC. I like this site too because of the absence of bickering. I am too old to argue about this stuff. I hope this thread helps someone who needs this info. I have no idea if my transmission works right or not, haven't put power to it yet. It may spen righ and shift gears nice on the bench, but the proof is if it does that under power. Spring is the time.

Thanks,
Chris

 

47v6: Can you tell me what your total cost was on this rebuild, including parts? I got a local quote to redo my 68 SM420 for $350. If the difference isn't too much, I might just pay to have it done

 

The rebuild kit is 135 bucks. it does NOT come with clutch keys or synchronizer rings. The brass synchro rings are 12 bucks to 20 bucks but you need to make sure you get the right ones. the Novak kit comes with them. I got all the parts for under 175 bucks with some assembly lube. You really need a press to get off the synchronizer clutch hub, but other than that its pretty straight forward for a refresh. This kit does not have the gear sleeves or any of that stuff and i am sure your guy would not be putting in new ones for you either at that price. It seems fair.

 

I don't have a press - my "shop" space is limited. So I might just farm this one out

 

For 350 bucks, it seems like a good deal. Remember, only the Novak kit comes with the synchros. You might want to make sure your rebuilder has new ones and the clutch keys seem to be difficult to obtain. If you know the transmission functions as it should and doesn't pop out or gear or grind going into 3rd or 4th, then you might not need a rebuild at all. It is a very robust transmission.