The Stovebolt Site Forums The Shop Area Driveline SM420 mating To Torque Tube

I got a hold of a bunch of AD parts a few years ago. One of the items was an SM420. (I have a '47 1/2 which has a 216 with a 4 speed (non-syncro). I plan to complete a rebuilt 235 that was part of the bundle of parts and install it in the '47 along with the SM420 and a torque tube rear end. I'm not sure of the year of the torque tube, but it does have Bendix brakes so I'm assuming these will all mate together properly.) The seller said the transmission probably has water in it. (It's been stored in my garage since I brought it home.) So I've finally started to clean the case and open it up and dig in to it. When I removed the drain plug, rusty water came out. There is an odor of gear oil but its odor is off. The shifter was frozen in first gear. The transmission had a drive shaft parking brake (which I understand was used in the big trucks with an open drive shaft). I removed the park brake (see attachment 1) and the rear cover of the transmission (attachment 2 and 3). I was able to get the top cover off and remove the shift stick. I sprayed the fork assembly with Thunder Bolt and the mechanism immediately freed up. I was able to put the top assembly into the 4 forward gear positions. Inside, the components don't look too bad. From the top view, the gears and synchro rings look to be on good shape (no broken or pitted teeth). So, I have a few questions which I'm sure that the great knowledgeable minds of Stovebolt will be able to help me with. I've looked at a shop manual but there are things I'm not sure about.
1. How can I remove and clean the shift rods and forks in the top cover?

2. How can I unfreeze the gears so I can start to take it apart, clean and rebuild? (I have a rebuild kit. I'm hoping the gears and synchros are ok but have a better idea once I can free things up.

3. What do I need for the rear of the transmission (see attachment 4) so I can hook it to the torque tube?

Thanks in advance!

You’ll need the parts on the back of an Sm420 that fit the torque tube. Looks something like this

There is one on eBay,
I’d buy it if I were you, you don’t see them often with the torque tube parts.

https://www.ebay.com/itm/184067418693

The forward yoke of the universal joint is unique to the SM-420 transmission. The one that fits the 3 speed doesn't work with the 420 transmission, and I believe the crash box 4 speed U joint is also different. That part is very difficult to find, and expensive when you do locate one. It also incorporates the speedometer drive gear into the U joint assembly. The U joint cross and the rear yoke are the same as the ones used on the 3 speed torque tube setup.
Jerry

I believe the crash box 4 speed U joint is also different
Need clarification ?

The four speed transmission front yoke for 1/2 ton torque tube,
is different from the 3/4 ton four speed transmission front yoke.

48-54 4 speed Front Yoke 1/2 ton Part # 3686323 (closed driveline torque tube)
48-53 4 speed Front Yoke 3/4 ton Part # 3686324 (I believe the 3/4 ton Front Yoke is enclosed, but the rear half of the drive shaft is open)
Technically both the 1/2 ton to & 3/4 ton u-joint assemblies are enclosed but different. The 1954 3/4 ton drive shaft is all open.
Source Grp # 5.555

xfiles

if your torque tube rear end has Bendix brakes, you may be good to go. The crash box transmission is about 2 inches shorter then the SM420. I would think that the SM420 would work with a 51 rear end (Bendix brakes) but would be to long for a 47 torque tube differential. Worth checking. Torque tubes come in a variety of lengths.

1947 was a one year only, and was longer than the 1948-54 torque tube.

Now that I'm aware the front yokes at the rear of the transmission are different for the 1/2 ton and 3/4 ton trucks (thanks, jorb), I was wondering if the retainer (I've been calling it a cover) (4.317 on the diagram) the same or different between the 1/2 and 3/4 tons? Also, see Tony292's first post for picture of retainer. Thanks.

I think they are the same, but I'm not a 100% for sure.
You should call these guys http://nwtparts.com/ ask for John,
He knows a lot about these old transmission.

In the meantime here is a 1950 3/4 ton SM420 rebuild
https://pbase.com/dennygraham/sm420_rebuild&page=all

1950 3/4 ton enclosed front driveline rebuild.
https://pbase.com/dennygraham/driveline&page=all

Because you're trying to mate this up to a 47 torque tube its a little tricky because its a one off year.

John will set you straight on what you have to do.

You need to find out your torque tube length and ID the transmission & rear axle.
Carrier Assembly: 1947, 1/2 ton, 62 1/2" long (Torque tube)
Source, Grp # 5.504

xfiles

I have a torque tube from a later truck. It has Bendix brakes. I plan to use that torque tube and not try to mate the tube from the'47. I'll do some measuring and give Northwest a call. Thanks for the links.

I measured the length of the tube and it is 60 1/8" which makes it '48 to '53. The casting number on the transmission is 591953 which makes it a '48 to '53. Have a call into John at Northwest transmissions and hopefully will hear from him later this week.

I talked to John at Northwest Transmission (nice guy) and he said the rear retainer (cover, tail housing) is the same for the 1/2 ton and 3/4 ton trucks. The front yoke (as previously mention is specific for the 1/2 tons with a 4 speed and is difficult to find. The rear yoke is the same for cars and trucks. The Filling Station listed a SM420 universal joint assembly (NOS) but it's very pricy ($400 - which Jerry alluded to) but it's not available currently. I'll post in Parts Wanted and hope for the best. Maybe. just maybe, I may find what I need.

Rusty, I had a front yoke for an enclosed sm420 last month and sent it to Australia, I'll dig around might have another.
Steve

Ok, thanks, Steve.

Good to hear you talked to John.
Did he have a rear bearing retainer cover, used with the torque tube drive line?
Did he say that the u-joint assembly for the 3/4 ton is different than the 1/2 ton u-joint assembly?

I'll hunt around on Craigslist to maybe find one.
I found that you are better off looking for the entire SM420 transmission, because most of the time
they still have the 1/2 ton ball housing and u-joint assembly attached.

Found the parts I need from a link jorb sent me. Thanks to all for your input.

Good morning all,

Just found this thread. I just bought a '40 1/2 ton. From what I can tell it's a bit of a Frankstein truck with parts from other trucks bolted on. It has post '47 rear fenders, a '53 235, the trans appears to be a Muncie SM420, pre '55 from what I'm reading (no fins on the housing). Thus far I haven't been able to ID the rear end. There's no torque tube on the drive line. I think I've decided to do a V-8 swap, possible a 327. Would the SM 420 work with that engine? Would I need an adapter to mate it to the 327? If the SM420 isn't a good choice any suggestions? What should I be looking for on the rear end to ID it? I've attached some photos I've taken so far. I'll be pulling the body off so I can sand blast/powder coat the frame and possible box it.

Joe

420 will bolt up to GM bell housing. Retainer dia. Should be 4.875”, so make sure bell housing is not 5.125”, otherwise need spacer from AA.

Ed

Hi Ed,

This one is bolted to a '53 235, I'm just not sure what the rear end is yet. Any idea why it won't go into reverse? It has the safety lever on it that you have to pull up to move it all the way to the left but once there it won't go back into reverse. I had it in there once while it was parked and not running but I can't seem to get it there now. It goes into all 4 forward gears just fine.

Also, if it's out of a '53, based on the engine casting number, shouldn't it be a closed drive system? This is why I'm not sure what the rest of the drive line is.

Joe

If the truck is not moving and the engine not running, the gears might not be lined up to go into reverse, crank the engine or move a wheel and see.

Ed

I’ve tried it both ways and still can’t get it into reverse. Looks like I’m going to have to open it up.

Morning Trad,

Whoever swapped out the 216 for the 235 also put in a 12 volt system with an alternator. It's looking more and more like I'll be going the "Modified" route with a 327, maybe a T5 and upgraded suspension (Mustang II front and maybe a 4 link rear. I'm pricing out all the goodies I'll need to come up with a plan. The Muncie, from what I'm reading is a really heavy duty transmission, just has a first gear that's useless (unless you're on a farm or hauling a very heavy load, and no overdrive gear. I may be able to sell the 235 and the Muncie to someone looking to do a "restoration". It's been suggested in another forum that the rear end may have been from a sedan given the 5 lug rims. Right now the plan is to put it in the garage up on jack stands and start disassembly and see what's actually there.

Joe

OK, before I pull this thing apart I have a question. I've been reading as many posts as I could find on the SM 420 and looking at as many photos and other info. I have not seen a photo of a 420 that has a rod coming up the side of the shift lever with a tab at the top to pull so that the shift lever can move all the way to the left to go into reverse. Mine has that rod. Everything else about it looks like a SM 420. I just want to make sure I've got the right transmission info before I pull it apart. This is what it looks like. The shift lever will not go all the way to the left without lifting up on the lever that pulls the rod up. But it still will not go into reverse, with the engine running or stopped.

Joe

the 420 is a great transmission with 2-3-4 synchronized, a big improvement on the earlier crashbox. As for your earlier question, I had a 327 with a later 420 behind it in my '63 c10. as far as I know it will work with any sbc with the correct bell housing. If I were messing with it anyway, today I'd opt for the t5 to get the overdrive ilo a 420.

The box in your photo is not an SM-420. It's the old crash box non-synchronized 4 speed, which is the only one with a reverse lockout. It will also have a different bellhousing bolt pattern, so a 420 will not bolt up without swapping to a 1948 or later bellhousing.
Jerry