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SM420 mating To Torque Tube
#1353187 Fri Apr 03 2020 11:08 PM
Joined: Sep 2007
Posts: 432
R
'Bolter
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!

Attached Files
Drive shaft brake.jpg (149.69 KB, 326 downloads)
Attachment 1
Transmission rear cover 1.jpg (168.92 KB, 329 downloads)
Attachment 2
Transmission rear cover 2.jpg (176.88 KB, 328 downloads)
Attachment 3
Transmission Rear view.jpg (137.29 KB, 333 downloads)
Attachment 4

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Re: SM420 mating To Torque Tube
Rusty Shackleford #1353204 Sat Apr 04 2020 12:28 AM
Joined: Oct 2019
Posts: 302
T
'Bolter
You’ll need the parts on the back of an Sm420 that fit the torque tube. Looks something like this

Attached Files
Re: SM420 mating To Torque Tube
Rusty Shackleford #1353207 Sat Apr 04 2020 12:32 AM
Joined: Oct 2019
Posts: 302
T
'Bolter
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

Attached Files
8B4961D7-AE74-4C3B-916E-9678F82E2C5F.png (1006.67 KB, 317 downloads)
Re: SM420 mating To Torque Tube
Rusty Shackleford #1353225 Sat Apr 04 2020 01:33 AM
Joined: Feb 2004
Posts: 23,217
H
'Bolter
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


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Re: SM420 mating To Torque Tube
Hotrod Lincoln #1353406 Sun Apr 05 2020 02:01 AM
Joined: Mar 2011
Posts: 1,509
J
Shop Shark
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 [chevy.oldcarmanualproject.com]

xfiles

Attached Files
1941-54 u joint .jpg (137.59 KB, 272 downloads)
Re: SM420 mating To Torque Tube
Rusty Shackleford #1353476 Sun Apr 05 2020 03:17 PM
Joined: Aug 2005
Posts: 247
D
'Bolter
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.

Last edited by Donf; Sun Apr 05 2020 03:21 PM.
Re: SM420 mating To Torque Tube
Rusty Shackleford #1353536 Sun Apr 05 2020 07:11 PM
Joined: Jan 2001
Posts: 5,145
P
'Bolter
1947 was a one year only, and was longer than the 1948-54 torque tube.


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Re: SM420 mating To Torque Tube
Rusty Shackleford #1353578 Sun Apr 05 2020 10:10 PM
Joined: Sep 2007
Posts: 432
R
'Bolter
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.


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Re: SM420 mating To Torque Tube
Rusty Shackleford #1353658 Mon Apr 06 2020 02:49 AM
Joined: Mar 2011
Posts: 1,509
J
Shop Shark
Originally Posted by Rusty Shackleford
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 [chevy.oldcarmanualproject.com]

xfiles

Attached Files
torque tube length.jpg (32.67 KB, 198 downloads)
3600 front drive shaft.jpg (83.57 KB, 191 downloads)
Re: SM420 mating To Torque Tube
Rusty Shackleford #1353676 Mon Apr 06 2020 04:18 AM
Joined: Sep 2007
Posts: 432
R
'Bolter
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.


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