The Stovebolt Site Forums The Shop Area Driveline SM 420 question

I've heard there's a way to couple up a SM420 to a torque tube rear end. Can someone provide some details on that? Thanks!
Jerry

If it was chosen as an option in the 1/2 ton. Prior to the extinction of the torque tube, about 54-55 right?
Standard in 3/4 ton which used a short enclosed forward shaft and then open rear shaft.
Info/parts in the regular parts catalogs.

Is the torque tube the same length for the 3 and 4 speed options in the 1/2 ton?

Yes, as far as I know.. but I've never had a 1/2 ton. Some friends have swapped these transmissions in their 1/2 tons without issue.

Yes - and, of course, they both use the same u-joint coupling assembly housing-cover.
The rear "housing" of the enclosed drive-shaft SM-420 4-speed [1/2 ton only in 1954] is the bolt-on cast-iron end-piece for a torque-tube.

The 3sp and 4sp u-joints yokes are different.

If you have pre-1954, you should use a rear-of-transmission mount on the existing frame cross-member.
A reproduction mount is sold by Classic Parts.
https://www.classicparts.com/1947-54-Rear-Transmission-Mount/productinfo/79-204/

Thanks! I'm asking on behalf of someone else who is considering a SM 420 swap for a truck that now has a 3 speed.
Jerry

The '47 second series 3100 torque tube and transmission were a one year and done configuration. I believe the '48-'53 are interchangeable.

Even with the SM-420 4-speed, he'll still drive it like a 3-speed...that 7:1 first gear is not much good unless you're in a parade.

Mike B

Yep, anybody who has ever owned one knows that- - - -some people just have to test the electric fence for themselves!
Jerry

As a recent convert from the 3sp to the sm420, There is that granny gear that is nice to play with but the best advantage over the 3sp is having the syncho on the (2nd) gear cluster, the gear you normally start out in. I hated to either have to make a complete stop to get it into first on the three speed.

Good Point sstock, I hadn't thought about it that way!

Mike B

My 1953 half ton had the SM420, liked it a lot.

Pretty bullet proof, but pretty noisy too but hey it is a kick @ss truck transmission that can take any abuse.

I just swapped out my sm420 for a 3 speed. Just wanted to put my truck in original configuration. (the 4 spd was installed later in its life). I wanted to go back to the 3 on the tree. It is an easy swap. Like others have mentioned the u-joint/yoke is different. The mount for the pre 54 is used for either transmission. The torque tube end is the same as well.

The 1st gear on the sm420 is def not gonna be used unless in a slow parade or working on the farm. My sm420 was pretty noisy, I was shocked how much quieter my truack is now with the 3 speed.

The sm420 is HEAVY so be careful installing it, the 3 speed is LIGHT.

Good luck!

There is one other good use for the SM420 1st gear on a "normal" truck (one that does not normally need to get a real heavy load rolling):

Win the "Slowest [non-stalling] Truck Race" at trucker shows/gatherings:
- set the idle as low as possible (no stalling is allowed)
- you can get out of your truck and walk along side of it, while you talk with onlookers
- they might have to end the race and declare you the winner, due to time constraints (if it is a 1/4 mile race course)

Two other good uses for an SM420:
- safely pull a VW beetle out of a ravine without burning a clutch (done that)
- drive up the side of a barn (almost tried that in an altered state-of-mind)?

I use 1st coming into a car show. Looks like I have an automatic.😉

Don

I pulled tree roots out when our front tree fell, using my SM420. And guess what that did.........it twisted the splined end of the TT driveline. I'll never forget that mistake. LOL.

Here's a pic of it:

[img]http://www.cmaynard.com/Truck/TwistedShaft2.JPG[/img]

Wow, you found the weak link.