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1971 sm420 swap with 1954 3 speed #1321670 Tue Aug 13 2019 03:37 AM
Joined: Nov 2013
Posts: 108
S
Sulli Offline OP
Shop Shark
Is it possible to take a 1971 open drive shaft SM420 and modify the u-joint and u-joint housing section to mate up to my 1954 torque tube drive shaft/differential? I currently have a 3 speed and am tired of it sticking in 1st gear. I'm hoping that only the housing for the sm420 u-joint and possibly the u-joint itself would need to be replaced. Any thoughts?
Image 1 is my current 3 speed. Image 2 is from Denny's SM420 rebuild with an enclosed driveshaft. Image 3 is the 1971 SM420 in question.
Any help would be appreciated!
mark

Attached Files
current 3speed.jpg (89.96 KB, 190 downloads)
DennySm420.JPG (140.15 KB, 195 downloads)
sm420.JPG (72.47 KB, 192 downloads)
Re: 1971 sm420 swap with 1954 3 speed [Re: Sulli] #1321681 Tue Aug 13 2019 06:52 AM
Joined: Feb 2001
Posts: 1,049
D
Dusty Offline
Shop Shark
The rear portion that you have circled in red comes off so you could swap with an earlier trans rear extension .

Re: 1971 sm420 swap with 1954 3 speed [Re: Sulli] #1321697 Tue Aug 13 2019 01:09 PM
Joined: Jan 2013
Posts: 1,523
S
sstock Offline
Shop Shark
Yes indeed as Dusty says.
You will discard the existing yoke and rear bearing retainer and seal, but have the speedometer drive gear pressed off the yoke and remove the speedometer driven gear from the rear bearing retainer.
Then find you a rear bearing retainer from a 1948- 1954 1/2 or 3/4 ton sm420, make sure you get the front yoke to go with it. You will need a new Ujoint https://www.fillingstation.com/detail/4114/Chevrolet_19401954_UNIVERSAL_JOINT_REPAIR_KIT.html. You will able to retain the existing rear yoke that you use with your 3 speed that slides into the torque tube.

The torque tube rear bearing retainers are starting to get scarce because everyone has threw them away but they are around. I have sourced at least one vendor that sells a complete new front and rear yoke as well but it is very spendy. https://www.fillingstation.com/detail/4985/Chevrolet_194854PU_4SPEED_UJOINT_ASSEMBLY.html

Good luck,
Steve

Last edited by sstock; Tue Aug 13 2019 01:10 PM.

1953 Chevrolet 3100
261 cu inch, sm420, 3.55 rear, omaha orange, still 6 volt, RPO green glass, side carrier spare, all done
https://www.youtube.com/watch?v=Bli...n05i04t1aokgm4p04jiwgffwhyyih5xbk0h00410
1964 GMC 1000
305 Big Block V6, sm420, the next cab off restoration
Re: 1971 sm420 swap with 1954 3 speed [Re: Sulli] #1321731 Tue Aug 13 2019 05:22 PM
Joined: Jul 2000
Posts: 745
G
Gord&Fran Offline
Shop Shark
They stopped making the sm420 in 1967. How did you determine 1971?

The 1967 SM420 was the only year with the backup light switch. (that can be seen in your photo)


Gord
----
1954 1/2 ton 235 4 speed
"I am not the kind of person who gets things done yesterday. I get them done when I feel like it. Often I do not get them done at all" - Joe Queenan
Re: 1971 sm420 swap with 1954 3 speed [Re: Sulli] #1321744 Tue Aug 13 2019 07:08 PM
Joined: Jan 2013
Posts: 1,523
S
sstock Offline
Shop Shark
They made the sm420 into the 80s on some specialized vehicles, I'm told


1953 Chevrolet 3100
261 cu inch, sm420, 3.55 rear, omaha orange, still 6 volt, RPO green glass, side carrier spare, all done
https://www.youtube.com/watch?v=Bli...n05i04t1aokgm4p04jiwgffwhyyih5xbk0h00410
1964 GMC 1000
305 Big Block V6, sm420, the next cab off restoration
Re: 1971 sm420 swap with 1954 3 speed [Re: Sulli] #1321761 Tue Aug 13 2019 11:36 PM
Joined: Nov 2013
Posts: 108
S
Sulli Offline OP
Shop Shark
Thanks Steve for what I would need to do in order to make it work. I was hoping it was along those lines of swapping out the backside. Pricey and scarce parts may kill the deal, and I may just look around for the right vintage unit instead. They pop up every now and then, but shipping is a pain/expensive and salvage yards in my area have no idea what I am talking about when I go looking... I've rebuilt the 3 speed shift box, replaced the bushings, made sure the levers were parallel and adjusted the shift lever clearances per the manual, but it always starts hanging up once the temp hits 185 degrees. Every time I think I have figured a way to fineagle it into working, it always lets me down. I'd be happy slapping a hurst floor shifter on it just to avoid all those troubles.

Gord and Sstock, good catch! I misread the ad... I just looked at it again, and it said that it came out of a 71 chevy pick-up, but was a 1960's SM420 unit. The casting code is 3896779 (picture is below), which probably aligns with your thought of it being a 67 because of the backup light switch.

You guys are good!

Attached Files
casting code.JPG (58.29 KB, 120 downloads)
Re: 1971 sm420 swap with 1954 3 speed [Re: Sulli] #1321812 Wed Aug 14 2019 04:26 AM
Joined: Jul 2000
Posts: 745
G
Gord&Fran Offline
Shop Shark
Glad that you know what you have. I brought it up because sometimes people who sell things can be misleading.

I have the same transmission. A previous owner of my truck removed the 3 speed and installed a 4 speed, keeping the torque tube. I found and installed the 1967 4 speed, ditching the torque tube and rear end; I wanted highway gears.

Thoughts:

Don't forget that installing the 4 speed means you will have to either find a new floor plate for the cab or chop a hole in the old flat floor plate.

Gearing wise, installing the 4 speed isn't really an improvement over the 3 speed. It's probably less work to simply fix what's wrong with your 3 speed.

If I was doing it all over again today, I would install a T5 instead of the sm420.


Gord
----
1954 1/2 ton 235 4 speed
"I am not the kind of person who gets things done yesterday. I get them done when I feel like it. Often I do not get them done at all" - Joe Queenan
Re: 1971 sm420 swap with 1954 3 speed [Re: Sulli] #1321848 Wed Aug 14 2019 04:28 PM
Joined: Jan 2013
Posts: 1,523
S
sstock Offline
Shop Shark
One improvement of the sm420 over the three speed is the ability to have a synchronized gear to downshift into to get the truck going again without stopping.


1953 Chevrolet 3100
261 cu inch, sm420, 3.55 rear, omaha orange, still 6 volt, RPO green glass, side carrier spare, all done
https://www.youtube.com/watch?v=Bli...n05i04t1aokgm4p04jiwgffwhyyih5xbk0h00410
1964 GMC 1000
305 Big Block V6, sm420, the next cab off restoration
Re: 1971 sm420 swap with 1954 3 speed [Re: Sulli] #1321905 Thu Aug 15 2019 12:49 AM
Joined: Nov 2013
Posts: 108
S
Sulli Offline OP
Shop Shark
Right about the floor plate mod and understood on the sm420's 1:1 ratio and the fact that it is still basically a 3 speed with an "optional" super low gear for first.

I've thought about the T5, and the only thing holding me up would be $$ and how to make the differential fit with the new leaf springs I have installed as well as finding the right one (Lugnutz's website has tons of info). I think I would end up needing to buy a differential, driveshaft and having to get the leaf spring perches fiddled with. More than I want to spend now even though the T5 benefits would be great.

The sm420 full syncro aspect would be really nice as well. Honestly, I had thought about finding one of those 100% non-synchro 4 speed crashboxes because I was so fed up with the 3 on the tree hang ups. I seem to remember reading about someone who rigged a wire or string through the firewall and around the 1st gear shift lever so he could yank it into neutral from inside the car rather than pulling over, turning it off, opening the hood, and pulling the lever each time it got stuck! Such a simple trick, I may go make that happen instead...

Re: 1971 sm420 swap with 1954 3 speed [Re: Sulli] #1321912 Thu Aug 15 2019 01:57 AM
Joined: Mar 2019
Posts: 387
G
glenns towing Offline
Shop Shark
Three on the tree hangups? Find out whats wrong and fix it. Linkage slop, motor mounts, adjustments, trans issus, etc? Theres something that should obviously jump out if the problem is happening as often as you seem to indicate. I drive a 51 2dr sedan with a 261 and 3 on the tree pretty aggressively without to many issues. But when something happens often, Its usually easy to duplicate and diagnose. JMO

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