The Stovebolt Site Forums The Shop Area Driveline 1971 sm420 swap with 1954 3 speed

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

The rear portion that you have circled in red comes off so you could swap with an earlier trans rear extension .

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

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)

They made the sm420 into the 80s on some specialized vehicles, I'm told

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!

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.

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.

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...

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

I've focused mainly on the shift box rebuild and shift lever adjustment per the manual. I've replaced the bushings, so beyond that I am at a loss as to how I could diagnose beyond those things. The linkage seems to be tight/not sloppy and the levers at the shift box seem tight as well. They all seem to operate smoothly if I grab them individually and move in/out of gear by hand.

I watched a 3-speed rebuild video and noticed on the inside of the cover plate, that there was a return or tension spring in there to pull on one of the internal levers. is it possible that is the issue if the spring is broken, or would it not work at all if that were broken? (The picture below is just an example I found on ebay for the spring I am wondering about)

You mentioned motor mounts, can those affect shifting? If so, what do I look for?

There is a little leakage at the shift levers on my cover plate. If that means worn bushings, and could that affect shifting?

Sulli: I just did this change on my 1953 model. I found the bearing retainer on ebay. The yoke was incredibly hard to find, another fellow Stovebolter bailed me out. Make sure you order the gasket set that goes over the torque tube ball. I would definitely seek the guidance from more experienced truck guys in order to get that connection correct.

Steve: I can't get that link to pull up a price for one of those U-joints. How expensive are they? You can PM me if that would be more appropriate.

Well, I took another look at the measurement between the steering wheel and the shift lever, and while I have been thinking that measuring from the thinnest part of the wheel was correct, I decided to try it from the thicker part between the finger grooves... That did the trick (thanks Glenn). Just drove around town for a while and did not have one hang up leaving stop signs. (Why I didn't try that sooner?) Lesson for me is that I shouldn't try to overcomplicate these things. The truck manual is pretty plain and straightforward and should be followed as such.

Green 98, I sent you a PM.

Thanks!
mark