Technical 1957 Chevy 3 speed to 4 speed swap

Hello, I just picked up a 57 Chevy pickup with the 3 on the tree and 235 engine. The tranny tends to keep popping out of gear and overall just never been a fan of these 3 speeds. Will a Muncie 4 speed Sm420 or Sm465 bolt right up on these trucks? Also will I have to change the bellhousing and or the driveshaft? Thanks!

 

The SM420 transmission will bolt right in, but you'll need a different driveshaft, one with a slip yoke built into it.

The 465 requires a bellhousing with a larger hole, and possibly some clearancing as it's a big transmission.

If you don't intend to use the truck to haul heavy loads, then there's little reason to put in that transmission. I'd be looking for something else.

 

If ya like log trucks, the sm420 is cool.
Will require the slip yoke DS as mentioned.
A Saginaw 4 speed would be the better choice for cruising. That 235 doesn’t need a Muncie. The Saginaw is much cheaper.
the current 3speed is simple to build.
An s10 5 speed is an option.
The sm420, Saginaw 4speed and s10 will work with your current bell housing
A latter full synchronized Saginaw 3 speed will work as well.
Chances are any choice other than the original would require a draft shaft shortening. Not difficult to do

 

The original driveshaft would work with the other car transmissions, with the addition of a fine spline yoke, most likely. The 57 3 speed is a full length transmission.

 

Years ago I pulled the three on the tree in my 6cy 55 PU and replaced it with a Saginaw 4 speed out of a Vega . I recall it went right in.
Cobbled up a floor shifter .

Made it a entirely differant vehicle.

 

I wasn’t sure on the trans length.
I’ve got a short truck 3 speed and a couple Saginaw full sync trans at the house.

 

The mid 60s trucks used the shorter Muncie 3 speed.

 

That’s the one I have.
Didn’t know if the 57 was similar.

 

So if I find sm420 tranny and driveshaft from a 55-59 truck I should be bolt and go? Thanks for the help!

 

Maybe. The trans is no issue.
I’ve only seen a couple SWBs with the 4 speed. So cutting down a DS might have to happen. You’ll also get a parking brake on the tail shaft from most 420s. All mine have one. Simple to remove if not needed. The trans is taller than the 3 speed. Wonder if there is a floor mod needed?
If you’ve never used a 420, they don’t shift or feel like a performance 4 speed. You’ll hardly ever use 1st.
1st and reverse aren’t synchronized
They’re tough as nails, noisy and heavy.
I dig em
Floor pic of 4 speed truck

 

The 3 speed floor cover is flat, the 4 speed has a lump as shown in the picture.

A driveshaft shop can make a driveshaft using a slip-stub assembly. The SM420 uses a 1350 series U joint, which is bigger than the 1310 at the rear end.

Most 420 transmissions are well worn out by now. Just a warning, you may not improve things!

 

Personally I'd go with a Saginaw 3 speed out of a mid 70's GM unless you are just hellbent a 4 or more speed. Sycro first gear and smooth shfting and pretty much the same length as the Muncie that is in it. You have to swap yokes on the trans but that isn't a big deal. I have one in my 48 that worked great for probably 70K before the engine got so worn out the pistons were deciding what hole they wanted to go up and down in.
I can't see a Granny 4 speed in anything that isn't going to be a real work truck. Unless you are just flat ass in love with them but they are in no way a hot rod trans even though a 465 shifts a lot smoother than a 420 provided that it isn't worn out. I'd expect to have to rebuild any SM 420 and most SM 465 trans that I found used.

 

Don't know where you're located--I'm in GA, have a full synchro 3 speed out of a '76 Chevy pickup. Has the long tailshaft. Will treat you right on it if you can pick it up.

 

Saginaw 3 with an overdrive.

 

My memory is the LWB trucks used the short tailhouse along with a 2 piece driveshaft and the SWB trucks used the long tailhouse with a 1 piece driveshaft.

 

I’m just curious, but was mentioned about the brake on the transmission. I’ve had 3 TF trucks (SWB) that had the original 4 speed, but all had the lever off the left of the column to set the brakes on the rear axle. Was the 420 an option and came with the transmission parking brake?

 

You will need to add a tailshaft support for a Muncie or Saginaw. They can't hang by the ears like the stouter boxes. Then where is the e-brake? Another consideration.

I like the granny gear boxes myself. With a 3.73 or 4.11 rear they are streetable. I have run them with a 5.14 to 5.38 rear... top speed with a 5.38 was around 50..

 

My last SM420 was in a 1965 C30 and the e-brake was on the transmission.

 

Maybe happened later than the TF pickups? I don’t know when the 420 even started in pickups, I was just going off the thread title.

Edit: wasn’t a C30 a 1 ton, maybe 3/4 ton truck?

 

I’ve put a ton of miles on a Saginaw hanging with behind a 235
Drug a truck with a sm420 out of a field a couple years ago. Changed the dope and zoom zoom. Nabbed another one out of a low mile bus.
Personally I’d don’t want a log truck trans in a 1/2 ton cruiser.
One other issue with using the 420 is adding a carrier bearing x-member. It might interfere with the current parking brake set up. But the tail shaft parking brake is kinda cool.
I picked up a 3speed OD for my 235 but also have a s10 5 speed.
But really would like to keep the column shift so I’ll probably use the Saginaw.

 

3/4 and larger trucks had the parking brake on the transmission, 1/2 tons had them on the rear end.

1/2 ton short bed trucks with the SM420 did not have a center bearing. I don't know for sure about long beds, they probably did. We could look in the parts book and see, eh?

http://chevy.oldcarmanualproject.com/1929_57chevyparts/index.htm

 

The Chevy trucks never had a tail support on a manual transmission until 1973, when they got rid of the bellhousing mounts.

 

Next question anybody near central California Or even PHX,Az area have a decent sm420 laying around? I know a lot of people replace them but I’m not seeing much out there for sale. Ive had these 4 speeds on a bunch of trucks and have been rock solid for me is why I’m shooting for it. I’ve been told these are a popular tranny for rock crawlers?

 

Naw, I’ll trust you, was just curious. Only other TF truck I was around was my Dads ‘59 3800? Dual rear tires, but never crawled under it, or if I did, didn’t pay attention to what was there.

 

My last good one went dirt cheap in the auction when we moved a couple years ago. Sorry.

 

The rock crawlers like them, they put them in Jeeps and other builds. The attraction is the 7:1 first gear. I have been lucky with used ones... but that was years ago. I'd look at a local truck parts yard for one. The SM465 is the newer version as Squirrel mentioned.

These guys are worth a look and may have resources. I was considering one of their hydraulic clutch kts a while back.
https://www.novak-adapt.com/

They even have a transmission rebuild service... hot damn...
https://www.novak-adapt.com/catalog/transmission/transmission-packages/sm420-package/

 

So I believe I located a sm420 in AZ. Would anyone know the length that the driveshaft that will be required once I do the trans swap? The guy I’m getting the tranny from has lots of parts for these trucks but mostly 60-72 gmc trucks. I’m assuming the drive shaft would be the exact same as size of the truck was originally a 4 speed? Appreciate any input!

 

I’m keeping this truck simple and just around town cruiser. Yes, I know the granny gear is useless but I’ve had a bunch of these trucks with it and it’s been perfect for cruising around town. If I can score a tranny for a couple hundred and drive shaft for another couple I’m fine with it. If I was to upgrade to the motor with a V8 I would definitely consider it more. It’s not gonna be street racing with the little straight 6 for sure lol.