The Stovebolt Site Forums The Shop Area Driveline SM420 to replace 3-speed in 57 3100 (driveshaft the same)

I picked up a 64-67 vintage SM420 4-speed that I want to swap into a 57 3100 with the original 3-speed set up intact. I don't know what size truck that the 4-speed came out of, just its approximate age. Did GM use the same driveshafts from 57 to the mid 60's? Same driveshaft in a 57 3-speed as a 57 with the optional SM420? There is a front yoke still attached to this mid 60's SM420 that I hope will attach to the original 3-speed driveshaft. I don't have the 57 in my possession yet to eyeball it in person. I'm hoping that there wasn't some special 55-59 only 3100 SM420 yoke to hunt down. Thoughts?

If your new SM420 has a driveshaft parking brake it came out of a big truck, the 1/2 ton units don't have a brake on the rear, they use the rear axle parking brake.

The drive shafts are not the same between 1957 3-speed and 4-speed, the 3-speed has the driveshaft slip joint as part of the transmission tail shaft (yoke slides into the trans with U-joints on both ends of the shaft) while the 4-speed has the yoke flange bolted to the trans output shaft and the slip joint as part of the driveshaft (shaft has U-joint at rear and slip splines on the front end).

Mike B

Mike B,
Based on your SM420 description, I have the 1/2 ton unit. The guy I got it from said it came out of a pickup. All of my transmission/U-joint experience up to now is all from 1946 to 1952 torque tube stuff. My first "modern" open driveline will be this 57. Does this mean that I'd have to find a 4-speed driveshaft to use this transmission? Jim Carter Truck Parts lists ME288 U-joint that fits 3-speed front and 3 or 4 speed rear. They also list ME680 (that they happen to be out of) for U-joint complete 4-speed front. Any way to use 55-59 front yoke and U-joint that fits 3 or 4 speed rear and retain my 3-speed driveshaft? Thanks!

SM420 is shorter than your 3 speed so you need a longer driveshaft. A drive shaft shop will be able to make one for you.

I still have the mindset that 3-speed and 4-speed transmissions are the same length and interchange as in 47-53. I need to readjust my plan. I got the SM420 cheap, but by the time that I get it rebuilt and have a driveshaft made for it, I might be better off just getting the original 3-speed rebuilt. I like the SM420 but will never actually do any heavy work with this 57. I've been searching out photos of the different driveshafts and consulting the Chevrolet Master Parts Catalog and I'm beginning to get the picture, I appreciate the help.

The original 3 speed transmission is about 22" from the bellhousing mounting surface to the end of the tail shaft.
The SM420 is about 14". As mentioned previously the front yoke and u-joints are completely different.
A modified or replacement driveshaft will be required.
Fred

One strange irony is that I purchased this truck specifically because it is very close to stock and my first impulse is to put a 1964-67 transmission and modified driveshaft in it. I didn't mention that a previous owner put in a fairly new, modified exhaust system that crosses over right behind the existing 3-speed that would have to be scrapped and replaced with a 4-speed installation. I think I'd be better off just addressing some deferred maintenance and driving it for a while. Thanks Mike, Gord and Fred for spelling out for me the ramifications of "improving" this driveline!

Dave

At the end of the day, both the 3 and 4-speed transmissions have the same 1:1 hi gear, so nothing is really gained speed wise. The SM-420 just gives you a really low Granny gear for pulling/hauling a load up a hill.

Mike B

A factory overdrive transmission is available for that series truck and you don't have to modify anything.
Some electrical components are required. Better performance, highway speeds, and less stress on
the engine.
Fred

Thanks guys!

Dave