sm-420 swap question

[thebeast61]

I did not find a thread covering this. I have a 61 gmc with the 305 and sm-420. I was thinking about swapping out the tranny. I was wondering if there are any trannys that i can swap to with minimal modifications. I would rather a bolt on. I just dont need the granny gear.
Also was wondering if putting a different tranny would improve the gas mileage.
I am having to rebuild my 420 and the parts are hard to find and expensive and would like to go with something more common.
Thanks for your help.

[Captainfab]

I'm assuming you're talking about the 305 V6? If so, I would think you could switch to something like a T5 5 speed. I'll let one of the GMC V6 experts confirm that.

[markeb01]

The engine is supported by side mounts near the front of the block and bellhousing mounts, so you can simply unbolt the transmission and replace it with anything that has the standard Chevy bolt pattern. If you have a tight budget any older non synchro 3 speed, later fully synchro 3 speed, or 4 speed Muncie or Borg Warner transmission will slide in place. The splines in the clutch plate just need to match up with the pilot shaft on whichever transmission is selected. The driveshaft may require a different front yoke and the shaft itself may need to be shortened. Some SM420 transmissions use a slip yoke while others use a bolt on flange with a slip yoke on the driveshaft. Most of the 3 speed manual transmissions use the same yoke used on the Powerglide and Turbo 350, while the 4 speed manuals may use either the 350 yoke or a Turbo 400 yoke depending on vintage. And again depending on the age of the parts, the front bearing retaining may need to be changed or have a spacer ring added to mate up correctly with the hole in the bellhousing.

A late model 5 or 6 speed can also be used but is a lot more expensive. There are numerous threads on this topic that can be found using the search feature.

Changing the transmission won’t improve gas mileage unless you add a model with overdrive in high gear, but it may make it more comfortable to drive the truck. Before tossing the 4 speed though, you might want to read up on it a bit. It’s a marvel of engineering and nearly bulletproof.

http://www.novak-adapt.com/knowledge/sm420.htm

[monolith]

I wish there was a way to re-gear and make those SM420's overdrive.

[Shaky]

Originally Posted by monolith

I wish there was a way to re-gear and make those SM420's overdrive.

There is, but it ain't cheap.

[thebeast61]

Thanks for the great info. Its very helpful. I have heard that the sm-420 is a great transmission. I like that it is the original tranny and motor so i might just spend the money and keep it. They must be bulletproof because i rarely see any for sale.

[Shaky]

Originally Posted by thebeast61

They must be bulletproof because i rarely see any for sale.

The Jeep guys snap them up since they're short, rugged, and have the rock-crawler first gear.