The Stovebolt Site Forums The Shop Area Engine Shop What kind of transmission is this?

I picked this 4 speed transmission up awhile back thinking that I might be able to use it to replace my three speed in my 1/2 ton truck. After looking at the transmission closer I'm not sure if it would work or be the best option. First of all, what kind of transmission is this?

Pic 1
Pic 2
Pic 3

Secondly, would this be a good option for a 1/2 ton truck? And if so, how much work would it be to swap this in to replace the 3 speed?

It looks like an SM-420 from a 1-ton or bigger truck. I think the band-type parking brake was not used on the 1/2 and 3/4 ton models, but that's just a guess. Other than finding the proper floorboard cover and a shift stick, it would be a pretty straightforward swap on any truck with an open driveline. You might need to modify the driveshaft to mate up with the tranny.
Jerry

Neil,

I agree with HRL, it looks exactly like a SM420. You would only gain a very low first gear, nothing else, plus it looks like it has been submerged in the ocean for awhile. Lots of work for not much gain in my opinion.

Stuart

3rd on the 420. Most likely a 1 ton trans. 1/2 and 3/4 used the rear service brakes for the parking brake, I'm thinking 1 ton and mybe 1 1/2 ton were the only uses that used the band style, with the larger trucks using an internal drum type.
Like Stuart said, not much gain for the work involved.

Check inside before installing it. Just a small amount of corrosion would be cause for rejection, it doesn't take much to make it sing or roar while going down the road. James

but some like to hear a truck roar...





not usually from the trans though.

hard tellin'
but
check for a pitted 'in' main shaft

hard tellin'
but
check for a pitted 'in' main shaft. a good tranny guy can pull it apart in about 20 minutes. it's always worth a look inside. B4 it's in the truck ..

Now can the band style e-brake be removed? And with my three speed I have a long tail attachment that is bolted to the back of the transmission (see here ) Is this something that can be switched over to the four speed, or will I need a new tail and u-joint attachment? I'd love to have a four speed in the truck, but I don't want to go through the extra work and expense right now if it's going to need a lot of parts for the conversion.

Thanks for all the help!

Actually, you also get syncro 2nd which is like having a full syncro 3-speed.

Thanks for all the help and replies, but I'm still curious... does the tail of the 3 speed bolt up to the end of this transmission? Or am I missing a part that attaches to those four studs sticking out of the back?

There's a 4-bolt flange on the front of the driveshaft with a universal joint in it that attaches there. Some drivelines have a splined slip yoke that bolts on, and the front end of the driveshaft has a male spline that slides into it.
Jerry

I have a sm420 in my 62 swb. It is identical to the one in the pic, including the parking brake assy. Mine has a 2 piece drivshaft with no slip yoke. I believe that will be your issue. You may need to swap in a 2 piece shaft to run this trans. It is an extremely strong trans, but the ratios are terrible for me. I have a 3.90 rear end, and my motor winds out to the moon on the highway. I think you would be better off using a t-5 if your motor is a 6cyl. I did swap a saginaw into my '63 years ago in place of the full syncro 3 speed, for a dramatic gain in acceleration, but this was before o.d.'s were common. I'm gonna run a t-5 in mine.