The Stovebolt Site Forums The Shop Area Driveline 3-speed tranny question

When was the last year Chevy made the exact same 3-speed tranny I have in my 1950 1/2 ton? Swapmeet season is here and I'm looking for an exact replacement to keep as a backup. If Chevy came out with a newer tranny that's even better & would bolt up to my 216 I'd be interested in that also.

Thanks!

SM-420's were made through '68, then SM-465's, either with short input(9" approx) will work. 465's aren't necessarily better, parts OK for both.

This is assuming you have an open driveline.

Ed

The 48-54 3 speed trans. are the same. The 55 up was a little stouter but they are built for open drive shaft. Like Pruss says you could use a 4 speed granny SM-420 but it will have to be from the same era because of the closed drive shaft. To use any other trans you would need to change the rear end & use an open drive shaft.

So to be clear.......I need to look for one of the AD truck transmissions from 47-54. Anything newer would involve changing to an open driveshaft.

Thanks Ed and Wrenchbender!

Chuck

Almost correct.

1948-1954, as posted by Wrenchbender (1947 is a different length).

Almost correct.

1954 1/2 ton only. The 3/4 and 1-ton 3-speed in 1954 were for fully open drive lines.

Thanks Tim for even more clarification. I appreciate it.

Even though the 1954 3/4 and 1 ton 3 speed were open drive, they used the same transmission with a goofy rear bushing, seal and yoke adapted to the same transmission as the torque tube style.

Thanks, Dave

I assume from (just now) re-reading in the 1954 manual, that no universal joint ball adjustment is needed on/with the 3/4 and 1 ton 3-speed truck?

I also see that the standard 3-speed transmission on the 1/2 and 3/4 ton 1954 trucks was not available on the 1-ton. A heavy duty 3-speed was available as an option on 1-ton trucks (4-speed was standard). Did that H-D 3-speed have the same rear "cover" as you described above?

Tim,
I was going by your post when saying "3/4 and 1 ton"(I did not look it up). I have a 3/4 ton example, but not a 1 ton.

Thanks, Dave - I had never read/noticed that oddity.

Is there a chance you have one you could photograph? This is interesting.

On the SM420(original 4 speed) transmission, you can use the "open" drive later with a torque tube. If you save the "tail" off of the older transmission. It bolts to the same pattern with out modification. The "ball" casting is bolted to the rear of the casing in the same place as the "seal" casting on the newer transmission. Same applies to use of the older SM420 in an "open" driveshaft. Just change the "ball" casting out for the "seal" casting and in it goes. I never, ever throw stuff out like this. Has saved me many times. You can use the 4 speed from 47 to early 60's if you need to. One part and all is good. I have not tried this with the 3 speed so no advise or opinion for that.

Tim, here you go, nine photos

Those are great photos, Dave - THANKS

Steve, what you describe is good information (for 1948 and later SM420s). However, was the SM420 used on 1947 Chevrolet trucks? It looks like the SM-420 was introduced in 1947, but a spur-gear (no-synchromesh) transmission is listed in the 1947 Truck Specifications Manual. I think the torque-tube length and transmission lengths changed in 1948? I am not sure about this - just asking.

I don't have a 47 to poke at. But the kind folks at Novak seem to think the 47-53 are the same(includes synchros?). The torque tube changes to open after 53? Here is a link to the "Information" part of the site: https://www.novak-adapt.com/knowledge/transmissions/manual/sm420
With out one to poke and mess with this is as good as I can do.

The 1/2 ton 1954 tucks still used a torque-tube (3.90 R&P vs 4.11 R&P in earlier years).

The SM420 front bearing retainer "snout" changed in 1954.

Some detail changes over the years. Loose bearings in the older versions. The snout as you said. The mounts, length and bolt patterns are the same. Some changes to the synchros, I think. But an old and a late will fit into the same place. The output shaft has the same spline count and the input as well(might be some "special" applications for non-GM applications) but over all you can use almost any year transmission in any year truck. Keep the tail casting that fits the application and off you go. The Jeep guys love the low stump gear for crawling. Was a good tough transmission.

1948 through 1954 use the same case with the same casting number. However in about 1952 they changed from a 2 lug to a 4 lug synchronizer, changed second gear and a few other things. You have to be careful when you rebuild one of these. The entire set of gears will move from one case to another, but you cannot mix and match other parts. The early and late is easily identifiable by pulling out the input shaft. If you expose a 2 lug synchro, you have the early model. If you expose a 4 lug synchro, you have the late model.

The HD transmission is a Borg Warner T89 which is very similar to what Ford used behind Y-Blocks and their FE engines.

The hardest part to find for the old 3 speeds is the synchro. At one time the brass rings were reproduced in the aftermarket. but the brass rings as well as the complete synchro is getting really hard to find.

So you're saying, all the main transmission case lengths are the same.
And the only thing that is different is the tail housing casting lengths?

Isn't the main output shaft a different length
when using a torque tube verses open drive shaft?

Jorb: The difference in length from "Tube" to "open" is all in the tail housing and the yoke. The yoke went from a trunnion to modern needle bearings. And the modern yoke is much shorter. The late rear casting has a seal and big diameter hole to accommodate the "open" style yoke. The casting is much shorter so the drive shaft is longer than the shaft in the torque tube.The out put shaft is the same. The case is the same. The output yoke changes and the rear housing. That is about it. Inside the case is as has been posted. Lots of changes over the years. Shaft splines, and length however stayed the same.

Wrenchbender wrote "The 48-54 3 speed trans. are the same."
My 51 Chev. 3100 3 speed is jumping out of gear at speed and sometimes hard to get in 1st and 3rd. Have a 54 trans. with engine standing by although no idea of internal condition.
Questions...any idea what is causing the issue? will that 54 attached to a 235 cu. in. bolt up to my 216.5 cu. in. or will modifications be necessary? I suppose by Wrenchbender comments the parts are interchangeable (as in tear down the 54 for parts)?
If I need to go elsewhere for parts, any suggestions as to who may have serviceable parts?
Thanks,
TexChev