The Stovebolt Site Forums The Shop Area Engine Shop '40 Transmission ID

I would like to identify the transmission from my '40 GMC. I believe it to be a 4 speed. Can someone direct me to a link where i can look up the serial # on the case? The #'s look like this:

1
GM
590474

Also if this tranny is a non synchro mesh unit did GM make one that is synchro'd that could be used in place of my existing unit? I want to keep the truck as origional as possible and there-for want to keep the enclosed drive shaft but want an easier shifting unit.

Whats the opinion out there relative to the ease of shifting with the non synchro tranny, are these things difficult to drive? When I was a kid my dad had a truck that had to be double clutched so im not unfamilier with this procedure but i would prefer to eliminate this headache.

Thanks, G. Lee

Gelee,

I've been driving my 41 with the 4 speed for the last six months. I drive it most days, peak hour through the city as well. You'll never use 1st gear (unless you need to pull down the side of a building ). It does take a bit of getting used to. Dusty gave me some good advise when I asked more or less the same question. Take it slow and easy and practice practice practice. You won't have to change gear as often as a modern truck. These old sixes have heaps of low down torque. It can be frustrating but I'd give it a go before you decide to dump the 4speed for something else. They are a pretty tough box. I also look on it along with the foot starter as an unofficial anti theft device. It certainly puts a smile on your face when you get a shift to snick straight in. It does have the ability to bite you in the bum though when you make a shift and it grates a bit going in and you scratch your head wondering what you did differently. Good luck

Bob

Gelee, the casting number you gave is listed on two websites ( this one and this other one ) as belonging to an SM420 4-speed.

It's my understanding that the newly synchro'd and helical-geared (except 1st and reverse gears) SM420 first appeared in 1947. However, the 1954 master parts catalog shows this particular casting number as belonging to the case for the earlier (and non-synchro) '31-'47 4-speed. So there is a bit of a contradiction here.

This casting number also came up in an earlier thread on this forum ( see here ). The consensus there was that it was the earlier tranny. Those two websites I gave above are probably mistaken.

It may not be hard to swap in an SM420 if you really want a synchromesh tranny, but I'll leave that determination to those who might know better than I.

Anybody know the moniker for the early 4-speed?

The 39-46 Chevy and GMC's were also available with a 3 speed transmission which had syncro's on second and third gear.

That 590474 case is the nonsyncro 4 spd. it was used 1931-47. 1948 is the first year for the SM-420.
Learning to shift it will take pratice and going slow between shifts.

OK, here's a real good opportunity for me to really [censored] someone off. You know this society laments the dumbng down of everything, the weakening of Men, the loss of feminity of women. Is everyone just a wimp now? Do we all need power steering because we can't handle a light well balanced truck without it? Do we need to have automatics and synchro trannies? Hasn't anyone ever driven a big truck? Screw it iif it's not synchroed. Have a sense of adventure. Double clutch it. If you want the modern feel, why do you want an old truck? Enjoy something with a little character. Any japjunk out there will give you a synchro tranny and that quick off the line acceleration. Go buy one fer cryin' out loud. Did your grampa give a crap about power steering. No, because he actually drove the truck before he had the opportunity to cut it all up and make it "better". Sorry to pee all over your thread, but this is representative of modern thinking. "The truck's 50 years old , man. It's gotta be a piece of crap. A torque tube? junk. 6 cylinder? Scrap it, you need a 350. It magazine-think that's going on here and everyone buys into it. This site has less of those guys than most places but still it's becoming more and more evident.
I have a '45 GMC 1/2 ton w/ 228 and a 4 speed. I replaced the original 3 on the floor with an NOS 4 speed.I have Patrick's 3.55's out back and the truck is a true pleasure to drive. It can do 70 on the Interstate if need be. It does not have power anything. It doesn't have a radio. It doesn't have a lot of things. But Character it has in spades. Real men know how to double-clutch!