The Stovebolt Site Forums The Shop Area Engine Shop 4 Speed Transmission

I have what I believe is the original unsynchronized 4 speed in my ’41 ¾ ton. I’m wondering what the manufacturer and model/part number for the transmission is. Also what years it would have been used in. And one more thing; Does it make the same whine in everyone’s truck?

Thats not whine, that's road music you get from the square cut gears. You get the same sound from an old tractor. They all sound that way. Not certain when the transmission was first used but believe that it was used through 47. Isn't it great belonging to the elite group of double clutchers.

I agree with Donf - that's pure music! But, it's not from square-cut gears, as all the gears in the 3 and 4 speed trannys from that era had helical gears. Here' a picture of your unit:
http://chevy.tocmp.com/shop/1941/41csm208.htm

It looks like a Muncie SM420 but they didn't come out until '47 and were used for many years after that.

I don't know what yours was called, maybe someone else with knowledge of the older transmissions will come by later. It

Sorry Waldo, but that aint it. If I knew how to link the page I would. It’s on page 215 of that manual. All straight cut gears.

Well, yuppers, them sure is straight-cut gears alright. Now that unit really looks like a SM420, it's probably the forerunner to it. The SM stands for synchro-mesh and it's likely that's the main difference between yours and the later ones.
Sorry I can't be of more help on this, I was hoping an expert on the earlier trannys would happen by.

I don't know much about your transmission but I can tell you it's successor, the SM420 is a rock crusher; in production for 20 years, military for 10 more; used in 1-ton trucks and busses; today's hot setup for rock crawling because of low gear.

I recently had a 3.55 R&P installed in my 1/2 ton and the SM420 takes on an entirely new persona. With the taller gear she still pulls away smartly in second gear with an equal gear change from 2 to 3 and 3 to 4. The syncro second is like driving a full-syncro 3-speed and second is tall enough to make it a useable downshift. If you like to put your truck to work it's nice to know you have that low low gear for dealing with heavy loads and/or steep terrain... plenty of pull without the instant red-line you get with the 4.11.

I'm really glad I decided to keep mine so if yours is the forerunner of the SM420 you have a sturdy transmission, and I hope someone comes along with more in-depth info on it.

BTT With any luck this link will show a cutaway of the transmission in my 1941 ¾ ton. I’m still interested in learning the proper name and years it was used 4 Spd.

BTT

I’m still trying to find the model number or name for the totally unsynchronized 4 speed used in a ’41 ¾ ton and the years it was used.

Chevrolet built their own 4 speed transmission with straight gears and a 6 bolt PTO cover from 1931 to 1947. They also used a Warner 4 speed trans in 35 & 36 for a short time due to a strike a the Chevy trans plant.

Hi Jim:

Thanks for the answer, although I’m still a little confused you mentioned “a 6 bolt PTO cover” I’m assuming that stands for Power Take Off, but I don’t think mine has such a cover. There is a top cover, a side cover and a bearing retaining cover at the back.

Where are you in Howell? That’s not far from here

The side cover is the PTO cover. The cover is removed when a PTO is installed.