The Stovebolt Site Forums The Shop Area Driveline SM 420 vs Crashbox

I know this is being discussed but I didn't want to highjack the other thread. I have a '47 1/2 ton with what I thought was an SM 420. The casting number (590747)shows it's a case for a '31 - 47. A 4X4 site lists the casting number as an SM 420. It has the 'lift lever' for reverse which indicates a crashbox but the Novak's contact said he has seen an SM 420 with the lever. It upshifts fine. Downshifting will grind unless I double clutch.From what I can see it sure looks like an SM 420. Does anyone have pictures of a crash box for a comparison? The more I look into it the more confusing it gets. I suppose the tranny could have been changed at some point. The tranny is in the truck and I can't really get into it due to recent surgery. Any information greatly appreciated. Thanks.

Did you copy the casting number right. It should be a 590474 for the 42-47 sliding gear gear type. If it has a reverse lockout lever, it is not a SM420. They used spring pressure for reverse lockout.

Sorry. The casting number is 590474.

So,if I don't have an SM 420, I do have some questions.Can a 3 speed fit in my truck? From what I can gather, these trucks came with a 3 speed (on the floor in '47) In '48 the 3 speed shifter was on the column.I read that the SM 420 is 2" longer than a crash box so I would have to shorten my torque tube if I want to put in an SM 420? Are rebuild kits available for the 3 speeds? Are they as easy to rebuild as the SM 420 supposedly is? Are the crash boxes easily rebuilt? Are parts available? Does the crash box have a number designation (like the Sm 420 or SM 465?)or is it just called a crash box or sliding gear transmission? Thanks.

I have never seen a number designation. I have heard them called sliding gear for 50 years. I first heard the term crash box on this forum. In order to install a SM420, you will not only deal with length, but also with a different bolt pattern on the clutch housing. They are very simple transmission. 3 and 4 speed of 47 and older will fit the same clutch housing.

It is amazing what you can learn from this site. I have done more searching and learned a few things. I would be interested in hearing from anyone who has a 1947 1/2 ton that came with an SM 420. I could be wrong, but I don't think they exist. (A transmission parts vendor told me that the 1947 Chev pick up didn't really exist!) Production of the AD '47 didn't start until May or June of 1947.My truck was built in November of 1947.Does this make it a late '47 which might have come with an SM 420? Does this make it a '48? I don't think so, Tim.When I get to it, I'll replace the clutch components, keep the 'crash box' and try to replace some bearings to lose the whine in 2nd and 3rd.(Questions about that to come) Thanks to all for replies and for other threads that have made for interesting reading.

It sounds as if you should be referring to your truck as a 47.2. As you are aware 47 was a split year production as GM disontiued their Liberty trucks (deco) and started to build the AD trucks.

You're right, Bigadswingdaddy, my truck is a 1947.2 (Advance Design) I know that 47 was a transition year and there were some crossover of parts over the year. This all started when I read that the SM 420 transmission was used from 1947 until 1967 when I think it was really used from 1948 to 1967. That's the fun about owning old trucks like these. You learn a lot while trying to get more information about your truck.

Ok now i have some questions.

I also jave a late 47 chevy truck . In my attempt to get it back on the road i now am at the transmission dilema. I have the early bellhousng it the truck noe. If i could get a floor shift 3 speed, all my problems would be solved. But i have a later 4 speed. I first thought about switching out the 216 for a 12v 235 that has the trans attached. As i was looking at it i realized switching the bellhousing us all i needed to right? But then my driveline will be too long and my clutch wont work. So i guess i am going back to my original plan of attack and that is search out a floor shift 3 speed.

Ok now i have some questions.

I also have a late 47 chevy truck . In my attempt to get it back on the road i now am at the transmission dilema. I have the early bellhousng it the truck noe. If i could get a floor shift 3 speed, all my problems would be solved. But i have a later 4 speed. I first thought about switching out the 216 for a 12v 235 that has the trans attached. As i was looking at it i realized switching the bellhousing us all i needed to right? But then my driveline will be too long and my clutch wont work. So i guess i am going back to my original plan of attack and that is search out a floor shift 3 speed.

I replaced the crashbox on my 46 1 Ton with an SM 420. I had to get another bell housing so the pedals have something to bolt onto. This may not be an issue with the AD's or possibly your year. I had to make sure the starter gears were the same as the flywheel teeth, the clutch lined up fairly well and works fine, and I had the intermediate drive shaft shortened. It was not that hard to do. It is worth the work.

General Motors, in their repair manual, calls their 3 speed trans the "The GM 3 speed transmission". And the 4 speed is lovingly referred to as "the GM 4 speed transmission". The 3 speed is synchronised in 2nd and 3rd gears. The 4 speed is not synchronised. I have an NOS 4 speed which I could not upshift without double clutching. I've driven semis for many years and know the technique, but I could never get it to shift clutchless, it was so tight.
3 and 4 speeds can be interchanged with each other as the 4 speed was an optional trans. The u-joint is different however. The front half remains the same but the rear half of the u-joint for the 4 speed is of a coarser spline. (10 spline I seem to recall)
I have one of each trans if anyone needs measurements.

I have a 47 and contrary to what a lot of people say I can say there were actualy 3 47 chevy trucks. The first being the 46 body style. Latter they changed the body to the AD trucks. The last 47 had a change in the parking break though only a few 100 of them even exist. This 3rd 47 had an SM420 of sorts. It's not like the ones of 48 and latter and its not like the crash box. This transmission is impossible to find parts for and from my understanding the trucks with it were a display only truck. Idk if they could even be driven but I have a funny feeling they can't.

All factory literature announces the new transmission (SM-420) in 1948. This includes the small mailers and the large full-line brochures. So until someone shows me some proof otherwise?
This is the 48 full-line which devotes significant space to the new transmission and relocated parking brake features.
http://www.flickr.com/photos/advance-design_parts_co/8131119337/in/set-72157631872034068
And rebuild slides for the "new" transmission.
http://www.flickr.com/photos/advance-design_parts_co/sets/72157632620943718/

Not a 47 1/2 ton but I do have a 1947.2 3800 one ton and that sure looks and shifts like like a SM420 ,same as a 1948 One ton 3800 and my 1951 one ton all sm420's No reverse lift lever like on my 1946 2 ton with the crash box.
Jerry

Jerry,

Is there a code on the transmission that starts with two letters?

Is the first letter a P or a Q or an R?

Still because there is a SM-420 in it now some 66 years after the fact proves nothing. Unless there is some documentation It has to be considered as a retrofit.