VCCA Home Forums TECHNICAL - 1945 AND EARLIER 1937-1945 39 Chevy truck u joint/ torque tube problem

Tony,

Assuming your trucks have not been modified from the original, the 1939 1/2 ton pickup wheel base is 113-1/2", and the rear end is a spiral bevel type, identical to the 1937 and 1938, except that the torque tube and propeller shaft on the 1939 are 1-1/2" longer than the 37, 38.

As long as you retain the 38 rear end in the 38 and the 39 rear end in the 39, the 3 and 4-speed transmissions will interchange on either truck. If you swap transmissions, you'll need to swap the clutch/brake pedal/master cylinder mounting brackets with the transmissions so that the 3-speed bracket stays with the 3-speed transmission and the 4-speed bracket stays with the 4-speed transmission. The steel floor pans have different cutouts to accommodate the transmission shift towers, but I don't think the floor pans will swap between the 38 and 39, as I believe the 39 is slightly longer (probably 1-1/2") than the 38.

Mark

They will not interchange, I have had both units dismantled and side by side on the bench everything is different. I must change all from the rear of the bell housing.
Tony

Years ago we used to remove 3 speeds and replace with 4 speeds in the dealership.

Thanks everyone for the info. Really appricate. I have located another rearend. I'm going to check it out tomorrow. I put the word out in my local community. I had a feeling that some one was hot rodding one close to me. I will post how it works out. Once again thanks so much for all the info
Ryan

So I pickup a 46 rearend other day. Correct length and width. When I got it back home, I went to put the ujoint on and the propshaft has different number is splines. Think 39 is 10 and the 46 is 17. Which I believe they changed in 41. Luckily the guy said he would sell me the 46 trans and ujoint. To correct everything. I've had to learn a lot on these trucks

Ryan,

If you've already installed the 46 rear end under your 39 pickup and have found that the torque tube/propeller shaft lengths are correct, then I can't dispute that. However, if you haven't installed it yet, I would expect the 46 torque tube/propeller shaft to be 1-1/2" too long, as the wheelbase on the 41-46 1/2 tons is 1-1/2" longer than the 39-40.

If the torque tube/propeller shaft is the correct length, with the transmission crossmember removed, and with the U-joint ball slid all the way back on the torque tube and the rear yoke of the U-joint pushed back into the torque tube as far as it will go, the U-joint should just clear the rear face of the transmission (by perhaps 1/4") when you lower the front end of the torque tube down. This allows removal of the transmission without having to release and slide back the rear end/torque tube assembly.

Installing the 46 transmission with the 46 rear end solves the U-joint/propeller shaft spline mismatch, and the transmission should bolt right up to your bell housing and accept the pedal mounting bracket and the emergency brake lever assembly with no modifications required.

Mark

The 1939 1/2 ton torque tube is 57 19/64" long. The 1946 is 55 5/8".

Ryan,

The torque tube lengths for the 37-46 Chevrolet 1/2 ton trucks given in the Chevrolet Master Parts Catalogs can be confusing and are not a good indication of whether or not a particular torque tube/carrier assembly will fit in your 39. This is because in 1940, the rear end in the 1/2 ton trucks was changed from a spiral bevel gear type to a hypoid gear type. The hypoid type rear end required a completely different carrier than the spiral bevel gear type, and this resulted in a change of torque tube lengths.

The best way to determine if a torque tube/carrier assembly will fit properly in your 39 1/2 ton is to measure the torque tube length from the front end of the torque tube to the face of the large flange on the carrier that bolts to the banjo axle housing. For the 39-40, this measurement should be very close to 68-9/32". I'm pretty sure you'll find that measurement to be about 69-25/32" (1-1/2" longer) on the 46 torque tube assembly.

Mark

With mine the banjo (axle housing) is different diameter where the centre bolts in and the axle tubes are different (may be same axles). The 39 is spiral bevel and 1/4" larger than the 38 hypoid bevel though both are the same 9:38 ratio. I think the front of the drive shafts are slightly different too but havent actually measured.
Tony

Tony,

It sounds like your 38 1/2 ton is some kind of hybrid--perhaps it has a 37-39 passenger rear end. I believe the hypoid type ring and pinion gear was introduced in the passenger car line in 1937, with two ratios available: 38 and 9 teeth for a 4.22 ratio, and 41 and 11 teeth for a 3.72 ratio. The hypoid type gear wasn't available in the pickup trucks until 1940, and it was 37 and 9 teeth for a 4.11 ratio. The 1937-39 1/2 ton pickups had the spiral bevel type ring and pinion gears which had 37 and 9 teeth for a 4.11 ratio, or an optional economy ratio of 42 and 11 teeth, or 3.82.

Mark

Mark
Thanks for the info. After doing some measurment I'm sure I could do some munipulation to everything and make the 46 work. Though I decided to start the hunt again for a 39/40 rear end. Any leads on one would be much appriciated. Ive been searching Michigan but nothing has popped up.
Thanks Ryan

Ryan,

You might be able to shoe-horn the 46 rear end into your 39, however, the U-joint ball housing may not slide back far enough on the torque tube to allow adequate access to the U-joint bolts, and even if you can get at them, once released, the U-joint won't slide back far enough to clear the rear face of the transmission to allow the torque tube to be lowered down out of the way, so that in the future, if you need to get the transmission out, for example, to replace the clutch or throwout bearing, you'll have to release the rear end and slide it back. Just make sure that when everything is bolted up, the propeller shaft or the torque tube has not shoved the U-joint hard up against the transmission output shaft. The propeller shaft and torque tube need to be able to slide slightly forward and back as the rear axle goes up and down.

Obviously, a 39 1/2 ton rear end would bolt right in to your truck. A 40 rear end should also bolt in, but, as you already discovered, the 39 U-joint yoke spline does not match the 40-46 propeller shaft spline. However, since you already have access to a 46 transmission, if you installed that in place of the 39 transmission, the U-joint should match the 40 propeller shaft.

If you can find a 1940 1/2 ton rear end, I'd recommend that, because together with the 46 transmission, everything should match up, and, you'll have the option of installing an after-market 3.55 ring and pinion gear in the future if you want to.

FYI, the 40 truck 3-speed transmission is a 1 year only design and is identified by two horizontally opposed ears on the back face of the transmission where the U-joint ball housing fits. The external dimensions and all the internal parts are the same as the 41-46 transmissions, so there's no problem with fitup, however, if you get a 40 3-speed, be sure and get the U-joint ball housing collar with it if available (the collar holds the U-joint ball housing into the back of the transmission). The collar has matching ears on it, and the earlier or later collars without the ears may not seal properly.

I don't know if you have had any prior experience with these Chevrolet closed driveline cars/trucks, but a common problem with them is that the original propeller shaft seal at the front end of the torque tube dries out and leaks, which results in transmission oil migrating back to the rear end. It's not a fatal problem, as long as you are aware of it and check the fluid level in your transmission regularly and top it off, as needed. If the seal is leaking, you'll also need to drain off the excess oil in the rear end periodically.

Mark