The Stovebolt Site Forums The Shop Area Driveline Driveline identification

My 1951 3100 has an open driveshaft....which means the torque tube was swapped for something else in the distant past. Not sure where the rear end came from, but it's 6 bolt and it appears to be pretty early.

The rear U-joint definitely needs replaced, so I got the rear shaft pulled this week. I figure we might just have to measure at the parts store to find the correct U-joint.

I would like to know what I am working with for future parts needs:

1) Am I right in thinking the transmission (4 speed) would also not be original to the truck? I have in my head that the torque tube version of the tranny was different than the open driveline. If so, where do I go about finding numbers to help me identify it? Everything is covered in a thick, protective layer of petrified grease and dirt....so need some idea where to start digging.

2) How about the rear end? Count bolts on the cover? Other numbers I need to look for?

Both open and torque tube would have had SM-420 trans'.

Ed

First thing is to put the truck on a diet and get the years of preserving muck off the truck. I know when I did mine the first time I wasn't sure I would ever get it all off. Thankfully that muck also kept things nice underneath. Once you are cleaned up find the casting numbers to help ID the parts you have. The transmission casting numbers will be easy to see, likely to start with 59xxxxx as mentioned in Tim's link above. The rear ID numbers if a GM axle will be on the passenger side near the pinion or U-joint.

The SM-420, the stock transmission used in these trucks, came both in closed and open versions. They also have conversion kits, so it is possible the transmission you have is the original.

Exactly what I needed! Thanks, SWEET!

Is it a sin to replace the rear u joint and not the front? Crawled back under the truck this evening and the front joint seems ok to my untrained senses.

Yes you can replace just one,but remove the drive shaft and check the front joint by moving the yoke side to side and up and down. This makes the ujoint cross pivot in the bearing caps. Do it slow and if you feel any roughness or tight spots then change it.

Thanks!

Dumb question: Does the front one have some sort of spring action? It always returns to center when you move it to one side or the other.

Now that I've messed with it some more, my spring action comment isn't really accurate. But it definitely does feel like there are some fairly positive "stop" positions at center, right, and left as you move it around.

Yeah that's a good sign it's on its way to being done...change it...as far as what kind of u-joints to get,if they are gm,they should be easy to match up at a parts store. GM is good at using standardized u-joints.as well as making cross over u-joints.it won't be hard to find something to fit.

Thanks. Neighbor came over today and looked at it. He agreed it needed replacing.

After dealing with three different parts stores today, finally found someone who could help me measure. Got the rear joint pressed into driveshaft. Still need to disassemble the front.

Are the little u bolts that hold the joint in the yoke a FLAPs item?These seem a little chewed up.

Casting # on tranny is 591665. So I guess it's probably original tranny that was converted to open driveline?

Yes, FLAPS should be able to get you the u-joints. I had to get some for my 53 1 ton. I'm replacing all of the u-joints. By the way, I found out when I had to replace the u-joint in my 09 Chevy Colorado, that the rear u-joint is two different sizes! The caps in the rear end yoke are a different size than the ones in the end of the drive shaft. Didn't know that was possible, but now I measure both ends carefully.

Tim

I did get the u joints bought. The rear was a 369 and the front was a 331, I think. Challenge to purchase without knowing exactly what driveline I'm dealing with. Definitely can't speak with just any person at the store.

I did my rear joints with MOOG 331's. Fit great.