Technical ‘63 Impala driveshaft measurement point

Hey guys so I’m converting a ‘63 impala into a 4 speed from a 3 on the tree. I asked on several forums what the length should be for a 4 speed ‘63 Impala and I got 24 5/8” for forward driveshaft. So I took the shaft down to the shop to have that part shortened to 24 5/8”. The guy asked me what point did i want that shortened from. I didn’t know what to say and told him that to measure from the center of the joints.
I hope this is correct that a drive shaft is measured in the middle of 2 joints?

 

This is one of the references I used for driveshaft length

 

That's how I measure them, but if you got a number from someone else instead of measuring it yourself, things will go wrong. Sorry.

 

I had a '64 Impala that started life as a 3 on the tree but then converted to Muncie 4-sp. I thought the 3 speed, 4 speed, and PowerGlides were all the same length? I broke a front shaft and replaced it with a PowerGlide one on my car.

 

I had a Saginaw 3 speed and it’s way shorter than the Muncie M20 4 speed I just bolted in. I had another impala guy that did the same conversion and he didn’t need to shorten it. Then there was another impala guy that had to shorten it.

 

The last car I did for a customer I had the car on jack stands under the rear end so car would be at ride height. I put a yoke in the transmission all the way in and pulled it out one inch. Then measured from center of u-joint to center of u-joint from yoke to the rear end yoke. The difference from length was taken out of the front half.

 

Won't work well on a 63 Chevy. The driveline is 2 piece and the center support is inside the frame tunnel.

 

The Saginaw 3 speed is the same length as the Muncie 4 speed. But the Muncie 318 3 speed that the car came with, is quite a bit shorter than both of them. (lots of guys think the original 3 speed transmission in these cars was a Saginaw, but it wasn't, that came a few years later)

 

You measure with the driveshaft out of the car. Then take the whole drive shaft to the shop and tell them to make it your measurement by taking the difference out of the front half.

 

Yeah what ever 3 speed it was. It was really short. I thought it was a sag maybe it was a Muncie

 

Been a while (like 30yrs give or take), but I did the same on a '62 Impala. 3 on the tree to a Muncie M-21. Shortened the front shaft an inch or two, wasn't much. Scribe an index before you cut, no balance needed.

 

We did the front of the front IIRC. Carefully cut it right where the front yolk fits in, (it's like a plug) pop it out. Make & true your length cut, index & weld'r up.

 

Another issue you can run into with shortening the driveshaft, is that the yokes are not all the same length. Since you have to change from co**** to fine spline, if you get a different design yoke, it could be a little longer, and might cause issues.

My driveshaft measuring technique is to get the car supported up in the air, but the rear end on stands (don't let the axle hang down on the suspension). Put the yoke in the transmission, pull it out 3/4 to 1 inch, so it's in it's normal operating position. Then measure center to center distance of the driveshaft, from front yoke to rear yoke. I hook the tape measure on the transmission yoke, and measure to the back of the U joint opening in the rear end yoke.

Having a two piece driveline complicates this, of course. If you remove the middle U joint, then put the rear half of the driveshaft in place, you can measure from the transmission yoke to the front yoke on the rear half of the driveshaft. Make sure sure that you get the actual center to center length, and that you have the transmission yoke sticking out the correct distance (3/4 to 1 inch from fully inserted).

Then I write this center to center distance on a piece of tape on the driveshaft, along with my name and phone number, before dropping it off at the driveshaft shop.

I've been temped to try to shorten driveshafts myself, but haven't actually done it. It's not rocket surgery to cut the weld open, pull the end out, then cut the shaft, and put the yoke back in. But you do have to make sure it's indexed to the yoke on the other end, so both U joints are in phase. And you have to make sure it's centered properly, which happens automatically when you put the yoke into the tube. But then you need to make sure it's still balanced...which also gets pretty close if you are just removing a few inches. And your weld wants to be uniform. Having a big enough lathe is really helpful, my lathe is kind of small.

 

^^^^^^^^all the above^^^^^^^

Operator error is another consideration too Jim.
I had a brand new 18x80" 10 hp lathe and a brand new Millermatic 200 MIG welder and had worked as a machinist for many years prior, had run a lathe mostly but was still in self-training as a weldor, yep f#@ked it up so had the big driveshaft shop in town redo it.