I currently have a 350/350 in my '71 Cutlass convertible with a 10 bolt 3.42 posi. Also in possession of a '72 12 bolt 3.23 posi and drive shaft from a 455/400 parts car.

I read an article about swapping diffs, but am still confused. (like that's new)

If I swap in the 12 bolt, do I use the TH-350 drive shaft or TH-400 drive shaft? How about a u-joint adapter and where can I find one?

TIA
C.J.

 

Lets take this one step at a time.
If you swap from a TH350 to a TH400,the driveshaft needs to be about 1" shorter.
If you swap from an 8.5" 10-bolt,to a 12-bolt Chevy,you might get away with reusing your existing driveshaft,but technically,the 12-bolt Chevy usually requires a slightly longer driveshaft,maybe 1/2" longer,but it depends on the type of yoke on it.Some of the yokes available for the 12-bolt Chevy are flatter,with less reach,and some stick forward farther.
So,if you do both swaps,transmission,and the rears,you "might" get away with using your existing shaft length,but it will likely still need to be shortened some.
Another factor is the TH400 yoke.They make different yokes,with different barrel lengths.If you have one already,measure that.
As far as adapter u-joints,yes,you can get a variety of different adapter u-joints.If you have the typical 1310 yoke on the 12-bolt Chevy,you would need a yoke that has 3R dimensions on one side,and 1310 on the other.You might want to deterimine the type of yoke you have on the 12-bolt Chevy 1st.
If you do have the 1310 yoke on the 12-bolt Chevy,I would suggest changing it to a 1330,like the SS cars,or the 1350.This would give you a beefier u-joint.
The 1310 would be fine,but you are sort of starting from scratch when doing the swaps,so now is the time to pick something different,if you feel the need.

 

Sorry...didn't make myself clear. I am using the 12 bolt and TH-350 transmission and was looking for info on which drive shaft to use and yoke adapters and where to get the adapter.

You are a real wealth of information, Brian.

C.J.

 

The 12 bolt is not a chevy it is a olds as there is no 3.23 gear for the 12 bolt chevy.

So if you have the 12 bolt in the car right now and want to swap the 8.5. You will not need to do a u-joint swap as the Olds 12 bolt uses the same u-joint as the 8.5 10 bolt. The probelm will be the drive shaft length.

You will bolt your rear in and measure your length from cup to cup with the slip yoke in the trans housing then take your measurement see which shaft is the right one. IF neither are correct take the longer one and cut it down and use that one. with the correct 400 or 350 yoke. Jim

That would be the most correct way to do it then guessing and hoping.

As far as u-joint strength goes ...we use a 1310 on a 10.90 car 1.35 60 foots pulling the tires and have never had an issue with strength. u-joint will fail when your drive line is incorrect angle.

They have conversion joints IF you have the chevy 12 bolt but from your description in gearing you have the 12 bolt olds rear.

IF your 12 bolt has factory bolt in axles it is a 12 bolt OLDS.

Good luck and take care, Jim

JD

 

Yoke adaptor...?? Do you mean conversion u-joint? cause that ios all you will need for the swap IF the rear is a 12 bolt chevy.

going from 12 bolt OLDS or 12 bolt chevy...your drive shaft MAY be too long. Install the 10 bolt 8.5 then try the drive shaft. IF you can not get it into the saddles with out prying it in then you will need to shorten the shaft some. Best to talk with the shop that will be cutting the drive shaft so they understand what you need done. Then your both on the same page. usually you will need about 1/2" to 3/4" slip on your trans slip yoke transition for the rear to articulate up and down.

Again best wishes on a smooth install...

Jim

JD

 

OK.I thought you were swapping both.Let'sfirst determine if you have a 12-bolt Chevy or 12-bolt Olds.

 

He said it was from a 455-400 parts car, so I'm guessing an Olds!

 

If you use a short tail 400 you only need to move your cross member backwards.
I believe all the A body frames are drilled for both transmissions.
You just need to change out the transmission yoke.

http://www.dennysdriveshaft.com/p102...turbo_400.html

http://www.dennysdriveshaft.com/

 

Been there,done that.No you can't.Your driveshaft will be a a bind.Even with the shortest yoke.Otherwise,they would have used the same shaft at the factory.
I know the diagrams show that the two different transmissions are almost the same length,but they are not exact.
If you swap from a TH350 to a TH400,you need to shorten the shaft.
If you swap from a small output 4-spd,to a large output 4-spd,you need to shorten the shaft.
If you swap from a TH350 to a 2004R,you need to shorten the shaft.
All frames have a variety of holes,for all the different makes of A-bodies,and all the different varieties of transmissions.