Switching a 7-1/4 for an 8-3/4 rear end

[DY31970]

I'm planning on installing an 8-3/4 rear end in my 1970 Challenger in place of the 7-1/4 . Can I use the same drive shaft?

 

[moparleo]

Not unless you have it modified. Standard or automatic ? Slant six or 318 ?

 

[DY31970]

Thanks moparleo. My car is a 318, automatic.

 

[pschlosser]

Study of the 1970 parts book suggests some things you may find of value.
1. The drive shaft for 8-1/4 and 8-3/4 differentials are the same length.
2. comparison of drive shaft lengths between 7-1/4 and 8-3/4 rear ends indeed shows the 8-3/4 is shorter by 2 and some fraction inches.
3. the drive shaft lengths look different between manual and automatic transmissions, same rear end.

Work out precisely the drive shaft you have (and it's presumed length), and what length you want it to be. The upside, is it looks like the correct one is shorter, so you may have an option to cut up the one you got.

Better still, find the correct OEM driveshaft, used, for this configuration.

 

[DY31970]

Thanks for the info pschlosser. The options of shortening the driveshaft I have or getting the correct length OEM one is the way I will go.

 

[Katfish]

Interesting chart, did the wheel base of the Challenger change after 71?
Why are the later driveshafts 4" longer? Doesn't make sense?

 

[pschlosser]

I checked the 1970 parts book more closely, and I came up with this:

• 1970 7-1/4 Axle, Engines 198,225,318, Trans A904 Length 49.89" 2996 146
• 1970 8-3/4 Axle, Engines 198,225,318, Trans A727, A904 Length 47.95" 2883 768 and 47.89" 2996 138

It looks like there is nearly a 2.00 inch difference between the two shafts. Get your longer one shortened by the proper machine shop.

 

[challenger6pak]

The axle tubes are different sizes. You will need the 8 3/4 shock plates and u bolts.