8 3/4" narrowing - Dimension Help

[dms321]

Hey all, I have a relatively stock 74 dart swinger, and am looking to swap out the original 7 1/4” rear end (3507881) with a 8 3/4" rear (2881489). I’m planning to have a local machine shop narrow the 8 3/4” housing for me, but was trying to figure out what the final modified housing width should be. Here’s a rundown of the measurements:

• Drum to Drum
  • 7 1/4": 56 3/4”
  • 8 3/4": 60 1/8”
• Housing Width (flange to flange)
  • 7 1/4": 51 1/2"
  • 8 3/4": 54 15/16”

The drum-to-drum difference is 3 3/8”. Does that mean the housing width of the 8 3/4" should be narrowed to 51 9/16” (54 15/16” minus 3 3/8”)?

I was planning to order new axles from Doctor Diff to put back in, but they are listing the stock A-body 8 3/4" housing width as 52 5/8”. That had me questioning my math. I was assuming I could order new axles for a stock A-body 8 3/4” housing, but the dimensions aren’t matching up. Perhaps that’s because the stock A-body 8 3/4" rears had the small bolt pattern and different drum brake dimensions?

Just want to make sure I don’t mess up a perfectly good housing (and waste my money)!!! If my math is correct and the housing should be narrowed to 51 9/16", I'm assuming I'll need to order a custom length axle package from Doctor Diff to stick back in.

Thanks!

 

[bb66cuda]

Factory A body housings are 52 5/8". When you change to large bolt pattern the drums will add a little width, but nothing to be concerned about.

 

[jbc426]

Cass a.k.a. Dr Diff is a great resource, and a very busy man. Have you considered just ordering the entire rear end from him? I did that on mine and it bolted right in. It showed up on a pallet at a local shipper.

Also, what are your goals with upgrading to an 8 3/4"? Is it just for strength? Are you converting to BBP? Are you converting to disc brakes? Are you trying to run wide rear tires under the fenders? All these decisions can play into what you are trying to achieve.

 

[rbkt65]

Dr. Diff axles are based on using 2 1/2 inch brake shoes(look at his chart and is says brake gap 2 1/2). Use his chart and print it out and take to your machine shop. His chart works really well.

 

[Professor Fate]

If you simply want to confirm what a stock A body 8.75's dimensions are, you need go no further than the sticky thread at the top of this very sub-forum.
An accurate 8 3/4" rear axle width list
If you stick with the factory 8.75 dimensions then Dr. Diff's axles will fit fine. Anything else and you're into custom axle territory.
And yes, factory A body 7.25s are a different width than A body 8.75s.
P.S.- If the 8.75 housing that you're starting with is from a B Body, chances are you only need to move the perches and use wheels with a bit more backspace. It's done quite frequently with perfectly good results.

 

[Dana67Dart]

What ever you do don't cut down an oem A body 8-3/4 housing.

Get a B,C,E, truck housing, in the 200 range vs the 700-1200 for a OEM A body housing

 

[crackedback]

Disregard the 7.25 rear lengths, do you no good.

You have an 68-70 B 8.75... good starting point.

You can use the axle as is, especially if buying new wheels, just need a bunch more offset. Want more dish, need to shorten and it's easier to get tires on/off the car with a shorter axle.

If you are putting the BBP axles in and want it the same overall length of a factory A body 8.75 (axle flange to flange), total length of the bare housing needs to be 52.075"
That should make the total width with brakes 57 1/8"

The difference in overall length at the 52 5/8 housing with BBP makes it right at 57.675, only a little over 1/4 further out on each side compared to the 57 1/8" factory SBP unit.

Axle tubes are the same length, there is NO OFFSET in the housing. The center section sits dead center and accounts for driveshaft offset. Chevy guys F this up all the time when shortening mopar 8.75 rears.

 

[AbodyBB]

So
To clear things up a bit, gents here are correct to say the width is 52 5/8 in. That is the housing width.

An accurate 8 3/4" rear axle width list

Her is a thread with an excellent list of axle widths. It includes housing flange to flange and axle flange to flange for the A body brake (2.38 offset) as well as the 2.5 inch offset for 2.5 width brakes. It also gives axle lengths. Thanks to 72bluNblu. I’ve used that list often.
Use those dimensions and you won’t go wrong.
The only dimension that changes with the BBP axles is the slightly wider track. .12 per side or about 1/4 track width

 

[tjpatte]

If you’re getting the axles through Cass just tell him how far apart you want the wheel mating surfaces and he will calculate the rest for you.

I used his spring offset kit then mocked my wheels under the car to fit 275’s centered in the wheel well and gave him the measurement and he did the rest

 

[crackedback]

Cass is a good guy, no doubt.

 

[tjpatte]

Here are the measurements for my axle and fitment on the car.