Eary VS. Late A-body rear end widths

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convx4

convx4

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So the 66-72 small bolt 8 3/4 has a width of 57 1/8"
The same housing with big bolt pattern axles has a width of 57 13/16"
Now the 73 and up big bolt 8.25/7.25 axle has a 56 3/4” width.
So the difference between big bolt pattern 8 3/4's is 1 1/16"

From my measurements the 8.25/7.25 width is as narrow a 8 3/4 can be made with out any other modifications allowing a 15x7 wheel to fit a dart swinger/scamp.

Why are people narrowing housings and not getting the maximum benefit from their effort?
 
Because narrowed axle housings are generally done in conjunction with spring relocation and tubs/minitubs.
You don't need to narrow a housing to fit 15x7s. Depending on the model (and therefore the rear wheel opening shape), you can even fit 15x8s with the right backspacing.
 
An accurate 8 3/4" rear axle width list

Don't order wheels and offsets off a chart. I have 7" wide 4 1/2" backspace on the rear with 255-60 15's. I might be able to run a 8" wide on one side but not on the other.

001.jpg
 
There are some pretty good wheel backspace calculators online if you take some time to search. They can help you with your measurements to determine what will fit. The one I had used in the past also had a tire chart/calculator that gave the diameter for various profiles and sizes which is pretty handy. I don't know how anyone figures all this out without using a resource like that as it is so complicated.

Here is an example, but I have never used this particular one.
Wheel Offset Calculator
 
convx4 said:
So the 66-72 small bolt 8 3/4 has a width of 57 1/8"
The same housing with big bolt pattern axles has a width of 57 13/16"
Now the 73 and up big bolt 8.25/7.25 axle has a 56 3/4” width.
So the difference between big bolt pattern 8 3/4's is 1 1/16"

From my measurements the 8.25/7.25 width is as narrow a 8 3/4 can be made with out any other modifications allowing a 15x7 wheel to fit a dart swinger/scamp.

Why are people narrowing housings and not getting the maximum benefit from their effort?
Click to expand...

I fit a 17"x9" into mine. No modifications, stock 8 3/4 sbp axle, stock spring location, stock wheel well. The fit was made through proper backspacing and offset on a custom rim. I have a 1/4" on spring side and 1/4" on fender side. I think the tire is just under 11" wide. I don't have a straight on rear photo, this photo makes it look a bit pushed out but its just the angle. They are tucked in there very nice, with a great stance. The fronts are 17x8. All stock there too, Kelsey Hayes setup. PM me if you want more details.
48F6289F-C360-4178-B79B-2C1DA38419BA.jpeg
 
Also, some people (like me) don't like the look of an axle that's narrower than necessary.

I didn't even like the factory axle width of my 73 Duster with Disk Brakes.

I put an uncut 67 B body 8 3/4 under it and to my eyes it looked more "correct" with the front and rear track the same.
 
69cudaownr said:
I fit a 17"x9" into mine. No modifications, stock 8 3/4 sbp axle, stock spring location, stock wheel well. The fit was made through proper backspacing and offset on a custom rim. I have a 1/4" on spring side and 1/4" on fender side. I think the tire is just under 11" wide. I don't have a straight on rear photo, this photo makes it look a bit pushed out but its just the angle. They are tucked in there very nice, with a great stance. The fronts are 17x8. All stock there too, Kelsey Hayes setup. PM me if you want more details.
View attachment 1715979545
Click to expand...

If my memory serves me right the face of the drum on a stock 8 3/4 rear measures pretty close to dead center of the leaf spring edge and the fender lip. Which makes sense for design purposes. My wheels had to be 5” backspace with 1/2” offset which is actually equal to a 4 1/2” backspace and centres a 9” wheel within the space available. (Wheel design affects offset as I recall)
 
Since there were no large bolt A body 8 3/4”, I would say that the offset of the aftermarket large bolt axles are made to accomodate the offset of B body brakes, making them slightly wider at the flange. Now, the front track of a 74 and later A body is wider, so I assume trust the rear was slightly widened to better sync that together.
 
convx4 said:
Why are people narrowing housings and not getting the maximum benefit from their effort?
Click to expand...
Well,
I narrowed mine to fit a standard 4.5 backspace wheel, up tight to the inner fender, less 3/8 inch. This lets me punch the wheels out until the rubber drags on the outer lip. My 68 Barracuda accepts up to 325/50-15s on tens, even lowered.
If I had not liked my Kidney-slots, aka slot-mags, aka Ansens and ET-IIs, as much as I did, this would not have been necessary. But now I can install any 4.5 or less backspace wheel. I have four sets of standard off-the-shelf rear wheels, of various widths, that I can now just bolt-on.

Of course, for me, the 8.75 was the only choice, because of the strength I needed and because of the Drop-out gear-carrier. At one time, I had a lotta lotta pumpkins; nearly every one from 2.76s to 4.30s in addition to a 4.88, and a 5.13..... or was it a 5.38, I forget.............. I never had a 4.56 nor a 3.73.
 
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