B body 8 3/4 in Demon

I have just installed a b body rear end in my demon and I was wondering what the best tire size and back spacing for this combo

Well, first, You are in for some fun.
This question has no answer, yet.
Is the B-axle stock width? What year B was it in?Or better is,what is the drum to drum distance?
The backspace Will depend on the width of the wheel, which will depend on the tire selected. You will also need to select the side of your car which has the smallest distance from the inner tub to the wheelhouse lip, as the rear ends are rarely centered.
The best idea is to map it all out on a strip of plywood or posterboard, using plumb-bobs to locate the various points of interference.
The car has to be jacked up under the dif, and be reasonably level from side to side. The map strip needs to be fixed to the floor so it cannot move, or indexed such that if it does, its easy to relocate.
After that its easy to find the maximum tub space, then what tire fits, then what rim-width works, and finally the backspace.
Any other way,is pretty much guessing.Unless someone else having the exact same combo, pipes up with what worked for him.
And finally a tip; If you're going for the max tire width, its best to make the bs slightly too deep, than the other way around.If it all ends up too wide, you're in trouble.But if its slightly narrow you can add a spacer between the wheel and the drum.
Work slowly and carefully. Good luck!

I have no info on this swap as mentioned above, there are a lot of variables. The best thing to do is pull out a tape measure and start measuring.

cuda guy said:

I have just installed a b body rear end in my demon and I was wondering what the best tire size and back spacing for this combo

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Springs moved in to the frame, or stock A-body width?

Or did you splay the springs out to the B-body perches? (not recommended)

Its all stock
relocated spring perch
I think its from a 69 b body
Nigel at Moparts told be it was 1 inch narrower than a stock a body rear end

cuda guy said:

Its all stock
relocated spring perch
I think its from a 69 b body
Nigel at Moparts told be it was 1 inch narrower than a stock a body rear end

Click to expand...

Uh, NO. The A-body 8 3/4 is 3" narrower than the B body 8 3/4 (if it's a 68-70 b-body).

The stock SBP A-body rear is 57 1/8" drum to drum (52 5/8 flange to flange). Spring perches are at 43" c-c (stock)

The stock BBP 68-70 B body rear is 60 1/8" drum to drum (54 15/16" flange to flange). Spring perches are at 44" c-c (stock)

As for people having solved this swap, yes, they have. I have a 68-70 B body rear in my Duster. Mine is not stock, my perches are at 42" c-c (1/2" offset) and I trimmed the outer quarter lip by roughly a 1/2" as well. I run 295/35/18's on 18x10" rims with a 7" backspace (38mm offset). If your spring perches are the stock A-body 43" c-c, and you haven't done any trimming or rolling on the quarter lip, the widest tire you're going to be able to safely run is a 275mm wide tire. But that should be a fairly safe option with a decent amount of clearance. I had 275's on my Duster without any modifications with no issues whatsoever.

As far as the backspacing, it will depend on what width rim you want to use. If you're looking for a 15" diameter rim you can go with a 15x8 or 15x9, either will work.

Assuming you really do have a B-body 8 3/4 out of a '69, and it's decently well centered with the perches moved to 43" c-c (stock A-body):

-For an 8" wide rim you'd want ~6" of backspace, and no less than ~5.75"

-For a 9" wide rim you'd want ~6.5" of backspace, no less than ~6.25".

If you're planning on a 17 or 18" diameter rim you'd need to use the 9" wide rim, the shorter tire sidewalls need the wider rim. You could even use a 9.5" wide rim, then you'd want ~6.75" of backspace, and no less than 6.5".

Not all of these cars are identical, the axles aren't all perfectly centered and the body's aren't all perfectly symmetrical, so it's obviously best to measure your car and see what you've got. As already mentioned, it's better to go with a little more backspace than a little less, if you have too much you can add a small spacer. If you don't have enough, well, your only recourse is to trim the quarters or buy new rims.

I ran a B body rear in my 68 Dart. They were only 7" rims. But worked good.

Darren said:

I ran a B body rear in my 68 Dart. They were only 7" rims. But worked good.

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If they were a stock 7" wide rim then I'd wager it was a '65-67 B-body rear, as they're narrower than the 68-70 B-body rears. Either that or they weren't stock 7" rims. I had 225/60/15's on a set of 15x7" cop rims (4.25" backspace) on my Duster with the 68-70 B-body rear before I went big, they barely fit. No way they'd work on a Dart, they would be well outside the quarters.

B-body rears had 6 different widths, depending on what year and what model B-body they came out of. So with a B-body rear it's VERY important to list the year of the donor as well.

Drum-to-drum measurements of B-body 8 3/4's

'62-'63= 58 1/2" (And '64 Max Wedge)
'64 = 60 7/8" (Exc.Max Wedge)
'65-'67= 59 1/2"
'68-'70= 60 1/8"
'71-'74= 63"
'71-'73 wagon= 64 3/8"

The rims were American racing, don't recall B/S. 55" flange to flange on rear rings a bell but I definitely could be wrong . I do remember rewelding the perches aprox 1" narrower than the stock B body rear.

See, I told you, you were in for some fun.
Just map it. spend an hour under it. Then you will know what will work.