moparmark65
Well-Known Member
I have a 69 dart gts that has a 7 1/4” rear end. I have found a ‘70 8 3/4” for B body. What will I need to do to make this fit properly?? Thanks for the input!
Swapping rear end question.
- Thread starter moparmark65
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Professor Fate
Push the button, Max...
A '69 GTS should have an 8.75, so I'll assume the 7.25 was swapped in or is just there to make it a roller...
Read this post carefully: An accurate 8 3/4" rear axle width list
A factory A body 8.75 housing is 52 5/8" wide. A '70 B body housing is 54 15/16" (let's call it 55 here for simplicity's sake).
That means the B body unit is 2 3/8 wider than an A body diff, or 1 3/16" per side.
B body brakes are wider than A body units, which increases the difference to about 3" overall, or 1.5" per side. This will all have to be made up for with wheel offset, especially in the confines of a Dart's rear wheel opening. The other option is to have the housing narrowed to A body specs.
As far as getting it mounted, the spring perches will need to get cut off and new ones welded on at the correct A body dimension, and keeping the pinion angle within spec. The 7.25 U bolts and shock plates are for 2.5" axle tubes, and the 8.75 has 3" tubes, so those will need to be replaced with the proper ones.
The driveshaft for a 8.75 is *about* an inch shorter than one for a 7.25.
The axles for a B body rear have a 5x4.5" bolt circle, all pre-'73 A bodies have a 5x4" bolt circle, which means your front and rear bolt circles (and wheels) will be different.
Read this post carefully: An accurate 8 3/4" rear axle width list
A factory A body 8.75 housing is 52 5/8" wide. A '70 B body housing is 54 15/16" (let's call it 55 here for simplicity's sake).
That means the B body unit is 2 3/8 wider than an A body diff, or 1 3/16" per side.
B body brakes are wider than A body units, which increases the difference to about 3" overall, or 1.5" per side. This will all have to be made up for with wheel offset, especially in the confines of a Dart's rear wheel opening. The other option is to have the housing narrowed to A body specs.
As far as getting it mounted, the spring perches will need to get cut off and new ones welded on at the correct A body dimension, and keeping the pinion angle within spec. The 7.25 U bolts and shock plates are for 2.5" axle tubes, and the 8.75 has 3" tubes, so those will need to be replaced with the proper ones.
The driveshaft for a 8.75 is *about* an inch shorter than one for a 7.25.
The axles for a B body rear have a 5x4.5" bolt circle, all pre-'73 A bodies have a 5x4" bolt circle, which means your front and rear bolt circles (and wheels) will be different.
HemiSSDart
Dartus Interuptus
The only B-Body rear that is almost a swap for an A-Body is a 65-67 unit and the spring perches are only 1/2" wider than and A-Body. I have done this swap a couple of times, it is pretty much a bolt in and the rear hangars can make up for the 1/2" difference. The biggest issue on a Dart is that the wheels will have to be made or found for the correct backspacing to bring the tire in closer to the frame rails, I had a set made. On a 65-69 Barracuda and a Duster/Demom it is much easier to find wheels as they wheel wells do not shroud the tire nearly as much as a Dart...
moparmark65
Well-Known Member
Mine is a mexican built Dart. They came wirh 270hp 318.May be why the 7 1/4. With all that has to be done, i will keep looking for an A body rear 8 3/4” i much appreciate the input. Makes my decision much easier. Thanks
Professor Fate
Push the button, Max...
Aaahh- that explains a few things!
Keep us posted on your progress, I really like reading about the oddballs! (non- North American cars, that is!)
Putting a 1/2" side load on the springs is dumb. And because the perches are 44" c-c on the B body rear and 43" c-c on the A body rear you're also losing spring clearance for the tires. In addition to all the side load on the springs/bushings/hangers.
And Barracudas, Dusters and Demons have an extra 1" of width in the outer wheelhouse, which is what makes the wider rear axle easier to deal with. The higher wheel opening helps as well, but the extra inch is a big deal.
moparmark65
Well-Known Member
Ya, I would have the perches moved to accommodate the 1” if thats the route i took. I have brand new leafs on it now, want it plum and correct! Thanks
HemiSSDart
Dartus Interuptus
I will take a guess that you have never done that swap. For a street car with limited track time it worked just fine...
The b swap without moving the perches has been done as long as these cars have been alive.
Is it better to have the perches at 43" yes. Can a driver car live a long life with them at 44", absolutely.
Is it better to have the perches at 43" yes. Can a driver car live a long life with them at 44", absolutely.
moparmark65
Well-Known Member
Your right I haven’t, but if I have to shorten the tubes why not just cut the perches off and place them in correct location. Not going to be that much more work.
moparmark65
Well-Known Member
If I used my existing 71/4” for measuring flange and perch location, wouldn’t that put me on track?
mygasser
Well-Known Member
you could fit an exploder 8.8 axle, much cheaper than an A body 8 3/4 for sure. buy a second short side axle shaft when you pick up the axle, or a new one from rock auto at $100 ish. then cut the long side of the casing down 2 7/8" to match which also centres the pinion. you end up with the overall length almost identical to an A body 8 3/4 but with several advantages. beefy 31 spline shafts, lsd, disc brakes and they're the same pcd at 4.5 x 5 the same as bbp mopar. plus because the 8.8 is widely used in mustangs parts are cheap and readily available. there's lots on here about the swap but you get the idea.
neil.
ps i did this swap in my 68 valiant. i got 8 1/2" slot mags with 295 50 15 tyres under the stock arches with no spring relocation or mini tubbing.
neil.
ps i did this swap in my 68 valiant. i got 8 1/2" slot mags with 295 50 15 tyres under the stock arches with no spring relocation or mini tubbing.
mopowers
Well-Known Member
That's pretty cool with no spring relocation at all. Do you happen to have a photo you can share? I've always thought 255s were pretty much the max with the stock spring location.
mygasser
Well-Known Member
here's one having gone back through my build it reminded me i did roll the arch lips up but that only gained i guess 1/2" at most. i'd fitted new ss rear leaf springs which makes it sit up at the rear. the front's high as i'd rolled it outside to turn it round ready for installing the engine/box so no weight at the time.
neil.
having gone back through my build it reminded me i did roll the arch lips up but that only gained i guess 1/2" at most. i'd fitted new ss rear leaf springs which makes it sit up at the rear. the front's high as i'd rolled it outside to turn it round ready for installing the engine/box so no weight at the time.neil.
I have a ‘68-70 B body 8 3/4 in my Duster. So yes, I’ve done it.
And I moved the perches, because I can weld and understand how the suspension is supposed to work.
I’m sure lots have people have done the shade tree install, and quite frankly I don’t care. Doing it wrong and lacking the ability to tell the difference does not make it right. Not to mention, you’re losing tire clearance doing it that way too.
mopowers
Well-Known Member
Thanks. Cool Valiant too!here's one
neil.
View attachment 1716486045
Not sure about others, but I think it's pretty damn cool that there are badass mopars like this all over the world. Cheers, from the other side of the pond.
HemiSSDart
Dartus Interuptus
Well you make the assumption that everyone has your abilities, most don't, I am glad you do. I'm just telling him the easy way to do it that works and has been done dozens of times over the years...
I haven't seen anyone mention the drive shaft. It will have to be shortened. I believe it's usually around 1.5".
More shade tree hillbilly stuff. You can open up the alignment pin hole in the perch as well to allow the spring to not be stressed if that is a concern.
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