7 1/4 to 8 3/4 rear end swap. HELP!!!!!

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318Duster'72

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I want to swap out the 7 1/4 rear end in my duster for an 8 3/4 inch.

The only issue is that on the car the distance between the bolt mounts (where the springs u bolt to the diff -- you know what i mean) .. that distance is 43 inches on the car and 44 inches on the 8 3/4 i want to put in. What i'm trying to say is that it is an inch longer than the diff currently on the car.

Is there any way to make this work?? Is it even a big deal?? I dont know!
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SEE ATTACHED PHOTOS AS WELL -- couldnt post new diff image too large, its a 489 non posi with no breaks on it atm.

Thanxx!!!!!

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Sounds like you have a B body 8 3/4 rear. Some folks say it's OK to just pull the springs out and just bolt it in. Will it work? Yes, but it is better to cut the perches off and weld them back on, or use new ones. In doing so it is important to maintain the correct pinion angle.

The disadvantage to just pulling the springs out, is it will prematurely wear the spring eye bushings.
 
probably best to move perches also youre going to need new shock mounts and u bolts as well because the axle tubes on an 8 3/4 are larger
 
You need to move the spring perches if you're going to do it right.

And ditto on needing new shock mounts and U-bolts, the 8 3/4 has 3" tubes and a 7.25 does not. You'll also need to have the driveshaft shortened by about 2.25", but you'll want to measure for the driveshaft length on YOUR car.

I converted my '74 Duster from a BBP 7.25 to a 68-70 B body 8 3/4. When I did it I installed a 1/2" spring offset kit, Dr. Diff sells them. Since I was welding on new perches anyway I figured I'd pick up the extra 1/2" of spring clearance.
 
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72bluNblu said:
You need to move the spring perches if you're going to do it right.

And ditto on needing new shock mounts and U-bolts, the 8 3/4 has 3" tubes and a 7.25 does not. You'll also need to have the driveshaft shortened by about 2.25", but you'll want to measure for the driveshaft length on YOUR car.

I converted my '74 Duster from a BBP 7.25 to a 68-70 B body 8 3/4. When I did it I installed a 1/2" spring offset kit, Dr. Diff sells them. Since I was welding on new perches anyway I figured I'd pick up the extra 1/2" of spring clearance.
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I dont understand why you welded on new perches and you moved the springs.. wouldnt you just do one or the other?
 
No, because you're not moving them in the same direction. The 1/2 offset kit makes the springs narrower, not wider. The center to center measurement on my car is now 42". The point is to add tire clearance, and I did it at the same time because the perches had to be moved regardless.
 
318Duster'72 said:
I dont understand why you welded on new perches and you moved the springs.. wouldnt you just do one or the other?
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If you do this, what I did when I moved mine was use a straight edge across the front of the pad right where it touches the axle tube, and I etched a line inward about an inch or so. If there is any weld spot there, grind it back gently so your straight edge butts up to the perch itself.

Then I cut the perches off very carefully, cleaned the area up and moved it in one half inch lining it up with that line I scribed and tacked it. Repeat on other side.

Check all of your measurements from axle flange to side of perch on both sides, making sure they are equal, then check center to center for the 43 inches.

The reason I did that was to keep the pinion angle the same. It worked for me. I checked it once in the car and it was perfect, no vibrations at all.

If you're on a tight budget, you CAN oblong the holes a bit on the shock mounts you have and use them until you can come up with the proper mounts. Just be sure there is enough meat left so they don't break.

You will need new U bolts, tightened to 45 FT LBS, with the weight of the car on the axle/springs.

Lastly, you will need to shorten your driveshaft and have it re-balanced.
 
Nobody has mentioned overall width. Won't the B-body axle push the rear wheels further out in the wells? This could be an issue with wide wheels -- you would need wheels with more backspace.
 
mvh said:
Nobody has mentioned overall width. Won't the B-body axle push the rear wheels further out in the wells? This could be an issue with wide wheels -- you would need wheels with more backspace.
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Yup, the B body rear end will push the wheels further out. Since the OP is talking about a Duster though it isn't as much of an issue, the stock 7.25" rear was significantly narrower than the front track width. It could be a problem if he's running aftermarket wheels with very little backspacing, but based on the pictures at the top of the thread the wheels appear to be some kind of OE steel wheel with the tires right up against the springs. When I switched my car over I was running 15x7" mopar cop wheels with a 4.25" backspace and 225/60/15's, I didn't have any issues with the wheels fitting. They were about as far to the outside of the wheel well as they could be, but they didn't rub.

Of course there are several different B body axle widths with a 44" spring perch, but I'm running the 68-70 B rear which is the widest of the group with 44" spring perches. If he's got a 65-67 B rear it would be a bit narrower and give a little more room. Now, if he wanted to really max out the tire width the backspace would have to increase compared to the stock wheels.
 
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