8 3/4 rear end swap

[droptop]

Getting ready to swap the rear in my 80 Road Runner to a 69 B body 8 3/4. Question about the brakes. Will the brake backing plates bolt up correctly from my original rear or will I have to get ones for the bigger rear?

 

[Oldiron440]

You can use the Fbody backing plates along with the rest of the parts.

 

[F body Deconstructor Jim]

Easy direct swap over.
80 should be 10x2-1/2" w 5 bolt flange.
Standard MOPAR brake "pattern" since 65 ish.
EXCEPT A bodies...ug...pain in the ass....lol

 

[droptop]

Thanks guys. That is what I thought, but wanted to be sure before ordering parts.

 

[BudW]

The 10” brake backing plate will be the same for the ’69 and to yours. The same might also apply to the brake shoes (not checked – so not exactly sure). The brake shoe hardware has been upgraded between the two car years though. The brake wheel cylinders are a slightly different size (diameter piston) and brake cables will not interchange.

The brake drums will interchange, as well. With that said, don’t let the weight of the differential (or car, for that matter) sit on the brake drum, or you can/will warp the brake drum. If there is a possibility that resting the weight of differential on it, take the brake drum OFF. The backing plate is strong enough to support the weight.

A couple of things to keep in mind:
The center of axle to the center of propeller shaft U-joint is different between the 7¼” and 8¼” as well between the 8¾” – so propeller shaft length will need to be cut shorter a bit.
Approximate distances from center of axle to center of U-Joint:
7¼” - 10.09"
8¼” - 11.69"
8¾” - 12.35"

Also, the spring perches have a centering hole in them. Going by memory, the 8¾” has a ½” hole in the spring perch (used by the centering bolt on leaf spring to keep differential in correct location). The FMJ’s have a larger centering hole, which is 1-5/8” in diameter (if I remembered the number correctly) because of the ISO-clamp system. Because of that, you have a choice to either get rid of the ISO-clamp (my recommendation for best suspension ride) or to make the perch hole larger (which is not an easy task and to keep hole in exact location).

The ISO-claim is a weak link on FMJ’s, and the rubber biscuits create a rougher ride. Also eliminating the ISO-clamp, you will raise the rear of car up about ½” without doing anything else (the thickness of the topmost rubber biscuit).

One last thing is the ’65-70 B-body 8¾” spring perches are 44” wide (center to center). FMJ’s perches are 44.46” wide. That is 0.23” difference per side (less than ¼”) - which the leaf springs can allow for without too much complaint.
BudW

 

[droptop]

Thanks everyone. I have completed this same swap many years ago but could not for the life of me remember if the backing plates interchanged or not. I remembered about the spring perch width and drive shaft differences. And if I recall correctly, I used the 69 8 3/4 shock mounting plates to get rid of the iso spring biscuits.

 

[BudW]

’60-73ish A, B, C and E-body rear shock plate (but not one for a 7¼” differential, which has a smaller diameter axle tube or ’70 E-body). A fairly popular part. Chrysler part number 1856346/1856347.

The FFI B-body ISO delete kit Firmfeel Mopar Suspension and Steering is a pretty complete kit but a bit pricy ($265 (US) plus freight). FFI’s FMJ ISO delete kit still uses the lower stud shocks which I really really dislike working with and will not work if you use (or plan on using in the future) air shocks or spring supplemented shocks. For the same price, I would recommend their B-body kit over the FMJ kit.

FFI's B-body kit

FFI's FMJ kit
If you don't know, the blue arrow is the leaf spring centering bolt (ISO uses a larger head and nut), orange points to washers which are 1-5/8" in outside diameter/½” inner diameter (to center the differential perches to springs), and yellow is for rear sway bar brackets (which most people don't need or use).

You will need new leaf spring centering bolts as well as U-bolts so, even if you found a pair of used shock plates (going price is maybe $125ish (US)), the FFI kit price might not be so bad after you price the other bolts.
BudW

 

[droptop]

I have the 69 shock mounting plates. Part of what my wife refers to as my million dollar inventory. LOL. All I need to buy is new u bolts. And hard brake lines.

 

[BudW]

Other things you will need is:
- Rear shocks (using ’69 Charger will help locate those)

- Rear leaf spring centering bolts (Again, ’69 Charger will help there as well), ½” size head/nut. The existing leaf spring centering bolts have a larger head (¾”, I think) and a larger nut. Now, a person could take a drill (¾”) and drill the shock plate center hole and differential perch hole larger, but I think the cost of new centering bolts will be cheaper than a ¾” metal cutting drill bit will be (if you can find one).

- Rear brake hose. If a car is over 10 years old and the brake hose you will be working with hasn’t been touched in that time frame, I highly recommend replacing the brake hose, as well. I “think” the metal lines will work with either brake hose – but don’t remember.
The 8¼” differential and 8¾” differential metal brake lines are bent differently – so new lines would be good.

The B-body rear brake hose looks like

And it goes to a 3-way brass block.

The FMJ rear brake hose has the 3-way block built into the hose.

I have never placed the two hoses side by side to see if hoses (with blocks) will interchange or not but I do know the FMJ hose will work.

- 8¾” pinion snubber will be needed and might already come with your new differential.
BudW

 

[Duke5A]

On mine the factory 8.75" pinion snubber was a mile away from the floor plate. I think Mancini sells an adjustable one.

The factory break line T isn't long enough either. It can be made to work if you pull the bracket that mounts the end to the floor board down, but I don't recommend it. Doctor Diff sells the correct length T if you can't find one locally.

 

[MiradaMegacab]

TTT