If you've ever considered upgrading the spindly 7 1/4-inch axle that likely resides under your A-Body, you probably already know that A-Body 8 3/4 rear axles are not very plentiful these days. They can still be found at swap meets and on eBay from time to time, but the chances of finding one during a stroll through the local junkyard are slim. Generally, when you do find one, the price reflects its desirability and rarity.
We were faced with this dilemma when planning a big-block swap into a Slant Six powered '68 Dart GT convertible. Then, when a friend tossed a B-Body 8 3/4 axle our way, we broke out the tape measure and began comparing. Granted, the B-Body axle is wider than the A-Body piece, but could it be made to work?
Once the axle housing was stripped of all its parts and the inside was cleaned, we cut the left-side perch from the axletube. Upon comparing the old perch with the new Mopar Performance PN P4120074, we found that the locating hole in our new perch was 5/8 inch in diameter, while the hole in our original perch measured 1/2 inch. Unless we made a spacer to fill the gap between the centering pin on the spring and the hole in the perch, the axle would not be properly located on the leaf springs. We cut 5/8 inch from each end of a short 3/8-inch steel pipe nipple and welded these small pieces into our new perches, ground them flush, then drilled them to 1/2 inch. Mopar Performance now offers leaf-spring perches with the correct 1/2-inch diameter hole, so be sure to check your application.
This is a typical junkyard...
This is a typical junkyard B-Body rear in all its grimy splendor, freshly torched from a '69 Satellite.
While at first it may seem ludicrous to even think of such a swap, there are a couple of good reasons to consider it. Aside from the aforementioned rarity and price tag of an A-Body unit, all A-Bodies through 1972 used a four-inch lug circle. The '73 through '76 A-bodies with disc brakes came with the same 4 1/2-inch lug circle that the larger cars had. Since the 8 3/4 was dropped from the A-Body at the end of 1972, all factory A-Body 8 3/4 axles have the small four-inch lug circle. Not only has the availability of aftermarket wheels in a four-inch lug circle always been a problem, but many owners of '72 and earlier A-Bodies choose to retrofit disc brake setups from '73 and later donor cars. Unless you want to carry two spare tires, you will need a pair of after-market axle shafts for your A-Body axle, but with flanges sized for the larger 4 1/2-inch lug circle. Custom axle shafts of this configuration are available, but they are sometimes costly, further raising the cost of retrofitting the beefier rear axle.
The B-Body 8 3/4 axle housing remained the same from 1962 through 1970. they are still more plentiful than the A-Body axles because nearly every B-Body built received one. Through 1964, the B-Body rear brake-drums and hubs were retained to the axle shafts via a large nut on each side. The '65 through '70 B-Body axles have shafts with the familiar flanges and slip-on drums, and this later style axle is the one you want.
The '63 through '72 A-Body 7 1/4 axle measures 53.2 inches from axle flange to axle flange, while the '62 through '70 B-Body 8 3/4 axle measures 55.0 inches from axle flange to axle flange. In addition, the leaf-spring center to center spacing of the B-Body axle is 44 inches, one inch wider than the 43-inch A-Body dimension. Not only would we have to address the 1.8-inch difference in overall width, we would have to cut off the leaf-spring perches and weld on new ones before the axle would mate to the leaf spring. Because of the extreme heat involved with cutting and welding, we drained the gear oil and cleaned the residue from the axle housing in order to avoid risking a fire or cooking the gear lube. The best way to do this is to disassemble the axle and clean the inside of the housing with solvent.
Disassembly begins by removing...
Disassembly begins by removing the brake drums, then the five nuts holding each axle retainer to the housing. Be careful not to lose the endplay adjuster lock on the right-side axleshaft.
With the nuts removed, the...
With the nuts removed, the axles can be slid out of the housing.
A pry bar works well to remove...
A pry bar works well to remove the old axle seals from the housing. Use caution to avoid nicking the inside of the housing.
With all the nuts removed...
With all the nuts removed from the carrier, it can be wiggled free from the housing. We caught the gush of gear oil in a drain pan. Although it isn't necessary to do so, we removed the backing plates at this point by slipping them off the axle retaining studs. Because we will be using all new brake lines and hardware, we snipped the old brake lines.
After cutting the left-side...
After cutting the left-side perch from the axletube we compared it to the new perch. The difference in the size of the locating hole is obvious. Perches are now available with the correct half-inch hole, so be sure to order the correct part and you can skip the next three steps.
Because the tapered, threaded...
Because the tapered, threaded portion of a 3/8-inch pipe nipple fits inside the locating hole of the new perches, we cut 5/8 inch from each end to use as spacers.