8 3⁄4 A-Body Blues
Unfortunately, the intro of the large bolt pattern brakes for the A-Body coincided with the departure of the fabled 8 3⁄4-inch rear axle assembly from the A-Body line. The 8 1⁄4-inch rear axle assembly was left as the stoutest A-Body rear available. The desirable 8 3⁄4-inch rear was never offered for the A-Body in big-pattern form, resulting in a twofold problem for A-Body performance fans. If going for the popular swap to the '73-and-up disc brakes in an earlier application, either to swap over from drums, or to take advantage of the more plentiful parts availability with the later discs, there were no production axle shafts available to bring an 8 3⁄4-inch rear to matching big-pattern status. On the other hand, if looking to upgrade from the 8 1⁄4- or 7 1⁄4-inch rears found as original equipment on later big-pattern A-Bodies, to the more performance oriented 8 3⁄4-inch rear, the same mismatched small pattern problem crops up. The only way out is to convert an A-Body 8 3⁄4 rear to the more useful 5-on-4 1⁄2-inch bolt spec.

There are a number of methods of converting an A-Body 8 3⁄4-inch rear to the big-bolt pattern, but there are some considerations to be weighed before deciding on the most advantageous approach. The most obvious and simple solution would be to pull the stock axle shafts and have them re-drilled for the big-bolt pattern. The sticking point is that the factory A-Body brake drums also have to be re-drilled to match the shafts. Drilling the axle shafts and drums is a precision operation, so forget about pulling out your trusty Black and Decker(tm) and centerpunch; you'll need to find a machine shop experienced in rear axle modifications. This approach to converting the bolt pattern has been used by Mopar fans for years. While the axle shaft flange has enough meat in its circumference (barely) to accept the big pattern, re-drilling the brake drums means that you now have a wear part which involves custom machining-not a good situation. Take this route, and you're faced with the prospect of having to go through a custom machining operation every time the rear brake drums need replacing.

Why not just pull a set of brakes from a large-pattern 8 3⁄4-inch as fitted to other Mopar car lines? It would be a nice solution- if it would work; unfortunately, the A-Body brakes used with the small pattern A-Body 8 3⁄4-inch rear were a unique narrow shoe/ drum arrangement. To match the narrow A-Body brakes, the axle shaft back-spacing from the flange to the housing was much narrower on A-Body 8 3⁄4's than with any of the large-pattern rears. The brakes from a larger car will bolt on the housing, but the drums will hit the backing plates well before they seat against the axle shaft flanges. Making matters worse, the center register which centers the brake drum on the axle flange face was also much larger with the big-pattern brakes than on the small-pattern axle shafts, making this swap a double no-go.

Another low-buck approach to a big-pattern A-Body 8 3⁄4-inch is to grab a set of large-pattern shafts from a large car (or truck) 8 3⁄4-inch rear, cut and re-spline the shafts to the A-Body length, and slap them in using large car brakes and drums. The backspacing on the shafts will now be correct for the brakes, and the center register will also be correctly matched. Be forewarned that the greater backspacing and larger brakes makes for a wider assembly than with the stock narrow A-Body brakes (about 0.8-inch overall). Tire clearance to the outer fender lip will be the critical issue here, moving the tires about 0.4-inch outboard per side, compared to the stock A-Body narrow brake arrangement. Other than the added width, the only pitfall in the shaft swap is that in many applications the shafts tapered down towards the spline end without leaving enough meat to machine new splines at the A-Body length. There were plenty of applications where the taper-down comes in at a point where enough meat is left for a re-spline job, with C-Body and early truck units being the best place to look. Measure-up before shelling out for a set of shafts for re-splining, to be sure. A final drawback to the cut and re-spline approach is that the new splines will cut through the factory heat treat, opening the door to potential axle shaft failure under heavy abuse. Getting The Shaft

Our own '69 Dart project car had long ago been upgraded from the factory Kelsey Hayes small pattern discs to 12-inch R-Body unicast rotors and sliding calipers running on factory late ('73-and-up) A-Body spindles. Since the brake swap had the front end changed over to the big bolt pattern, the rear needed to be changed to match. We took the easy and cheap way out then, simply bolting in a boneyard late A-Body 8 1⁄4-inch rear to replace the factory-fitted small-pattern 7 1⁄4-inch. Two years later the 8 1⁄4-inch was still serving us well, taking all the punishment our mild small-block would dish out. Even though the 8 1⁄4-inch was still functioning flawlessly, to facilitate dragstrip gear changes and the addition of a Sure Grip limited slip, we were intent upon swapping the rear for a bulletproof 8 3⁄4-inch.