How To Install A Big-Bolt Pattern - Small Bolt Swap-Out

One of the odd things about pre-'72 Mopar A-Bodies was the use of five lightweight 7⁄16-inch wheel studs on a small 4-inch bolt circle. While Chrysler used the common 4 1⁄2-inch bolt pattern and beefy 1⁄2-inch studs on virtually every other car line (the early Imperial's 5 1⁄2-inch pattern being a notable exception), the early A-Bodies were saddled with an econo-box wheel pattern. Going back in the genealogy to the A-Body's origins, as Chrysler's compact economy entry in the early '60s, the five-on-four pattern didn't look so bad compared to the competition's compacts, which usually sported even more miserly four-bolt patterns.

In 1973, the big-bolt pattern was finally made available on disc brake-equipped A-Bodies. Previously, the disc brake option consisted of the small-bolt pattern Kelsey Hayes four piston fixed caliper brake set-up, unique to the A-Body car line. The "new" brake set-up brought in for '73 was basically an adaptation of existing large-car components to the diminutive A-Body line. With the large car disc brakes also came the "big car" five lug on a 4 1⁄2-inch bolt pattern, and stout 1⁄2-inch wheel studs. Of course, to go with the large pattern up front, the rear was also revised to include the same large bolt pattern, as well as off-the-shelf "big car" drum brakes, replacing the skinny rear binders previously used exclusively in A-Body applications.

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.

The first order of business was to locate a suitable 8 3⁄4-inch rear for our conversion. Being highly sought after, A-Body 8 3⁄4 rearends tend to be pricey at the swaps, and are always thin at the boneyards. Looking for the low buck score, we checked the pickings at our local wreckers and found an early Valiant with an 8 3⁄4-inch perched on its springs. Pre-'65 8 3⁄4-inch rears used the old style tapered output axle shaft, with a separate hub secured by a large nut, as opposed to the modern flanged output shafts used from '65-and-up. The rear had obviously been sitting there for quite some time, overlooked by all takers because of the oddball tapered shaft and hub output. Since we knew we'd have to change the shafts and brakes anyway, we stripped out the stock old-style shafts and low-performance differential. We brought home the bare housing at the bargain basement price of $19. These old tapered-shaft rears can be to the modern flanged output axles simply by using the corresponding late-type axle shaft seals and bolting in the modern axle shafts. The housings themselves are essentially the same.

Although any of the above discussed approaches to converting the bolt pattern would get the job done, when we were ready for the job of changing over our 8 3⁄4-inch, we picked up the phone and ordered a set of custom axle shafts from Moser Engineering. The Moser shafts are 4340 steel forgings, ensuring they'd be up to any dragstrip punishment we would subject them to. Since they are a custom axle, Moser can build the shafts to whatever specification is required. Want to keep the original narrow A-Body brakes while moving to a beefier large-flange shaft? Just order the custom shafts with the stock backspacing (you'll still have to drill the drums to match). Our choice was to have the shafts built with the wider, large-car backspacing, which allows us to run the much more common and beefier big-car brakes (we went with mondo 11x2 1⁄2-inch units). For those who want to take either of the bucks-down paths to the changeover, Moser can redrill the stock A-Body 8 3⁄4-inch shafts, or perform the cut and re-spline job on a suitable set of used replacement axle shafts.

With the new axle shafts, our conversion was a true bolt-on task with no custom machining or one off components. The only trick was to locate a set of parking brake cables of the correct length to match our A-Body chassis. Some boneyard digging and measuring, and we were able to locate cables to fit our application (taken from a '76 Dart with 11x2-inch brakes). The cables must come from a braking system of the same diameter (but not necessarily the width) as the drums you are going to run, since the length at the apply end must match the diameter of the drums (e.g. cables from 10-inch brakes if running 10s or from 11-inch brakes if running 11s). With all of the components in hand and a new high performance gear set (see "Pig Hunt," Mopar Muscle, July '99), putting together our modified A-Body 8 3⁄4-inch was reduced to a simple nuts-and-bolts task. The photo captions outline the basic wrenchwork.