There are several versions of the 8 3/4 centersection, and up until the mid-'60s, the differential had a bolt-together pinion yoke, making those undesirable for performance builds. The most common casting numbers for the 8 3/4 centersections end in the numbers 741, 742, and 489, with the 741 case being the least desirable due to its small diameter pinion bearing. Inside the cases, there were two types of Chrysler Sure Grip, commonly called the clutch style and the cone style, used to apply power to both axles during hard acceleration, but let the inside axle slip to maintain control when cornering. Both of these differentials worked well, but the clutch style is considered better of the factory Sure Grips because of its simpler design and its ability to be rebuilt when worn out. And speaking of wearing out, any factory 8 3/4 that has been used extensively, especially in a muscle car, is likely worn out by now. Since the rear in our Road Runner was suffering multiple ailments, including gear whine and chatter during tight turns, we decided to replace it.

Because our Road Runner will see mostly street driving, with some aggressive driving thrown in during weekend trips to the dragstrip, we decided that a Sure Grip unit would be the perfect choice for our B-Body. Additionally, since our A-833 four-speed transmission ends in a one-to-one ratio (no overdrive), we needed a set of gears that would allow highway cruising, counting on the relatively low first gear of the four-speed to get our weighty B-Body out of the hole. Knowing we're planning a future engine change to a larger and more powerful engine than the 383 currently in the car, and having experience driving cars with a multitude of engine and gear combinations, we decided that a 3.55 gear ratio would be about right. This gear ratio is a bit of a compromise on both ends, sacrificing a little bottom end and a little top speed, but is a great all-around gear that will allow us to drive the speed limit on the interstate while keeping our engine at a reasonable rpm.

With our decision made, we called Randy's Ring and Pinion to see what our options were. Building rear differentials for everything from industrial applications to all-out high-horsepower racing vehicles, the team at Randy's has good experience and quickly suggested, although they did have aluminum centersections for the 8 3/4, that we should build a differential using a factory 742 case for our application. Inside, Randy's installed a new Auburn Sure Grip unit, new heavy-duty 3.55 ratio ring and pinion set, and all new bearings and seals. Additionally, we chose to install a billet pinion yoke, taking away the weak link of the somewhat brittle factory yoke.

Once ordered, Randy's Ring and Pinion shipped the completed centersection directly to us, and local shop Inline Performance Specialists was nice enough to let us use one of their lifts to install it. The difference? We'll just say dramatic. The new differential is far quieter than our worn out 3.91 geared 489 case factory rear, and we even took away an annoying vibration that we later found was caused by a worn pinion bearing in the unit we removed. Even better than our newfound smooth and quiet operation, the 3.55 gears allow us to cruise at highway speeds without excessively revving the engine. Not bad for a phone call and a couple of hours on the lift.

Inline Performance Specialist
Randy's Ring & Pinion (Yukon Gear)
10411 Airport Road SE
WA  98204