We chose a billet yoke that...
We chose a billet yoke that utilizes U-bolts instead of straps to hold the universal joint in place. The factory yokes and straps were a weak link, and this eliminates them from our combination.
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.
With the pinion gear installed,...
With the pinion gear installed, the housing can be flipped so the ring gear, Sure Grip unit, and bearings can be put in place. Shims are used for proper side-to-side alignment.
With the caps installed and...
With the caps installed and tightened to the proper torque, a dial indicator can be used to check for proper gear backlash. If necessary, shims are utilized to gain the proper backlash, ensuring a quiet gearset.
As a final step, the ring...
As a final step, the ring gear is brushed with non-drying yellow dye, then turned to check for proper depth of pinion engagement.
Again, shims can be used to...
Again, shims can be used to correct any discrepancies. Proper gear engagement ensures the stress of acceleration is placed on the proper area of the ring gear so the gearset will hold up. After this last step, the centersection is ready to be shipped.
With our new differential...
With our new differential on the way, we put the Road Runner on the lift to drain the gear lube and remove our old centersection. Fortunately, this is an easy task on the 8 3/4, requiring only that the axles be slid out slightly so that the differential can be removed from the front of the housing.
After removing the old centersection,...
After removing the old centersection, we cleaned off the sealer that had been used in lieu of the proper gasket.
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.
Silicone sealer does work...
Silicone sealer does work OK here, but using the correct gasket is a better choice.
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.
The new rear differential...
The new rear differential from Randy's Ring and Pinion can now be slid into place. It should mate flush to the housing and is held in place by ten studs and nuts that should be tightened in a star pattern.
Chrysler driveshafts only...
Chrysler driveshafts only had two sizes of universal joints, and in our case our small U-joint driveshaft didn't match up with our large U-joint rear end yoke. Fortunately, our local driveshaft shop stocks a U-joint that incorporates both sizes, so we only had to install this rear U-joint before reinstalling our driveshaft.
Despite what some might tell...
Despite what some might tell you, the pinion snubber is an integral part of most Mopar rear suspension. We suggest installing an adjustable pinion snubber and setting it about 3/4 to one inch from the floor when the car's weight is on the wheels.
Filling the rear end with...
Filling the rear end with the proper amount and type of lubricant is essential to gear life. Since our differential also features a Sure Grip, a special additive must be used to prevent chatter when turning the car.
With our new centersection...
With our new centersection in place and secure, we lowered the car but left the wheels off the floor. By running the car in fourth gear for about 20 minutes, then in reverse for several more minutes, the initial gear break-in can be performed without a load on the wheels.
Randy's recommends driving...
Randy's recommends driving 500 miles on new gears before towing or using full-throttle, so that's what we're doing. After the initial break-in, we'll change the fluid and be ready for some aggressive driving and possible swap to a more powerful engine. For now, we're enjoying the quiet ride of the new gears, as well as a reasonable rpm while cruising on the highway or interstate.