Although it's about as useful...
Although it's about as useful as a one-legged man in a kicking contest, we had this 4.10-geared, 489-case centersection with an open differential just lying around. The gearset was pulled from one of the Mopars that passed through our hands over the years, and though the low ratio offered the potential for strip performance, the lack of a limited-slip differential negated the dig offered by the ratio.
One of the most appealing things about a Mopar 8 3⁄4 rear, besides its ruggedness, is the drop-out center-section. With this design, ratio changes can be made with relative ease and speed. And, setting up a fresh gearset can be done on the bench rather than under the car. It's not uncommon for Mopar fans to have a couple of extra sets of gears, packing away the hardware for the task at hand. Centersections for 8 3⁄4 rears are available ready to go from the many rebuilders and wholesalers, but with a little know-how, you can build your own from a junkyard core and the required parts. There is a certain amount of mystery in setting up a set of gears, but with a little time and care, a capable enthusiast can roll his or her own.
The first hurdle in tackling such a job at home is the seemingly endless list of special tools for the job. These items add up to an investment far greater than the average Joe can justify when putting a gearset or two together every other year. While lacking a boatload of professional tools won't stop a successful rear-gear build, it may slow you down. Other than a dial indicator to set the gear lash and an inch-pound torque wrench to check the bearing preload, it can be done with just the basics. In fact, we've known guys with the magic touch who can set the lash and bearing preload by feeling it. Here's the basic rundown of what needs to be done.
Dismantling an 8 3⁄4 centersection is fairly easy to do. Begin by removing the adjuster-lock tabs at each bearing cap, loosening the main cap bolts and then backing off the adjusters at each side of the bearings with a suitable spanner wrench. We use a simple homemade spanner tool fashioned from a length of bar stock with a pair of hardened 1⁄4-inch bolts welded to the end to engage the drive slots in the adjuster.
Getting Loaded
The pinion gear rides on bearings placed at each end of its shaft and is tightened into the case by the pinion nut. Between the two bearings is a spacer that sets the minimum distance allowed between the bearings, acting as a stop to keep the pinion nut from crushing the bearings as it is tightened. Early 742 case rears used a solid spacer and shim arrangement, which is simple to understand. Tightening the nut increases the pressure applied between the bearings and the races until the bearings come to a positive stop against the spacer. By allowing the spacer length to adjust via added shims, the effective length of the spacer can be set to stop the bearings where they are preloaded correctly against the inner bearing races. This is the bearing preload. By placing an inch-pound torque wrench on the pinion nut-after it is tightened-and turning the nut, the torque wrench measures the resistance the bearing is subjected to. If the preload (inch-pound number) is too high, adding the appropriate shim will back them away from the races, lessening preload. If the pinion is too loose, removing some shim will allow them to bear more tightly into the races, increasing preload. To determine the required shim thickness, the pinion may need to be removed and installed a few times until it's correct, but the procedure is pretty simple.
The 489 case centersections that came later did away with the spacer and used a crush sleeve instead. Think of the crush sleeve as a variable spacer. As the pinion nut is torqued to spec, the crush sleeve collapses. Tightening the nut allows the crush sleeve to compress enough to let the bearings press against their races with the required preload, measured in the same way as the above spacer. The trick is to stop tightening when the required preload is reached. Problems arise if the crush sleeve is compressed too much, allowing too much preload. Backing the nut off will relieve pressure, but the crush sleeve is compressed and will not expand. A crush sleeve is designed for one-time use, and even removing and installing a new yoke can throw off the tension. What's worse is crush sleeves have been known to lose tension under extreme abuse, causing the pinion nut to lose its torque, which leads to the pinion flopping around like a fish out of water, trashing a set of gears. Fortunately, aftermarket shim-adjusted spacers are readily available for the 489 case-an upgrade we strongly recommend.
 With the adjusters extracted,...  With the adjusters extracted, remove the main cap bolts and caps; the ring gear and differential assembly can then be lifted from the case. |  The ring gear will need to...  The ring gear will need to come off the old open differential if the same gearset is being re-used. The attachment bolts are left-hand thread. |  The ring gear is a light-to-tight...  The ring gear is a light-to-tight press fit over the register edge of the differential carrier, so it usually takes a little persuasion to get it off. A few taps around with a copper mallet had ours free. |
 For the 8 3⁄4, limited-slip...  For the 8 3⁄4, limited-slip differentials are available from a variety of sources and in a variety of styles, including original and replacement Sure-Grips, lockers, and even spools. We happened to have a decent looking cone-clutch Sure-Grip on our shelf, begging to be mated to our gears. |  Cone-style Sure-Grips are...  Cone-style Sure-Grips are worn out when the cone clutches bottom-out in the case. To inspect this unit, the differential was dismantled and then reassembled with some clay at the clutch-cone ends to gauge the remaining clearance. We found .060 inch, which indicated plenty of service life left. If the clearance is near or at zero, the clutches won't grab, and the limited-slip function will not work. Machining the cone ends and adding spacer shims behind the side gears can breathe new life into a worn cone-clutch Sure-Grip. |  With the inspection passed,...  With the inspection passed, our used Sure-Grip was simply cleaned and reassembled with gear oil as assembly lube. |