Mopar 8 3/4 Rear-Gear Buildup - Gearing Up

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.

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.

Pinion Depth
This is the most difficult portion of a do-it-yourself gear change. Gears are designed to mesh at a precise distance between the ring gear and pinion gear, and small production variations in gear-case dimensions, or the gears themselves, can throw the alignment off. The ring gear rides on the centerline of its bearings, so the position of the pinion in the case is designed to be adjustable. Behind the large bearing at the gear end of the pinion, a shim (or shims) sets the exact position of the pinion gear in the case. More shim brings the pinion gear closer to the ring gear, while less allows it to sit deeper in the case. Only when it is at the right height will the gear mesh show the correct pattern (assuming the backlash is at spec-more on that later). To determine if the pinion height is correct, specialized pinion-setting tools are used. Unfortunately, not many of us can justify this equipment to do an occasional rear project every few years. If the same gearset is going back into the same case with the same bearings, no problem, but the more things are changed, the more likely the shim thickness will need adjustment.

Like we said, only the correct pinion height will provide the correct gear pattern. We can set up the gears, look at the pattern, and see if it is right. If you get it right on the first try, great-it's your lucky day, and it's done. If not, the pattern tells us which way the pinion needs to move to get it right. The pattern shown on the ring gear will show which way the pinion needs to go, but not how much. Now it comes down to disassembling the gearset, making a shim change, putting it back together, and checking it again. It may take a few rounds of disassembly, but this is guaranteed to zero-in on the correct pinion height. It's a trial-and-error process that takes time, but in the end, a perfect pattern shows that the gear setup is correct, no less so than if done with a setting tool. Taking this approach virtually requires that a crush sleeve isn't used during trial fitment since the rear may need to be assembled a few times before it's exactly right. The best place to start is a trial with the original amount of shim behind the pinion bearing, and then trial assembly and checking from there. If starting from a bare case, .030 inch is an average amount of shim.

Ring Gear Backlash and Preload
Although it seems we're dealing with two topics, backlash and preload are set at the same time and are related. The ring-gear carrier (differential) rides on bearings at both ends. Their races float in the housing bore, and threaded adjusters bear on the outside of the races at each side to set their position. Like the pinion bearings, the differential bearings need preload, which is set by the adjusters. Tighten them toward each other, and the bearings will be squeezed together against their races, giving preload. Turning the adjusters requires a spanner wrench-a reasonably priced tool.

Backlash is the play between the ring and pinion-the slop felt when the ring gear is rocked back and forth with the pinion held so it's not moveable. This free-play is required and must come in at correct specifications. Measuring backlash is simple: Rig a dial indicator against one of the teeth in the ring gear, and rock the ring to see how much free-play is recorded. Moving the ring closer to or farther from the pinion changes the backlash. The adjusters provide for the side-to-side travel, working the ring gear over to the correct backlash with a spanner wrench, while at the same time cranking up the preload good and tight. The photo captions go into more detail.

So that's it-four adjustments to nail down, and that 8 3⁄4 rear can be set up like a pro. Really, the only trick part is homing in on the correct pinion-depth setting. With the rest, it is pretty easy to tell when it's right while the adjustments are being made. Not such a daunting task once it's all broken down, is it?