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?