Like we said, only the correct pinion height will provide the correctgear pattern. We can set up the gears, look at the pattern, and see ifit is right. If you get it right on the first try, great--it's yourlucky day, and it's done. If not, the pattern tells us which way thepinion needs to move to get it right. The pattern shown on the ring gearwill show which way the pinion needs to go, but not how much. Now itcomes down to disassembling the gearset, making a shim change, puttingit back together, and checking it again. It may take a few rounds ofdisassembly, but this is guaranteed to zero-in on the correct pinionheight. 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 thanif done with a setting tool. Taking this approach virtually requiresthat a crush sleeve isn't used during trial fitment since the rear mayneed to be assembled a few times before it's exactly right. The bestplace to start is a trial with the original amount of shim behind thepinion bearing, and then trial assembly and checking from there. Ifstarting 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 preloadare set at the same time and are related. The ring-gear carrier(differential) rides on bearings at both ends. Their races float in thehousing bore, and threaded adjusters bear on the outside of the races ateach side to set their position. Like the pinion bearings, thedifferential bearings need preload, which is set by the adjusters.Tighten them toward each other, and the bearings will be squeezedtogether against their races, giving preload. Turning the adjustersrequires a spanner wrench--a reasonably priced tool.
Backlash is the play between the ring and pinion--the slop felt when thering gear is rocked back and forth with the pinion held so it's notmoveable. This free-play is required and must come in at correctspecifications. Measuring backlash is simple: Rig a dial indicatoragainst one of the teeth in the ring gear, and rock the ring to see howmuch free-play is recorded. Moving the ring closer to or farther fromthe pinion changes the backlash. The adjusters provide for theside-to-side travel, working the ring gear over to the correct backlashwith a spanner wrench, while at the same time cranking up the preloadgood and tight. The photo captions go into more detail.
So that's it--four adjustments to nail down, and that 83/4 rear can beset up like a pro. Really, the only trick part is homing in on thecorrect pinion-depth setting. With the rest, it is pretty easy to tellwhen it's right while the adjustments are being made. Not such adaunting task once it's all broken down, is it?
In lieu of the crush sleeve, a solid spacer can be substituted into a489 case, which is the better way to go for several reasons: The solidspacer holds its dimension better under high-load use; it can be reusedmultiple times; and it allows the drive yoke to be removed and replacedwithout the possibility of altering the gear setup. Most importantly,though, the solid spacer can be installed normally during the trialassembly, and if the mesh pattern checks out, you're done. The spacer(right) from Randy's fits in place of the stock crush sleeve (left), anduses shims to fine tune the distance between the inner and outerbearings. This provides the required preload on the bearings when thepinion is installed. A good baseline setting for the spacer can beestimated by measuring the length of the old, used crush sleeve andadding shims to the spacer to match. Install the pinion and tighten thenut to specs. The tuning torque should measure between 14 and 19 in-lbfor new bearings, or between 6 and 10 in-lb if the old bearings arebeing reused, as measured with a beam-type inch-pound torque wrench. Ifit's too tight, add shim; if it's too loose, take some shim out. | 
With the pinion side of the equation in place, the differential andring-gear installation is next. Place the differential into positionwith the bearing races in place, and loosely install the bearing caps.Thread in the adjusters, making sure the threads are lubed and theyaren't cross-threaded, and run them in until they square up the bearingraces. Make sure excessive play is taken out of the bearings, and rockthe ring gear to make sure the ring isn't binding against the piniongear. | 
Fully torque one bearing-cap bolt on each cap, while snugging down theother. Turn the adjusters in until there is a small amount of preload.At the same time, keep a noticeable amount of backlash in the gears. Youcan feel the adjuster tighten and remove any bearing play. Starting witha large amount of backlash, turn the left adjuster (opposite side of thegear teeth) as tight as you can, pulling on a 12-inch long spannerwrench. Tighten the right-side adjuster to make sure it's tight, andthen measure the backlash. Randy's Ring and Pinion recommends .006-.010inch of final backlash, easily checked with a dial indicator. Backlashshould be checked at four different positions by rotating the ring andpinion. If the backlash is too wide, back the right (ring-tooth side)adjuster off a notch, and then bring the left back in to full preload.Again, wrench the right adjuster to make sure it's tight, and recheck.Work the ring gear toward the pinion in this manner until the backlashis within specs. Once the backlash is set, torque the remaining main-capbolt on each cap. |