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.
We ordered a bearing and seal...
We ordered a bearing and seal kit for our rear from Randy's Ring and Pinion. The kit contains the required replacement bearings, races, seals, gasket, and crush sleeve, as well as a selection of shims and gear-marking compound needed to set up the gears.
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?
The bearings simply press...
The bearings simply press onto the differential. We used a pressing tool that bears on the inner race of the bearing. Pressing on the outer basket will ruin the bearing.
Although we reused the same...
Although we reused the same gear set, the pinion gear was removed to change the pinion bearings and clean everything up. Step one after reassembly was to drive in the new bearing races using a socket as a driver.
Since our pinion was being...
Since our pinion was being reused in the same case, we didn't change the pinion shim, which sets the pinion depth, a critical spec for proper gear mesh. The shims slip between the pinion gear and the bearing before the bearing is pressed on. Required pinion depth is easily checked, but only if the elaborate set of special tools required is on hand. Without these tools, some trial and error is needed. It's best to just set up the gears with the same shim that came out and then check the gear-contact pattern after the rear is assembled. The contact pattern will reveal if the pinion depth is off and needs adjustment.
As stated, setting up gears...
As stated, setting up gears without the special tools for measuring pinion depth generally requires a full assembly and then a check of the gear-mesh pattern to determine if the pinion depth is correct. If the pattern shows the gear mesh is off, the gears will need to be removed to change the pinion shim. That is why the crush sleeve is not installed until final assembly. In final assembly of a 489 case like ours, a new crush sleeve is positioned on the pinion, and the pinion nut is torqued to collapse the crush sleeve until the preload on the bearings results in a turning torque within specs. Since the crush sleeve cannot be reused, if the pinion depth is off, installing a crush sleeve on initial assembly will ruin it. It is common practice to leave it out during the mock-assembly stage and simply tighten the pinion nut to reach the specified turning torque. Once the correct shim is confirmed by the gear-mesh pattern, tear the rear down to install the crush sleeve, and do the job all over again. To prevent the pinion from turning while torqueing the pinion nut, we used this homemade pinion flange holder that bolts to the yoke.
In lieu of the crush sleeve,...
In lieu of the crush sleeve, a solid spacer can be substituted into a 489 case, which is the better way to go for several reasons: The solid spacer holds its dimension better under high-load use; it can be reused multiple times; and it allows the drive yoke to be removed and replaced without the possibility of altering the gear setup. Most importantly, though, the solid spacer can be installed normally during the trial assembly, 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), and uses shims to fine tune the distance between the inner and outer bearings. This provides the required preload on the bearings when the pinion is installed. A good baseline setting for the spacer can be estimated by measuring the length of the old, used crush sleeve and adding shims to the spacer to match. Install the pinion and tighten the nut to specs. The tuning torque should measure between 14 and 19 in-lb for new bearings, or between 6 and 10 in-lb if the old bearings are being reused, as measured with a beam-type inch-pound torque wrench. If it's too tight, add shim; if it's too loose, take some shim out.
With the pinion side of the...
With the pinion side of the equation in place, the differential and ring-gear installation is next. Place the differential into position with the bearing races in place, and loosely install the bearing caps. Thread in the adjusters, making sure the threads are lubed and they aren't cross-threaded, and run them in until they square up the bearing races. Make sure excessive play is taken out of the bearings, and rock the ring gear to make sure the ring isn't binding against the pinion gear.