Carrier bearing preload is closely associated with backlash in that it's adjusted via the same shims. Essentially, after the point where there are enough shims for zero side-play of the carrier, you add a certain amount more to the shim back to "squeeze" the bearings even more. That means you will have to force the carrier/bearings and shims into place in the housing, or spread the case with a case spreader. Most times, you will add .005 to .008 inch (or whatever is specified) to the shim pack for preload. You would add this thickness after you have achieved the correct backlash and gotten a good pattern. Then, on the final assembly, you add the extra thickness and then carefully drive the assembly in with the plastic mallet, or drive in the shim separately with a special driver as shown here (available at Randy's). You should recheck both backlash and the pattern after this, but usually it won't change.
A case spreader is nice but not really necessary except in the case of the bigger axles (Dana 70 and 80, et al), where it's more difficult to "drive" the carrier in with enough shims to adequately preload the bearings. Shown is a Randy's spreader on a Dana 60. A spreader is easily found for Dana axles, but more difficult (or not applicable) for Ford or GM units.
While most axles use shims and spacers for carrier-bearing preload and backlash adjustment, some use adjusters. Most "drop-outs," like the Ford 9-inch, the Mopar 8.75, the Toyota and Samurai diffs, use this setup. A few integral carrier units also use them, including the GM 14-bolt, and the newer AAM 10.50 and 11.50 units in late model 1-ton GM and Dodge trucks. The basic setup specs don't change, but instead of changing shims, you work the adjusters to obtain the correct backlash and preload.
Reading the pattern is your final setup check. Paint three or four teeth on both sides in three places around the ring gear. Rotate the axle in both directions through the marked areas. The key element in getting a readable pattern is to apply rotational resistance to the ring gear as you turn the pinion. That will give you a nice, clear pattern. On standard-rotation axles used in the rear, look first to the drive side (convex, curved outward part) of the tooth as the primary indicator. This would apply also to reverse-cut (high-pinion) axles used up front. For standard-rotation axles used up front, or high-pinion axles used in back, read the coast (concave) side of the tooth first. The drive and coast patterns may look completely different, so if your drive side is perfect and your coast side is so-so, go with the drive-side pattern. Shown in living color is a good pattern.
We've also included the Yukon chart that shows acceptable and non acceptable gear patterns. There is one key "real world" element to keep in mind, and Randy Lyman, founder and owner of Randy's Ring & Pinion, is adamant about it. "The most important part of the pattern, the one that tells you about the pinion depth, is the position between the face and flank," he says. "It can be very difficult or impossible to get the heel-to-toe position centered because there are so many nonadjustable variables that affect it, like housing alignment and the way the pinion-bearing bores were machined. Changes in backlash can move the pattern, but you have to stay within the manufacturer's specs, and often the few thousandths you have to play with won't be enough to change the pattern much." In general, just so you know, contact biased towards the toe can be moved closer towards the center by increasing backlash, and heel contact can be moved down to the center by decreasing backlash.
Setting the backlash is one of the two major elements involved in getting a correct pattern. Backlash is essentially the clearance between the teeth of the pinion and the ring gear. It's measured in thousandths of an inch. You can't get an accurate pattern if the backlash is out of spec. The backlash is adjusted by the number of shims on each side of the carrier. | 
To reduce backlash, you decrease the shim thickness on the bearing farthest from the ring gear and increase it on the side closest to the ring gear. To increase backlash, do the reverse. (Image courtesy General Motors.) | 
Measuring backlash is done with a dial indicator with a magnetic base. It should be checked at three places around the ring gear and the readings averaged. A wide discrepancy between the readings that you can reproduce is a cause for concern and could indicate too much runout in a carrier or a ring gear, or other problems. |