The first things you’ll need to set up a rearend are the basic rebuild parts, such as new bearings, seals, shims, bolts, a crush sleeve and a nut. We used Reider Racing’s “full” installation kit for our 12-bolt.

If you’re starting with an assembled rearend, there are a couple of tips regarding disassembly that you should know. Always re-install the case bearing caps in their original location. To avoid mixing them up, mark the case and caps with a punch. We used two marks on one side of the case and cap (arrows) and one mark on the other side.

Here’s a trick for installing the pinion bearing races. To get the race started evenly in the seat, use a plate that reaches both sides of the race and drive it in. You’ll still have to tap it in the rest of the way with a punch, but the plate makes it easier to get it started.

The hardest part about setting up a 12-bolt is getting the pinion depth correct, which ensures that the ring and pinion mesh with each other correctly. Pinion depth is changed by adding or subtracting shim thickness (arrow) between the pinion gear and the bearing. The thicker the shim, the closer the pinion is moved to the ring gear.

The large bearing is press-fit on the pinion (which means that a hydraulic press is usually required), so a hot tip is to get a second bearing and have a machine shop take a few thousandths off the inside race so that it will slip on and off the pinion by hand. This allows you to determine exactly which shim needs to be used without having to make several trips to the machine shop to remove and re-install the bearing. Our machinist used a connecting-rod hone to machine the tough bearing surface.

This pinion-depth checking kit from T&D Machine makes the procedure relatively painless. It costs about $150 and works on GM rearends as well as those from Ford, Chrysler, AMC and Dana.

Different gearset ratios require different pinion shims. This chart shows a pretty accurate shim recommendation for various gear ratios. In our case it was right on, as we ended up with a .021-inch shim for our 3.08:1 gears.

On most aftermarket gears, the pinion depth is stamped on the end of the pinion, but O.E.M. gears, like the GM gears we used, must be measured to determine the proper pinion depth. There are two different size pinions used in 12-bolts with different bearing-surface thicknesses—1.625 inches in our case. The instructions in the T&D kit tell how to determine correct pinion height for all the different rearends, and knowing the pinion thickness gives you a master housing dimension (4.670 inches in our case, or 4.556 if the pinion is 1.438 inches). Measure the pinion-head thickness, shown here, and subtract it from the master housing dimension, which gives you a pinion depth to shoot for. With a pinion-head thickness of 1.9468, we ended up with an ideal depth of 2.723 inches.

After it’s calibrated according to the instructions, the T&D tool is installed in the housing and torqued into place. The micrometer is moved across the face of the pinion gear and stopped at the highest reading, which should be written down. You’ll notice that the mic is reading off a small magnet on the pinion gear. Because the pinion isn’t centered under the mic, the magnet gives the mic something to read off of as it swings through the arc. The magnet is manufactured to a precise thickness, which is taken into account when determining pinion depth. The dial indicator showed .648. Subtracting that from the tool’s calibrated pinion depth of 3.375 (for all 12-bolts) tells us that the pinion depth is 2.727. Since our ideal pinion depth (from above) is 2.723 inches, we need .004 inch less shim thickness. We had installed the pinion with a .025-inch shim for a baseline, so a .021 shim will give us the correct pinion depth.

Once you know which shim is necessary, have a new bearing pressed onto the pinion, making sure to slip the shim on first. The arrow shows the crush sleeve, which is centered between the two pinion bearings. When the pinion nut is tightened, the bearings sandwich and slightly crush the sleeve, and the tension in the sleeve provides preload on the bearings. The more the nut is tightened, the more the bearings compress the sleeve, providing more preload.

This is the second-hardest part: tightening the pinion nut enough to compress the crush sleeve. This requires a hefty impact wrench, but we tried to use a homemade tool that someone told us would work. The long handle of the tool rests on the ground and keeps the pinion from turning while the nut is tightened. The cheater pipe broke the breaker bar (now a broken bar) before we even got close to the required force on the nut. A big impact wrench with 160 psi of air pressure is required to tighten the nut enough to crush the sleeve. Also, always use a new nut and washer.

Preload must be checked with an in-lbs torque wrench. Most in-lbs wrenches start measuring at 20 in-lbs, but the correct preload on a 12-bolt is 17 to 18 in-lbs, so we had to step up to this wrench from Snap-On. Take the reading as the wrench is turned through a gentle arc and not as soon as you start turning it. The breakaway reading will be much higher than the true preload reading.

The ring gear is a tight fit on the case (the case locates the gear, not the bolts), so it doesn’t just slip on. To make it easier to install the ring gear, use three guide pins as shown. Push the gear as far on as possible by hand, then use a rubber hammer to carefully tap it into place. Another trick is to heat the ring gear on a hot plate for a few minutes, which makes it expand slightly and makes it easier to slip on. It’s a good idea to take a sharpening stone to the mounting faces of the ring gear and the case before installation to ensure that they are perfectly level.

With the ring gear and differential case installed, the next check is backlash, which is the amount of clearance between the ring and pinion. A dial indicator and magnetic base are required. Set the dial indicator to read the ring-gear movement off a gear tooth as shown.

Backlash is adjusted with shims on either side of the case. The total thickness of the shims depends on what case you have. Most 12-bolts need a total shim thickness (both sides) of .484 inch, but our earlier case only takes .170 inch. Correct backlash for a 12-bolt is .005 to .010 inch. Adding shims to the ring-gear side moves the ring gear closer to the pinion, which decreases backlash. Conversely, removing shims from this side increases backlash. As shim thickness is removed from one side, it should be added to the other side to maintain the proper preload on the case. We ended up with .080 inch total on the left side and .090 inch on the right.

The shims are also available in several different thicknesses from GM, as shown.