GM 10 Bolt - How To Switch Gears

Switching Gears

Rebuilding a GM 10-bolt rearend

By Jefferson Bryant / Photography by Jefferson Bryant

If transmissions are shrouded in mystery, then the rear differential follows on its coattails. Unlike transmission building, rebuilding a rearend is basic nuts and bolts stuff, but that comes with a caveat — the setup is the tricky part.

Backlash, pinion depth and wear patterns are science and art mixed together, and you do need a few specialized tools. We set out to rebuild an aging GM 8.5-inch 10-bolt rearend.

Before you start, you need to identify your parts. Some people will trash talk the GM 10-bolt as a “junk” rearend, and if they are talking about the 8.2-inch, then they are not far off. But the 8.5-inch 10-bolt is actually just as strong as a 12-bolt or Ford 9-inch rear, with only a fraction of an inch difference in the ring gear diameter. Since both the 8.2 and 8.5 10-bolt rears have the same cover, you have to look for some key signs as to which unit you have (short of taking it apart and measuring). All 8.5-inch 10-bolt rears have two squared-off ears at the 4- and 6-o’clock positions on the bottom of the pumpkin — that is the No. 1 telltale sign. If you don’t have those, you don’t have an 8.5-inch 10-bolt. Up until 1988, all GM 8.5-inch 10-bolts had 28-spline axles, which are thinner than and not as strong as the upgraded 30-spline axles. Finding 30-spline axles is easy, you just have to take some measurements and order them. You should also check to see if the rear carrier is posi or open-style; the easiest method is to jack up the rear of the car and spin the wheels — if they both spin the same direction, you have a posi; if they spin in opposite directions, you have the classic pegleg rear.

Our 8.5-incher had 28-splines and an open carrier, not to mention some clanking, groaning and general worn-out feel, so we ordered some parts. While we could have stayed with the open carrier and 28-spline axles with no real consequences, posi is so much better. We ordered a new TrueTrac posi unit from Eaton (the makers of the Detroit Locker) so that we would have the excellent noise-free turning and straight-line traction that we need. The TrueTrac comes in both 28- and 30-spline versions and we chose the 30-spline. That meant we needed new axles. We ordered the rest of the goods — 30-spline axles, 3.73:1 gears (an upgrade from the 3.07s) and all the bearings, seals and gaskets.

To get the rear set up properly, you will need some tools — specifically, a dial indicator with a magnetic base and some calipers. These are used to check the backlash of the gears and the thickness of the shims respectively. Backlash is the amount of play the ring gear can move before the pinion moves. Each rear-differential type (as in 12-bolt, 10-bolt Dana 60, etc.) has its own specs, and these are usually included in the gear-set instructions. The 8.5-inch 10-bolt should have between .006 and .010-inch of backlash — we are not talking about much, and the only way to check this is with a dial indicator. We ordered our dial indicator kit; it came with a nice plastic case, too. This is the basis for the rest of the install, so it needs to be checked first and it needs to be right. Next the contact pattern must be checked, which is done with colored grease (included as part of the new gear install kit).

Once you have the basics down, swapping gears is not that difficult, it just takes time to learn and a lot of trial and error. Setting the backlash and pinion depth (dictated by the contact pattern) means installing and removing the carrier and pinion. On this swap, we did this about 15 times, for various reasons. The end result is better traction, more power (though the engine does spin a bit faster at speed because of the lower gears) and all-around better drivability. Whether you want more performance and capability or you just need to replace some worn-out parts, rebuilding your rear differential at home can save you money and be one more thing that you did yourself.

1 Drain away: We raised the car up on our new lift from Quality Lifts. You don’t have to have a lift, but it is really nice for a project like this. With the wheels off, we removed the driveshaft and cracked the seal on the rearend. Surprisingly, the gear oil was not as bad as expected. One note — don’t wear basic shop clothes for this, you want to wear something that you won’t mind throwing away; nothing smells worse than used gear oil.

2 C-note: This is a C-clip rear (most Buicks and some Pontiac 8.5-inch 10-bolts had bolt-in axles), so the drift pin must be removed to gain access to the C-clips. The drift pin is held in place by a small bolt to the side of the carrier, and then the drift will slide out. Don’t let it fall, as these are not easily replaced.

3 Don’t drop: A magnetic tool is the easiest way to get the C-clip out and not drop it. Once you have these out, the axles slide out of the carrier.

4 Mark it: Before pulling the carrier caps, mark them for side and position. You can’t swap these side-to-side or flip them upside down, so make a solid mark on each one. Some caps have arrows pointing towards the outside, which makes it easier.

5 Punch-out: The pinion nut requires a large socket; we used a 3/4-drive set from Gearwrench on an impact gun to remove it. The pinion gear must be hammered out, but use a drift punch, don’t just smack the pinion stud, it will become unusable and could warp enough that is gets stuck permanently.

6 Code cracker: If you are unsure of the original gear ratio, it is stamped into the face of the pinion. The 13-40 represents 13 pinion teeth and 40 ring teeth, which breaks down to 3.07:1.

7 Tough stuff: You can see a big difference in structure when comparing the carriers. The stock open carrier to the left is not as solid as the new TrueTrac from Eaton (right). The TrueTrac is a positive-traction limited-slip unit, meaning both wheels are driven under even loads, but when cornering, the carrier allows the inside wheel to slip a little, which is a good thing. And it lets you do big smoky burnouts with both tires. The TrueTrac requires different bearings from the stocker, which set us back about $50 for the bearings and matching races.

8 Measured analysis: We learned a few things along the way with the axles. There are two ways to measure axles, from the outside wheel surface to the C-clip flange or to the end of the axle. Different companies measure different ways, so you need to have both specs and check with the tech department before ordering axles. Also note the slightly larger diameter of the splined area for the top axle; this is the 30-spline, versus the smaller 28-spline stocker. The 30-spline will handle more torque.

9 Bench-press: The main pinion bearing (as well as the carrier bearings) have to be pressed off, so we took them to the local NAPA machine shop for that. You don’t want to toss the old pinion bearing, you need it for the setup. Using a fine carbide burr, grind out the inside of the bearing until is slips onto the new pinion shaft easily. This is so you can check the fitment of the gears without destroying bearings.

10 Torque stop: The new Yukon ring gear was bolted to the carrier and torqued in a cross-bolt pattern (like you torque a wheel) to 65 lb-ft. No threadlocker needed.

11 Trust but verify: Many times the original pinion shim will get you there in terms of pinion depth, but you can’t trust that – not by a long shot. Always start with the stocker and go from there.

12 Test-tight: The pinion gear is installed with the old bearings, no crush collar and the pinion nut tightened down firm enough to get just a bit (enough to tell it’s there, no slop, but not tight) of drag on the bearings, using the original pinion nut. The new nut has flat spots on it that serve as locks; you don’t want to use the new one for this.

13 Carrier shims: As with the pinion shim, the stock carrier shims often get you closer but, in this case, not even. We used one factory shim and then stacked up a set of the new shims to get us closer. The carrier should be tight, but you don’t want to have to beat it into place either; consider it “hand-tight.”

14 Backlash: The dial indicator was set up to read 90 degrees off the coast (back) side of the ring gear. Once zeroed, the pinion is held tight and the carrier rotated until the pinion is engaged, both forward and reverse. The carrier should move easily — it should not bind, grind or be difficult to move. Here we had .006-inch play, which is spot-on. Adjusting the carrier side to side moves the gear closer or further from the pinion, reducing or increasing the backlash. An adjustment of .010 in shims is worth about .007 of backlash. Be patient. Once the backlash is set, don’t change the shims.

15 Grease trap: Next the grease comes out. Spread a little grease over four to five ring gear teeth, then rotate the carrier (not by the pinion) back and forth over the pinion gear about five times. Then bring this section back to the open side and check the pattern. The instruction booklet has a guide for the acceptable patterns. You are not looking for centered placement, more the relation of the markings to the teeth. The drive side has a flat edge in the deep part of the gear, showing that the pinion is too close to the ring — it needs less shim.

16 Shim it: Using the supplied pinion shims, we measured the stock shim and reduced it by .015. You want to make big changes (.01-.15) at first, then work your way to smaller changes (.03-.04).

17 In the house: Once the pattern came out how we wanted, we had the new bearing pressed on, slipped on a new crush collar and dosed the pinion threads with some threadlocker and put the pinion in the housing. The front (smaller) pinion bearing was installed, along with a new seal before the pinion was installed.

18 Preload: The last step in the pinion install is setting the pinion preload. The specs for this particular unit are 6-8 pounds per inch, which is barely anything. You can’t rent lb-in torque wrenches (at least not in our town) and they cost about $100 to buy one, and that one wasn’t even small enough to hit 6 lb-in. We talked with a few local experts (not shadetree guys, real experts) who told us that they use an impact to get the pinion nut to the shoulder (you need the torque for that job) and start to crush the collar, then carefully tighten the nut until there is just a bit of drag on the bearing. It does not take much, and you get a feel for it during the setup process, so don’t freak out. If you go too far, you need to pull the pinion and get a new crush collar. The kit comes with two for this very reason.

19 Tap dance: Once set, the pinion nut needs to be hit with a pointed punch in two spots. This keeps it from backing off, which will happen if you don’t do this. You don’t have to kill it, just a few taps with the hammer will do.

20 Carrier: Next the carrier is reinstalled with the correct shims and then torqued to 65 lb-ft.

21 Hammer time: The new axles don’t come with wheel studs, which we forgot. We had to put the job off till the next day so we could get the studs. If you have never done this, it’s easy. Just use a brake drum and a 3-lb sledge. Knock the studs flush. When you torque the wheels for the first time, give ’em a little extra and make sure the studs are seated. Recheck them after a few miles.

22 Last look: Changing bearings can tweak the settings, so it’s a good idea to check it one last time before buttoning it all up. This pattern looks good. The TrueTrac carrier uses a different locking system. A hardened steel disc rests between the axles, keeping them from sliding to center; this replaces the drift pin on the older unit.

23 Plug it: The plug is inserted and retained with a large snap ring – much more secure. The previous 10-bolt we built had a broken drift pin that almost destroyed the housing. It managed to kill the stock posi carrier in the process.

24 Cover-up: The rear cover cleaned up a little, was treated to a new gasket and reinstalled.

25 Fill ’er up: We filled the rear with Royal Purple gear oil. The oil needs to be changed after the first 500 miles. Once filled, the driveshaft went back in and the job is done. Take it easy for the first few miles; the gears need some time to heat- cycle to break in.