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The GM 10-bolt with the 8.5 in ring gear, and its later 8.6 in (8.625 in) version, sits under a huge spread of GM rear-drive vehicles: Chevy and GMC 1500 pickups, Tahoe, Yukon, Suburban, plus older Camaros, Chevelles and G-body cars. It is a strong axle for its size, it takes common C-clip 30-spline axle shafts, and gears are easy to find. The catch: there are several different GM 10-bolts, and buyers regularly order parts for the wrong one.
Step 1: Confirm 8.5 vs 8.6 (and Rule Out the 8.2)
Count the cover bolts first. Ten bolts means a GM 10-bolt, but the 7.5, 7.625, 8.2, 8.5 and 8.6 all fit that description. The cover alone is not proof — measure the ring gear or read the axle tag before you spend money.
- 8.5 in: the older design. Found in 1970s and 1980s GM cars and light trucks and the half-ton pickups and SUVs up through the late 1990s.
- 8.6 in (8.625): the 1999-and-later full-size half-ton axle under the Silverado, Sierra, Tahoe, Yukon and Suburban 1500. Later covers use 8mm metric cover bolts, which matters if you buy an aftermarket cover.
- 8.2 in: looks similar from behind, does not share gears. If your ring gear measures 8.2 in, stop and buy 8.2 parts.
The good news: the 8.5 and 8.6 share ring and pinion sets. Where generations differ is the bearing kit, so buy the install kit that is listed for your axle and years. For the ratio you have now, check the RPO sticker in the glovebox: GU4 is 3.08, GU6 is 3.42, GT4 is 3.73, GT5 is 4.10. A G80 code means you have the factory locker, which adds labor and some care during teardown.
Step 2: Pick the Ratio by Tires and Use
Do not pick gears off a forum signature. Pick them off the tire you actually run, the weight you actually tow, and your transmission.
| Ratio | Best Fit | What You Give Up |
|---|---|---|
| 3.42 | Stock to 31 in tires, light towing, mostly highway; the common step up from 2.73 or 3.08 | Feels soft with 33s or a loaded trailer |
| 3.73 | 31 to 33 in tires, regular towing, the best all-around half-ton ratio | A few hundred more RPM at 70 mph |
| 4.10 | 33 to 35 in tires, heavy trailers, mountain driving | Highway RPM and fuel economy on stock-size tires |
Our opinion: if you went up one tire size and tow anything, 3.73 is almost always the right answer. And whatever you pick, regear the front axle to match on a 4x4 — the front and rear ratios have to be identical. After the swap, recalibrate the speedometer and shift points so the transmission is not shifting off a wrong vehicle speed.
Step 3: Carrier Break and 30-Spline Axles
Like most axles, the 8.5 uses a carrier break. Very low highway ratios use a different carrier than the 2.73-and-up gears. Moving between 3.08, 3.42, 3.73 and 4.10 keeps you on the same carrier, which covers almost every truck regear.
The 8.5 and 8.6 both use 30-spline, C-clip retained axle shafts. To get the carrier out you pull the cross shaft, push the axles in, remove the C-clips, and slide both shafts out. While they are out, look at the bearing surface on each shaft. If the old axle bearing has grooved the shaft, replace the shaft or run a repair bearing; a new axle bearing on a pitted surface will not last. A puller and seal driver set like our 13-piece rear axle bearing puller and seal driver kit makes the axle bearings and seals a 20-minute job instead of a fight.
Step 4: Gears Only or Gears Plus Master Install Kit
Buy the master kit. Every time. The pinion bearings have to be pressed off the old pinion to get the shim pack, so they are getting disturbed anyway. Putting new gears on 150,000-mile bearings is how a fresh regear starts howling at 5,000 miles.
A proper master kit includes pinion and carrier bearings with races, pinion seal, pinion depth shims, carrier shims, crush sleeve, pinion nut, ring gear bolts, gasket or sealant, and marking compound.
| Part | What It Is | Price |
|---|---|---|
| GM 8.6 3.73 Ring & Pinion + Master Kit | Motive Gear 3.73 (41/11) with master bearing kit, 1999-2008 trucks and SUVs | $455.99 |
| GM 8.5 3.42 Ring & Pinion + Master Kit | Motive Gear GM10-342A (41/12) with R10RMK Koyo bearing kit, 8.5 and 8.625 | $399.99 |
| GM 8.5 3.73 Ring & Pinion, Gears Only | Motive Gear GM10-373A, 30-spline pinion, 8.5 and 8.625 | $244.99 |
| Allstar ALL68521 Install Kit | Bearings, shims, crush sleeve, seal, bolts; listed for 1971-1984 8.5 axles | $159.99 |
The math is simple: the 3.73 gears-only set plus a separate kit lands close to the price of the bundle, and the bundle removes the risk of mismatched bearings. On a late truck, buy the bundle. On an early car axle, the gears-only set plus the Allstar kit is the cleaner match for the years.
Crush Sleeve vs Solid Spacer
Stock 10-bolts use a crush sleeve to set pinion bearing preload. It works fine for street trucks when it is crushed slowly and checked often. Overshoot preload and you cannot back off — you have to start over with a new sleeve. A solid spacer (crush sleeve eliminator) is worth it on a drag car or a truck that sees hard launches, but it means shimming preload by hand and takes more setup time. For a daily driver that tows, a properly crushed sleeve is fine.
Step 5: Setup Is Where Regears Live or Die
Order matters. Set pinion depth first with the shim behind the inner pinion bearing. Then set pinion bearing preload by crushing the sleeve. Then set carrier backlash and carrier preload with the side shims, and read the contact pattern with marking compound. Most 10-bolt setups land around .006-.008 in backlash, but use the spec card that ships with your gears.
Buy or borrow setup bearings (honed-out bearings that slip on and off) so you are not pressing a new bearing on and off three times while chasing depth. A dial indicator, an inch-pound torque wrench and a bearing puller are the minimum tools. If you do not own those, this is a job to hand off. Our backlash and pinion depth setup guide walks through reading the pattern.
Step 6: Upgrade the Cover While You Are In There
The stock stamped cover is thin and sits right where a rock, a hitch or a stuck winch strap finds it. A nodular iron cover like the Yukon Hardcore cover for 8mm cover bolts adds real protection on a truck that goes off pavement. Check your cover bolt size first; the 8mm version is for later axles.
DIY vs Shop Cost
| Approach | Parts | Labor | Total |
|---|---|---|---|
| DIY, gears + master kit | $400-460 | A weekend, plus tools | $400-460 + tools |
| Shop, rear axle only | $400-500 | $600-1,200 | About $1,100-1,700 |
| Shop, 4x4 front and rear | Two sets | Two axles | Roughly double |
If you have never set up gears, your first one will take a full weekend and probably a second trip to the parts counter. That is normal. If you need the truck Monday, take the parts to a shop that does gears every week.
10-Bolt Gear Sets and Master Install Kits Ship Same-Day
We stock 3.42 and 3.73 gear sets, master kits, install kits and covers for the GM 8.5 and 8.6. CoreTransmissionParts carries the broader drivetrain catalog if you need something we don't stock direct.
Shop Gearbox Insider → Shop CoreTransmissionParts →FAQ
Do GM 8.5 and 8.6 10-bolt axles use the same ring and pinion?
Yes. The 8.5 in and 8.625 in 10-bolt share ring and pinion sets, so a 3.42 or 3.73 set listed for both will fit either. The 8.2 and 7.5 in 10-bolts do not share gears with them. Confirm the install kit is listed for your axle and years, because bearing kits are where the generations differ.
Do I need a new carrier to go from 3.42 to 3.73 in a GM 10-bolt?
Usually not. Moving between 3.08, 3.23, 3.42, 3.73 and 4.10 keeps you on the same carrier. The carrier break on the 8.5 is at the numerically low end, so only an axle that came with a very low highway ratio may need a different carrier for deeper gears.
Can I reuse the old bearings when I regear?
You can, but you should not. New gears on tired bearings is the most common reason a fresh regear starts whining a few thousand miles later. The pinion bearings have to come off the old pinion anyway, so buy the master install kit and replace them.
What does a GM 10-bolt regear cost at a shop?
Expect roughly $1,100 to $1,700 for gears, a master install kit and labor on a single rear axle, depending on your area and whether the carrier has a locker. Doing it yourself, the parts run about $400 to $460 for gears plus master kit.