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The AMC 20 has a bad reputation it only half deserves. The center section is strong: an 8.875-inch ring gear, a big pinion, 29-spline axles and a 12-bolt cover. What gives it the bad name is the factory two-piece axle shaft, tubes that were pressed into the center casting and can twist, and decades of owners running bigger tires than it was built for. Fix those weak spots and an AMC 20 is a good axle for a lightly built Jeep.
Which Vehicles Have an AMC 20
- Jeep CJ-5, CJ-7 and CJ-8 Scrambler, 1976-1986. The one most people are dealing with. Two-piece shafts, offset center section, usually 2.73, 3.31, 3.54 or 3.73 gears.
- Full-size Jeep: SJ Wagoneer, Cherokee and J-series pickups. Many from the mid-1970s into the 1980s got the AMC 20 rear. Some had a Dana 44 instead, so check the cover.
- AMC cars. V8 Javelin, AMX, Matador, Hornet and similar cars used the same Model 20 center section.
Identify it by the cover: 12 bolts on an irregular, almost round shape with a flat spot, and a 20 cast into the housing. A 10-bolt cover is a Dana 44 or Dana 35; see our Dana 44 guide if that is what you have.
Two-Piece Axle Shafts: Hub Spin and Key Shear
On the factory axle, the hub and drum flange is a separate piece pressed onto a tapered shaft end. A key in the taper locates it and a castle nut with a cotter pin holds it on. When the nut loses torque, the hub starts to walk on the taper. The key shears, the keyway wallows out, and you get a clunk on every throttle change.
Check it: jack up a rear wheel, put the transmission in gear, and rock the wheel hard. Any clunk or rotation of the hub against the shaft end means trouble.
If it is caught early, clean the taper, install a new key and torque the castle nut to at least 250 ft-lb, then advance to the next cotter slot. Never back off to line up the pin. If the taper or keyway is chewed up, the shaft and hub are scrap. Removing a hub needs a real hub puller, not a slide hammer or a torch. A pressed hub should not go back on a different shaft; the press cuts serrations into the hub that only matched the original.
The One-Piece Shaft Conversion
The real fix is a set of one-piece 29-spline shafts with the flange forged on. Once in, the hub, key and castle nut are gone, and so is the failure.
The one-piece shaft uses a pressed-on axle bearing and retaining ring with a new outer seal, so you need the matching bearings. The Yukon AMC Model 20 axle bearing and seal kit for one-piece shafts ($157.59) covers both sides.
While the shafts are out, plug-weld or bead-weld the axle tubes to the center casting. AMC pressed the tubes in, and on a Jeep with big tires and a locker the tubes can twist, leak gear oil at the tube joint and shift the camber of the axle.
Pinion and Carrier Bearing Whine
Like most light axles of its era, the AMC 20 sets pinion bearing preload with a crush sleeve and the pinion nut. Worn pinion bearings whine under load and go quiet on coast. Worn carrier bearings give a steady rumble that changes in turns. Our whine on acceleration vs deceleration guide walks through telling them apart.
Do not just crank the pinion nut to cure a leak or a whine. If the yoke has play, the crush sleeve has lost tension. Do it right with a new sleeve, nut, seal and bearings. The AMC 20 differential master bearing kit with Timken bearings ($191.99) has all of it. A pinion flange holding tool ($35.99) holds the yoke while you crush the new sleeve in small steps, checking rotating torque as you go. For a seal-only job, read our pinion seal leak guide first.
Pinion depth is set with a shim behind the inner pinion bearing race, and carrier preload and backlash with shims behind the carrier bearings. Pulling the carrier bearings without damaging them takes a bearing separator and puller set ($269.99), and you need a dial indicator with magnetic base ($47.99) to read backlash.
Trac-Lok, Truetrac and Lockers
Many AMC 20s came with a clutch-type Trac-Lok. After decades the clutches are flat and it acts like an open diff, or it chatters in turns because nobody added friction modifier.
For a trail and street Jeep that needs no clutch packs, the Eaton Detroit Truetrac for AMC 20, 29-spline ($883.99) fits 3.08 and numerically higher carriers. A full locker is stronger off-road, but on a short-wheelbase CJ on wet pavement it can be twitchy. Our open vs limited slip vs locker guide covers the tradeoffs.
Regearing an AMC 20
Factory gears ran from about 2.73 up to 3.73 depending on engine and transmission. A CJ on 31s or 33s with 2.73 or 3.31 gears is badly undergeared. Common choices are 3.73 or 4.10 for 31s and 4.10 or 4.56 for 33s. Our regear after bigger tires guide has the math.
Setup notes: the pinion depth shim is the critical one, so start with the old shim and check the pattern. Backlash usually ends up around .006-.010 inch, but follow the gear maker's number. Our backlash and pinion depth setup guide goes step by step.
What It Costs: DIY vs Shop
| Repair | DIY | Shop (rough) |
|---|---|---|
| Pinion seal and crush sleeve | About $30-$70 | About $250-$450 |
| Two-piece hub key and nut re-torque | About $20-$50 plus hub puller | About $150-$300 per side |
| One-piece axle bearings and seals | About $160 plus press time | About $350-$600 |
| Full bearing rebuild, existing gears | About $190-$300 | About $800-$1,300 |
| Regear with limited slip | Parts about $1,300-$1,700 | About $1,800-$2,800 |
| Regear, limited slip and one-piece shafts | Parts about $2,000-$2,700 | About $2,500-$3,800 |
DIY assumes a dial indicator, a beam or dial inch-pound torque wrench for pinion preload, a flange holder, a bearing puller and a press. Fill gear oil to the bottom of the fill hole, and add friction modifier for a Trac-Lok. A fresh Yukon chrome cover for the AMC Model 20 ($68.73) is worth it if the old one is bent from rocks. Gear oil is in our fluids section.
Parts and Tools for the Job
| Need | What We Carry |
|---|---|
| Full bearing rebuild or gear swap | AMC 20 master bearing kit, Timken ($191.99) |
| One-piece shaft bearings and seals | Yukon AMC 20 one-piece axle bearing kit ($157.59) |
| Limited slip, 29-spline | Eaton Detroit Truetrac, AMC 20 ($883.99) |
| Holding the yoke | Pinion flange holding tool ($35.99) |
| Reading backlash | Dial indicator with magnetic base ($47.99) |
| Pulling carrier and pinion bearings | Hydraulic gear puller and bearing separator set ($269.99) |
If the noise turns out to be further forward in the transfer case or transmission, CoreTransmissionParts.com carries the rebuild parts.
Rebuild It Once, Rebuild It Right
New Timken bearings, a fresh crush sleeve, seal and shims in one kit for the AMC 20.
Shop the AMC 20 Bearing Kit → Gear Oil and FluidsFAQ
How do I tell if my AMC 20 has two-piece or one-piece axle shafts?
Pull the brake drum. If you see a castle nut and cotter pin on the end of a tapered shaft holding the hub on, it is the factory two-piece design. If the drum slides over wheel studs pressed into a flange forged onto the shaft, it already has one-piece shafts.
Why does my AMC 20 hub keep coming loose?
The two-piece hub is held on a tapered shaft by a key and the castle nut. If the nut loses torque, the hub walks on the taper, shears the key and wallows out the keyway. Once the taper is damaged, a new shaft and hub are the fix. Keep the nut torqued to at least 250 ft-lb.
Is the AMC 20 worth rebuilding or should I swap to a Dana 44?
With one-piece shafts and welded tubes, an AMC 20 handles 33 inch tires on a lightly built Jeep fine, and the 8.875 inch ring gear is bigger than a Dana 44's. A Dana 44 swap makes sense if the housing is bent, you want 35s or larger, or the shafts and gears all need replacing anyway.
How much does an AMC 20 rebuild cost?
A DIY bearing and seal rebuild with a master bearing kit runs about $190-$300 in parts. A shop bearing rebuild with your existing gears is usually $800-$1,300. A regear with a limited slip runs about $1,800-$2,800 at a shop, and adding a one-piece shaft conversion pushes it to $2,500-$3,800.