Building a Killer AMC Axle

Instead of making the wheel flange and axleshaft in one piece (left), the AMC 20 axle has a two-piece design, where the shaft is splined onto the separate hub and held on with a nut (center). This is the major weak link of the 20, as the splines and itty-bitty keyway can’t stand up to big tires and horsepower, which rips the splines out of the hub (right).

Tube strength in the centersection is another downfall of the 20. The tubes twist out of the housing, and also bend or break. Tri-County Gear replaces the tubes with thicker units, and designed an over-the-housing truss to be welded onto the new tubes to stabilize the tubes without reducing ground clearance.

New tubes with a thicker wall are pressed into the centersection and welded securely to prevent twisting. Regular 20, wide-track 20, or Quadratrac 20 lengths are all available.

Tri-County Gear welds the Dana 35 axle ends onto the new tubes, since they are the same bolt pattern and a tad thicker than the stock 20 style. Since this is a custom unit, you can select nearly any end style you want to use, as long as you have the axles and brakes to fit it.

Here’s what shaving a 20 for ground clearance is all about. Not only do you gain nearly 1 1/5 inches of clearance, but the cover is also modified to strengthen the housing where the casting is removed. This also prevents the leakage-prone cover from peeling back from the housing after catching a rock and losing precious 90-weight.

If you’ve already painted yourself into a corner after buying a locker and axleshafts for your 20, the upgraded shaved 20 housing is the way to protect your investment. We happen to like 20s since they are plenty strong and look unique, so we slapped in new 4.10 cogs from Precision Gear for reliability.

A new Air Locker from ARB gives us the choice of an open or locked differential. We like being given a choice of operating modes, rather than being a one-legged peeler or a solidly locked spool. The ARB allows us to decide which is right for the driving conditions, and how we want to handle any obstacle.

Unlike other shaved axles, you don’t have to machine down the 20’s ring gear, so all the strength is retained. Plenty of clearance is left between the casting and gear for proper lubrication and oil flow, and even for those nasty chunks that might someday be floating around inside the diff.

With the internal gasket glued in place with 3-M Weatherstrip Adhesive, the modified cover is bolted on. Four counter-sunk Allen head screws hold the bottom of the cover on, which allows for a smooth bottom for sliding over rocks.

We selected the Warn full-float kit for this 20, which gives us the option of running locking hubs should we desire to flat-tow the Jeep. The kit comes complete with new hubs, spindles, axles, and attaching parts. There are also many different brands of one-piece axle upgrade kits available, all of which are a zillion times better than the stock axles. Don’t even think of using the stock axles. Bad thought! Bad!

With the axles installed, the spindle is bolted to the axlehousing flange. The lower bolt hole is drilled into the flange using the spindle as a pattern. This greatly strengthens the spindle mounting by going from four to five bolts. Once cleaned up, the spindle is removed and an O-ring is installed to seal the spindle to the housing before the spindle is bolted back on.

The Warn disc brake kit is a premium upgrade, but you can retain your stock drum setup. The Warn brackets need to be trimmed a tad for the bigger housing ends, but Tri-County Gear can set you up with its own versions that bolt right on.

If you opt for disc brakes, the studs in the Warn hubs need to be pressed out, and the rotors installed on the hubs with new studs. We ordered up some stock Jeep rotors from Crown Automotive for a ’78 CJ to work with our caliper and Warn bracket setup. Brake choices vary, so plan ahead to make sure you have the right combo of parts before you put this type of system together.

After packing the wheel bearings and installing the hub and rotor assembly, the bearings are adjusted. Warn uses the standard four-slot nuts found on most early 4x4 spindles, so service of these parts is a snap.

Special thrust washers are installed to prevent the drive flange from coming in contact with the end of the spindle. If you decide to run locking hubs, these washers aren’t needed since the hub body is recessed for the spindle.

The solid steel drive flange slips into place on the hub, and we doubt you’ll ever break this part. An external snap ring retains the flange, and then a plate is installed with an external snap ring to retain it.

Stylish-looking Warn caps bolt onto the plate in the hub sealing the unit from water and dirt. If you wanted to run locking hubs, carry a set of flanges just in case you get a little rowdy with the throttle.

Slap on a caliper and some pads and you’re ready to roll. Hoses, parking brake cable, and the like are all specific to your vehicle’s year and options, so check with Tri County for the right combo you need.

Whackity-pop, zizzz, zizzz, and a whole bunch of expletives are what’s heard on the trail when a rear axleshaft lets loose. In the case of many 4x4s, too much right foot or too large a tire are the main culprits, rather than a poor axle design or weak material. In fact, most OE axles in 4x4s are designed for light- to medium-duty use, not the stress and strain we put them through. We constantly tell you in these pages to upgrade your axles to something stronger, just so you don’t get stranded on a trail. But what if you’ve already poured a ton of money into a semi-weak axle, like the AMC corporate 20? Start over with a Dana 60 and junk the parts? Actually, there is an answer to your question about these dilemmas.

The AMC corporate 20 axle has been given a bad rap over the years, even though some of that rap is well deserved. AMC started using this axle in the Jeep line in 1976 when the first CJ-7s came out. They continued until the end of CJ production in 1986. While it featured a large ring gear diameter (8.875 inches) and fairly beefy axleshaft spline count (29), the two-piece axles would twist in the hub, the axletubes would twist in the housing, the low-clearance housing allowed the cover to peel away and leak fluid, and the axletubes would bend into a smiley face. Still, there are zillions of them out there that haven’t given up the ghost, and quite a few aftermarket fixes to beef up the weak points.

So what do you do if you’ve invested too much in your 20 to get rid of it? A new shaved housing from Tri-County Gear in Pomona, California, is the most obvious answer. Jason Bunch explained how he takes a good centersection from your axle, shaves the bottom off for clearance, retubes the casting with stronger tubes, trusses the axle over the top for clearance, and welds on beefier axletube ends for a super strong unit. The whole deal ain’t cheap, but it should be the last axlehousing you need to buy for your Jeep. Check out the details of what we put together in an afternoon and see if it’s right for you.