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Build a Stronger Model 20 Axle

Posted in How To: Transmission Drivetrain on January 28, 2019
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The AMC Model 20 rear axle was at one time a common component in Jeeps, some American Motors cars, and as IFS/IRS differentials in the Hummer H1. Easily identified by a round cover with 12 bolts, inside is an 8 7/8-inch-diameter ring gear. Stock gear ratios ranged from 2.56:1 to 4.10:1, and aftermarket cogs are available up to a 4.88:1 ratio. Spline count on the shafts was 29 splines. There was also a narrow-track AMC 20 used in Jeeps from 1976 to 1981; it measured 50.5 inches from flange face to flange face. A wide-track version of the axle was used from 1982 to 1986; the axle width was increased to 54.4 inches.

The AMC 20 axle was delivered from the factory with a two-piece shaft design. While some of these axleshaft assemblies have survived fine for decades, some others have failed when the splines between the axleshaft and the mating wheel flange shear, causing the hub to spin on the shaft. Long-term wheeling or larger tires can bring on this failure, resulting in the loss of drive to the tire connected to that axle.

The relatively weak welds where the pressed-in axletubes mate with the centersection of the housing are another vulnerability of Model 20 axles. These welds can break and cause the axletube to rotate in the centersection. Also, the axletubes are considered by many to be relatively weak, and some owners have retubed these housings with thicker-walled tubing.

Our 1976 CJ-5 had the stock narrow-track AMC 20 axle and was still running the original two-piece axleshafts. We didn’t feel we needed to move up to a larger axle, but we did want to address several weaknesses in the current one. We aimed at running 32- to 33-inch tires and installed a one-piece rear axleshaft conversion kit (including high-tensile strength 4340 chromoly shafts) from Alloy USA. The shafts also use cold-rolled splines that are stronger than typical cut splines. Our stock differential cover was still straight, but we opted for a Rugged Ridge armored cover. This axle was already populated with an air locker and 4.11 gears, so we left the differential internals alone.

Removing and taking the stock axle apart was the first step. We needed to reuse the drum brake backing plates that were captive on the stock axles between the wheel hub and the pressed-on wheel bearing. A large jaw puller wouldn’t pull our two-piece axle assemblies apart, and we were unsuccessful pressing them apart on a shop press without doing damage to the backing plates. We finally cut apart the wheel bearing cages and dissected the taper bearings until the bearing assembly came apart. Then, the backing plate could be slipped over the remaining inner race of the pressed bearing.

This AMC 20 sits under a 1976 CJ-5, and a previous owner had installed a skidplate/truss structure that was overly large and ate up a lot of ground clearance. It was not reused.

While the axle was partially disassembled, we wanted to strengthen the housing a bit. Fortunately, someone had already rewelded the rosette welds on the tubes; otherwise we would have done that to help keep the tubes from spinning in the housing. We did opt to add a simple over-the-axle truss built from box tubing to further support the axle structure and as a second measure to prevent axletube twisting.

Most of the drum brake components had aged on this axle, so we sourced nearly all replacement pieces from Omix-ADA to make the brakes as good as new. Installing the new axleshafts was relatively easy, but does require use of a shop press for the bearing installation. We can now drive the Jeep with greater confidence in the rear-axle strength and brake condition, and we know that the differential is armored against rock encounters.

We started disassembly of the axle by removing most of the brake components and then the four bolts that secure each backing plate to the axlehousing flange. We used a large slide hammer to release each axle and slid them free of the housing.
The Alloy USA conversion kit includes new one-piece shafts, lug studs, bearing components, spacers, and seals.
Here you can see one assembled and one unassembled two-piece stock shaft. The large nut holds the wheel hub tightly onto the splined shaft. On the right is a one-piece Alloy USA shaft.
With the axleshafts and all the outer parts removed from the housing, we used 1 1/2-inch, 0.180-inch-wall square tubing to fabricate a basic truss over the top of the housing. Care was taken to weld the truss over some time to prevent excessive heating of the housing that could cause it to warp. We also moved the axle vent hole and rear brake tee locations to accommodate the truss.
We sourced new brake parts and brake hose from Omix-ADA to replace the old ones on our axle. Additionally, we opted to install a Rugged Ridge cast-steel rear cover.
Each brake backing plate is reused so they must be extracted from the original axleshaft assemblies. New brake wheel cylinders were installed, but we did not install the remainder of the brake parts until the axleshafts were secured in the housing. Note that the driver- and passenger-side plates are different.
Alloy USA provides new lug studs for the axleshafts. We applied red thread-locking compound to the threads and torqued the studs into the axle flanges.
A new bearing retainer plate with its seal goes onto each shaft behind the flange; then the brake backing plate fits over it.
The old inner axlehousing seals were pulled from the axletubes and new ones pressed into place. It’s important to make sure each seal seats fully in the axle bore so the following spacer fits correctly.
Steel bearing spacer rings included in the kit slip into the housing and seat up against each inner seal.
Each wheel bearing is preloaded once the axle assembly is bolted to the axletubing flange. To obtain the proper bearing preload, the outer edge of the bearing should protrude from the axle flange by 0.020 to 0.060 inch. We checked this measurement using a flat edge and feeler gauges. If the bearing protrudes excessively, the bearing spacer can be shortened, or flange shims can be added to obtain the correct distance.
The Timken USA bearings with the conversion kit were loaded with EP grease and then pressed onto each axleshaft along with a retaining ring. Note that the bearing installs in a specific direction. Each backing plate should also be mated to its correct short or long axleshaft. Upon assembly, RTV silicone was applied to seal the backing plate to the housing and bearing retainer plate.
We pressed each bearing and retainer ring onto its shaft using a 20-ton shop press.
Each axle assembly was slipped into the axlehousing and then secured using four bolts per side. Tightening these bolts preloads the wheel bearing so that the outer race does not spin in the axletube bore. The Alloy USA wheel flanges have a hole in them to allow access to the bolt hardware.
We completed assembly of the drum brakes using Omix-ADA replacement parts. The parking brake cables were hooked in later, once the axle was back on the Jeep.
Since we had the rear cover off the axle, it was easy to fill the differential without having to pump fluid into the fill hole. We added about two quarts of Red Line 75W90 GL-5 synthetic gear oil as a start and checked the final fluid level after the axle was mounted back on the Jeep.
The stock differential cover is made from thin, stamped sheetmetal. It’s vulnerable to catching on rocks and peeling the lower edge backward, or to being dented from a direct hit to the face of the cover. We swapped over to a Rugged Ridge heavy-duty, cast-steel cover. A second benefit to the new cover is that of a bottom drain hole.
With the axle bolted back into the CJ-5 and plumbed with a new rubber brake line, the brake shoes were adjusted and drums installed.
We needed to bleed the rear brake cylinders. Our crusty master cylinder was filled with old fluid. We siphoned out the dirty fluid and put new fluid through the lines. We’ll be doing a front brake upgrade soon, so this master will be replaced shortly.

Amazon Affiliate links are our attempt to show you real-world pricing and availability for the products we review and install, and while the Amazon links are separate from editorial and advertising, the Four Wheeler Network may receive a commission on purchases made through our posts.

Sources

Rugged Ridge
www.ruggedridge.com
Red Line Synthetic Oil
Benicia, CA 94510
707-745-6100
http://www.redlineoil.com
Omix-Ada
Suwanee, GA 30024
770-614-6101
www.omix-ada.com
Alloy USA
Suwanee, GA 30024
800-449-6649
www.alloyusa.com

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