Why Put Good Money Into an Axle That Appears to Have So Many Weaknesses?Posted in How To on January 1, 2001 0) (
If there is one Jeep part that gets more badmouthing than a Backstreet Boy in a biker bar, it has to be the AMC 20 rearend. So why put good money into an axle that appears to have so many weaknesses? Because its already in your Jeep, and in most cases it can be made strong enough for your type of wheeling. Most of the AMC 20 failures occur because of poor installation procedures, abusive situations like jumping your Jeep, or extreme rockcrawling. Besides, complete axle swaps can be costly, and in some instances certain components of the new axle may be weaker than what the AMC 20 offers.
Over the years, several mods have become available to build the 20. We compiled some of them, found the advantages of each, and noted what to look for when working with an AMC axle. There are three basic versions of the AMC 20 that can be found in the 76-86 CJs. The 76-81 CJs are narrow-track, 82-86 ones are wide-track and are about 3 inches wider. CJs with the Quadra-Trac transfer case have a narrow version of the 20 with an offset differential. Be sure you know what version you have before buying parts. Check out the captions to find out how to make lemonade out of your lemon. The What Do You Need? sidebar will help you decide what modification best suits your needs.
Getting the Shaft
The biggest problem with the AMC unit is the two-piece shafts. The design is actually quite strong when new, but the long-term durability is terrible. The axles consist of a flange, which the wheel bolts to, and a shaft with splines on one end and a serrated taper and keyway on the other end. A large bolt keeps the assembly together most of the time. These shafts were conceived in an effort to save money when building axles of different widths for the AMC and Jeep lines. Producing a separate flange that could bolt to various-length shafts (depending on the application) allowed AMC to cut the cost of building the shafts. It essentially eliminated the need to forge or weld any of the pieces. Needless to say, several companies have produced axle fixes for the 20. The two schools of thought are one-piece axles or a full-floater kit. Both methods are many times more durable than the factory shafts. One-piece axles replace the shear-prone keyway design with solid shafts. If a one-piece shaft should break, however unlikely, the wheel will remain on the vehicle because it is a semifloating design. The wheel will certainly be out of position because the shaft carries the weight of the Jeep as well as the power from the differential. A broken shaft will cause severe pressure on the axle bearing. The Jeep will only be able to travel a short distance with a broken one-piece shaft before the bearing fails. One-piece axles rarely break off at the flange, which would cause the wheel to fall off of the Jeep. In reality there is more to consider than axle strength when choosing a one-piece axle kit. For example, some kits come with roller bearings. These are not as durable as tapered bearings.
When shopping for a two-piece kit look for forged axles. These shafts start as solid rods and are pounded into shape. The flange is formed by thousands of pounds of pressure. Avoid friction-welded shafts. These are formed by spinning the shaft end and forcing it into a machined flange. Heat builds until both materials melt and the two pieces become welded together. Friction-welded shafts are not as strong as forged axles.
The way the splines are made is also an important aspect to consider. Rolled splines are preferred. Rolled splines are essentially pressed into the existing shaft material. The shaft is then heat-treated for strength. Generally, heat-treating is only on the outside of the shaft. The process does not penetrate all the way through to the center of the shaft. If a shaft were to be heat-treated all the way through it would make the material brittle and it would snap. Rolled splines are about 35 percent stronger than cut splines. A cut or hobbed-spline axle is produced by taking a heat-treated blank and using a mill to machine each spline. The process cuts away much of the heat-treated area and can make the splined portion of the shaft weak.
Full-float kits include new spindles and bearing hubs similar to what can be found on the front of your CJ. The shafts transmit power from the differential to the wheels. They are not subjected to the weight of the vehicle like two-piece and one-piece axles. Full-float axles such as a Warn kit are substantially stronger and more durable than the factory axles. However, the locking hub on the end of the axle may end up as the weak link. Locking hubs can be replaced with solid drivers for strength if desired. Regardless of whether a hub or shaft breaks, the wheel will remain on the vehicle and it can be driven with the offending pieces removed if need be. Besides the novelty of having locking hubs on the rear axle of your Jeep, a full-float kit allows a Jeep to be flat-towed without the driveline spinning. A full-float kit usually costs twice what a quality one-piece axle kit does.
The AMC axle can be difficult to work on. Setting the factory axle bearing endplay to 0.004-0.008 inch is time consuming and impossible if you dont have the right tools. If set up incorrectly the bearings will fry and destroy the axleshafts and possibly the housing. Many older one-piece aftermarket axle kits used the same method of obtaining the correct bearing preload making backyard repair less appealing. The fact that AMC 20 housings vary up to ½ inch in width also adds frustration to the installation of axles that use shims for endplay adjustment. Most modern-day kits like the one from Superior Axle & Gear have an improved design that allows the new one-piece axles to be installed by anyone with common handtools and access to a bearing press. Installing a full-float kit can also be done with common handtools. The endplay measurement is not needed since the bearings are not a part of the shafts. They ride in a separate hub just like the wheel bearings on the front axle. A press is not required for the install of a full-floater kit. However, some full-float kits interfere with e-brake componentry.
Another weak spot for the AMC 20 is the axletubes. Your AMC housing is probably a little distorted unless your CJ has been in a vacuum for the past 20 years. Some bending is OK. The tubes are made from small-diameter light-duty tubing. For years it has been known that the housings bend. However, Superior believes that the tubes will have to bend into an obvious smile before any related axle failure will occur. Warn says that a bent housing will cause oil leaks and if bent enough it will cause the locking hubs of a full-floater to get chewed up. Replacing the hubs with drivers will solve the hub munching but could cause an axleshaft to snap due to misalignment. Warn recommends checking the straightness of your 20 before installing a full-floater.
The tubes have also been known to spin inside the centersection when the assembly is subjected to high-torque loads. This is often remedied by welding the steel tubes to the cast centersection. If not done properly (well tell you the right way), the welds can crack and the tube will spin anyway, possibly destroying the housing at the same time.