How to Identify an AMC Model 20 Rear Differential Caseby Thomas West
American Motors manufactured most of its own mechanical parts, including rear axle assemblies. The model 20 rear end is a straight axle with enclosed axle shaft tubes that is suspended from the vehicle with leaf springs. The model 20 was used in several Jeep models, such as the 76-86 CJ, Wagoneer and Cherokee, as well as some AMC models, such as the Eagle four-wheel-drive car. It may be beneficial to know how to identify the differential case of the AMC model 20 when obtaining a replacement or when ordering repair parts.
Slide under the rear of the vehicle.
Observe the rear cover of the differential case from the rear of the vehicle looking forward. The cover on an AMC model 20 is round -- some enthusiasts indicate that the cover resembles a helmet used by World War I soldiers. Note that if the rear cover on your differential case appears flat on one side, if may have been manufactured by Dana or by the Chrysler Corporation, which purchased AMC in the late 1980s.
Count the number of bolts that secure the cover to the differential case -- there should be 12 bolts on an AMC model 20.
Narrow or Wide-Track
Measure the width of the rear end from the inside edges of the wheels with a measuring tape.
Know that the narrow-track model 20 axle assembly used in 1976 through 1981 Jeeps should be 50.5 inches wide.
Know that the wide-track axle assembly used in 1982 through 1986 Jeeps should be 54.4 inches wide. The difference in width is in the axle tubes of the differential case, which houses the axle shafts.
Model 20 Decoding
Locate the single or double letter -- indicating the axle ratio -- stamped into the rear of the housing to the right of the differential cover.
Determine if your model 20 differential is a narrow-track or a wide-track, as the same code can have different meanings on each variation.
Consult a shop manual for your particular Jeep or AMC vehicle to determine what the code indicates on your rear end, as the multiple variations available preclude listing all the codes here.
- Aftermarket one-piece axle shafts can be used to replace the factory two-piece units, eliminating the worry of the key shearing off as in the original design. The old two-piece units can also be replaced with a heavy-duty full-floating rear end -- such as used in large trucks -- at considerably more cost than switching to one-piece axle shafts.
- When used gently in an American Motors passenger car, the model 20 rear end may give many years of trouble-free service. If your Jeep has a model 20 rear end, you may be in for costly repairs if you use your vehicle for extreme rock climbing or jumps. The axle shafts often lead to many failures in these rear ends. While many rear ends have one-piece axle shafts, American Motors used a two-piece design in the model 20. This may have been done to save tooling costs when manufacturing different lengths of axles shafts for different applications. The flange -- the piece the wheel bolts to -- is a separate part from the shaft. A keyway keeps the flange from turning on the shaft. The key may shear off during hard use and cause the shaft to spin within the flange, which will get you nowhere.