Step 9: Address the AMC 20 Axle Shafts

If your CJ has an AMC 20 rear axle with factory two-piece axle shafts, this step is not optional. It is a safety issue and it belongs in the first month of ownership, not on the someday list.

Understanding the failure

The factory AMC 20 uses a two-piece axle shaft design where the outer stub shaft is pressed into a separate hub. This design made manufacturing easier and allowed hub removal without pulling the axle shaft. The problem is that over time and with repeated thermal cycling, the press fit loosens. Once the fit loosens, the stub shaft begins to rotate inside the hub under torque loads. This creates more wear, which further loosens the fit, until eventually the shaft spins completely inside the hub and the wheel separates.

This failure does not happen gradually with warning signs. It happens suddenly. The Jeep loses drive to the rear wheel, the wheel angle changes, and control is lost. It has happened to CJ owners at highway speeds on flat dry pavement with completely stock tire sizes.

The fix — one-piece axle shafts

The solution is straightforward, inexpensive, and permanent. One-piece axle shafts from aftermarket suppliers such as Moser Engineering replace the two-piece design with a forged one-piece unit that cannot fail in the same way. The conversion typically costs $200-250 for a pair of shafts and takes a couple of hours to complete — most of that time is removing the old hub assembly.

One important note: Crown Automotive sells AMC 20 axle shafts that are commonly available and may seem like a budget solution. These shafts are actually welded two-piece units — not forged one-piece. They are not an appropriate safety upgrade. If you are buying one-piece shafts, confirm they are forged one-piece units from a reputable supplier.

After installing one-piece shafts the AMC 20 is a capable and reliable rear axle that handles 33 inch tires without issue on street and light trail use.