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Rear Axle Swap Tips

Posted in How To: Transmission Drivetrain on September 1, 1998
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Photographers: Rick Péwé

Swapping parts is a time-honored tradition among 'wheelers, and the rear axle redo is a high priority for many. That's why we figured that you could use some important tips on the whys and wherefores for your next project. Of course, not all of these gems apply to every rear swap, but they do cover the basics. Remember that changing to a different rearend may be the hot ticket, but planning the swap in detail and doing it right can save a lot of grief.

While some swaps are a true bolt-in, these are few and far between. Our "14-Bolt Swap" in this issue shows how close yet far a near bolt-in can be, and some are even more complicated. For instance, Dana 44 rear axles used in Jeep CJs come in just three basic styles, which encompass two widths, two offsets, two tube diameters, two spring-pad locations, and three different housing ends-not to mention five different axle styles and brake assemblies. Many people think that these axles are all the same and can "bolt right in" to different CJs, but this is not always the case.

The most common swap is to use a completely different, and hopefully stronger, axle assembly than the one being replaced. One exception is the Dana 60 axle, which comes with a variety of spline counts. For example, the standard 30-spline axleshaft of a Dana 60 is the same as a Dana 44 axle. This means that only the gears, carrier, and housing are stronger than the 44's. If you are snapping 44 axles, this might not be the best swap. Even so, you'll gain a full-float design. However, the 60 is also available in the 35-spline size, which is considerably bigger and stronger and makes a great swap.

Bearings carry the weight of the vehicle whether the axle is a full-float or semi-float style. As a rule, the larger the axlehousing the bigger the bearings. Because they're usually stronger and more durable, cylindrical rollers such as these handle radial load well but not side loads. Tapered rollers are better for side loading and adequate radial load, while ball bearings seem to be the best all-around style. Custom axle ends can be installed on a housing to change styles, but the axleshaft must be a non-C-clip type to use tapered or ball bearings. If you change axle ends, you can eliminate some C-clip axles.

The basic consideration of any axle swap is to figure out what best fits your application. If you own a 1/2-ton Chevy with a 12-bolt axle, swapping in a weaker 10-bolt unit is going backward. Likewise, a 2 1/2-ton military axle is overkill unless you're into professional mud bogging. The next consideration is the overall width, or the distance from one wheel-mounting flange to the other. Unless you're trying to increase or decrease this dimension, measure carefully to retain the original track. This is especially true if you're only swapping rearends to avoid different track widths front and rear.

Alternatively, most axlehousings can be cut down to size and axleshafts cut and splined if they are too long, however, this should usually be left to knowledgeable machinists, which adds cost to the swap. Differential offset is also important, since the driveshaft needs to be in as straight a line as possible. If you have an offset axle and try to swap in a centerset diff, the driveline may not be able to be hooked up. Finally, the miscellaneous brackets and spring pads need to be in the same place or cut off the housing and moved. This is usually the easiest task of all, as long as the pads are welded on in the correct position for driveline angles.

Mechanically, the gear ratio is also a concern. If the axle you select for swappage contains your current gear ratio, all the better. But what if you want to change ratios someday and that ratio isn't available for your new axle? Planning ahead in this department can also save bucks.

Another major problem can be brakes. Changing an axle can lead to bigger or smaller brakes than the stock offering, which can cause improper braking. Converting to disc brakes or adding an adjustable proportioning valve can help remedy these situations. Also, the stock emergency-brake cables may not hook up to the new brakes, so be prepared for some fabrication time if this is the case.

While most axles on modern 'wheelers have the differential placed in the center of the housing, early Jeeps and some others have the pumpkin offset to one side. A direct swap between models is hampered by such a design, and cutting down a fullsize rearend to fit these applications means that one axle tube and axleshaft gets whacked more than the other, because the replacement housing sometimes isn't long enough for the offset long side. Careful measuring is the important key when customizing.

Even with all these details to consider, swapping in a stronger or more suitable rearend is an excellent idea. Parts from the aftermarket abound for you to do the job yourself, and plenty of reputable companies can custom-make an axle assembly that will virtually bolt right in. But whether you do it yourself or farm out the job, take heed of these tips and tricks and get it done right the first time.

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