A Super-Strong 11.5 AAM Rear End For Your GM Or Dodge
Diesel trucks are built to handle a lot of abuse, but diesel enthusiasts like us tend to push way beyond the manufacturer’s intended performance levels. We love more power and more torque and are not afraid to put it to the test pulling a sled, drag racing or in the dirt and rocks off road. But problems can develop when we reach the limits of the factory components. Differentials, gears, axle shafts and even the axle housing itself can prove our undoing at the most inopportune times. In this article we’re going to highlight some of the best aftermarket products available to improve the strength of the 11.5-inch AAM rear axle found in most Dodge and GM 3/4- and 1-ton trucks.
We took our high-mileage 2001 Chevy 2500 HD to the pros at RPM Offroad in Bristol, Tennesee. The truck’s rear end was operating like an open diff, causing one tire to spin just about any time we got on the throttle hard, especially in turns. We also wanted to increase the track width to match the front of the truck and make sure the housing was strong enough to handle the serious abuse of desert racing. RPM
Offroad’s Jason Nelson took the lead, tearing down the housing then
setting up the gears as it went back together.
To ensure equal traction to both rear tires with the ability to turn well when driving on the street, we chose to run a Yukon Grizzly locker rather than a full spool. Since we needed custom axles that were longer than stock, we opted for the 38-spline Grizzly locker over the stock 30-spline configuration so we could take advantage of stronger, larger diameter axle shafts.
We considered two different axle options: Chromoly flanged axles from Yukon that can be cut to the proper length and Branik Motorsports 300M double-splined axles built to length. For the added strength we opted to run the Branik 300M axles but will have the Yukon axles as spares in case we break one.
Since we’re running large 37×12.50R17 Goodyear Wrangler MT/R tires, we wanted to change the gear ratio while the rear end was apart. We contacted G2 Axle & Gear and ordered a set of 4.56 gears for the front and rear of the truck. Using an EFILive V2 module from SoCal Diesel we were able to change the ratio in the ECU and TCM to keep the shift points the same as when Brayden Fleece originally tuned the truck. This also allows the speedometer to read correctly.
The axle tubes on many of the GM versions of the 11.5 AAM axle are another weak point. They’re built using a tube that’s almost 4 inches in diameter at the housing and reduces to 3.5 inches out to the spindle and only about ¼-inch thick. Under heavy loads the tubes can bend and deflect. Newer Chevy and GMC trucks as well as the Dodge trucks we have seen use the 4-inch diameter tubing from center to spindle, but we haven’t cut one apart to determine the tubing wall thickness.
We decided to use 4-inch diameter 1/2-inch wall thickness steel tubing to replace the factory tubes. The 4-inch tube was cut 3 inches longer than the factory tubes to give the truck the desired extra 6 inches of track width. To retube the housing we cut the stock axle tubes about an inch away from the spindle backing plate and again near the cast center section of the AAM housing. We then took the housing, spindles and new tubes down to Twin City Auto Machine where Jack Sorah machined the old axle tubes out of the housing. Steven Hensley turned down the inner end of the axle tubes to a 0.003-inch interference fit with the housing for a secure fit that would be finished with a full weld. To make the spindle-to-axle tube junction stronger he machined down the outside diameter of the remaining factory axle tube on the spindle side and enlarged the inside of the axle tube to create another 0.003-inch interference fit.
Once the parts came back from the machine shop the RPM team installed the tubes in the housing by heating the housing with a torch and freezing the tube ends with dry ice. This causes the opening in the housing to expand while the tube contracts and allows the tube to drop right into the housing. The assembly was placed on a workbench and the spindle ends were installed using the torch and dry ice again. To make sure everything stayed properly aligned and dead straight, they installed machined bushings into the bearing journals and slid a large stainless steel shaft through the length of the axle assembly with caps threaded onto the ends of the spindles. Then Michael Powell was able to fully weld the axle tubes and spindles, alternating positions of the welds then moving on to another spot to prevent warping the steel.
While the new tubes are definitely stronger than the stock ones, we knew we would be jumping the 7,000-lb truck and wanted to make sure we wouldn’t bend the axle assembly. We contacted the team at Artec Industries and ordered one of their new fabricated axle trusses for 11.5 AAM axles. The low-profile truss is CNC cut to fit tightly along the top of the housing and is strong enough to be used for link mounts, with the top plate made from 3/8-inch thick mild steel supported by interlocked 1/4-inch thick mild steel gussets. The truss would be a good foundation for a multi-link rear suspension setup should we decide to ditch the leaf springs in the future.
Once Powell finished welding the axle he applied a fresh coat of black paint then worked with Nelson to install the locker, gears and axles with the housing in the truck. Before installing the internals, they had to open up the spindle bores to fit the larger 38-spline axle shafts. Fortunately, Yukon makes a kit that bolts onto the spindle snout and keeps the bore running true in the housing. During this installation the spindles were bored after the assembly was welded back together; in retrospect, we’d recommend boring them before cutting the spindles off as it will make it much easier to clean the filings out of the housing. It took us hours to get all the metal shavings and filings out of the housing, which we did using brake cleaner spray and passing wadded-up rags through the axle tubes with a broom handle.
After hanging the axle on the leaf springs, RPM Offroad general manager Jereme Miltier fabricated a new set of shock hoops to install the shocks on the outside of the frame for better handling and more ground clearance. He could have cut the shock mounts off the old axle tubes and welded them onto the new ones, but the new mounts will work better for off-roading. To finish off the installation, a G2 Torque differential cover was installed that features load bolts to prevent the bearings and differential from trying to walk out of the rear of the housing under load. We could have opted for the G2 Brute cover for better looks and increased fluid capacity, but felt for our application the Torque would be the best choice. To keep the gears and bearings well lubricated, the housing was filled with Royal Purple 75W-90 Max Gear synthetic oil. Don’t forget to drain and refill the axle after the 500-mile break in period for the gears and diff.
If you plan to perform these upgrades yourself you will need a well-stocked toolbox as well as fairly substantial DIY skills and a good welder. If you don’t have the tools, equipment and expertise to complete the transformation, you can still perform the labor-intensive parts of the project yourself (like removing and stripping down the axle assembly) while having your local diesel performance shop handle the welding and gear setup. Then you can install the complete rear axle back in the truck yourself.
Remember, this article is an overview of the products and processes that the team at RPM Offroad used to build our Ultimate 11.5-inch AAM axle assembly; we do not have space in the magazine to cover every step of the process in detail.
With the upgrades we performed on the rear axle we feel that it should hold up to just about anything we can throw at it. If you’re interested in sled pulling, drag racing or serious off-roading with your Dodge or GM diesel truck and want to make sure that your rear end can handle the abuse, these are the upgrades that can make it happen. Additionally, by retubing the housing and using custom axles, you can configure the rear axle width to work with your particular truck to best meet your traction needs and work with your wheel and tire combination. DW
G2 Axle & Geara
Twin City Auto Machine
Woodward Race Products/WRP Threadlocker
Yukon Gear & Axle