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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.

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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.

1 To start the Ultimate Rear Axle Build Jason Nelson puts the big Duramax truck on one of the twin post lifts in the RPM Offroad service bay and then removes the rear wheels.

2 With more than 360,000 miles on the clock it’s no wonder that the underside of the truck has a little surface rust—we were worried that the diff cover might actually rust through! Notice how the axle tubes are larger at the differential housing than the rest of the tube and how the U-bolts and shock mounts hang down below the axle housing just waiting to get hung up when off road.

3 The GM 11.5 AAM axles feature a drain plug that can be used to easily drain the gear oil. For some reason the Dodge versions do not; the diff cover must be removed to drain the oil.

4 While the old gear oil is draining Nelson begins to tear down the axle and removing the calipers, rotors, hubs, backing plates and parking brake cables.

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.

5 The driveshaft must be removed as well.

6 After removing the axle bolts, the factory 30-spline axles can be removed from the housing.

7 Comparing the factory axle (center) to the Yukon chromoly axle (top) and Branik Motorsports 300M axle (bottom) it’s easy to see that the 38-spline Yukon and Branik axles are both larger and stronger than the skinny factory shaft.

8 Nelson also removed the factory differential and ring gear to make way for the Yukon Grizzly Locker and 4.56 G2 gear.

9 Nelson uses an air hammer with a punch attachment to remove the pinion gear from the housing, then drives out the bearing races that will be replaced along with the bearings from the G2 master installation kit.

10 Then he can remove the U-bolts and lower the axle assembly down from the truck with the transmission jack. If you don’t have a transmission jack a couple of strong friends will work, but you may need to supply pizza and beer.

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.

11 After taking careful measurements to determine the leaf spring pad location as well as the precise axle tube length, Nelson uses a portable band saw to cut the axle tubes. The saw makes quick work of the 1/4-inch thin wall tubing.

12 Looking at the cutaway factory axle tube leaves little doubt as to how axle tubes can bend when trucks are driven or worked hard.

13 The 4-inch replacement tubing we chose to use is not only larger diameter through the full length of the tube it also has a thicker wall to make it much stronger than the factory 3.5-inch tubing.13 The 4-inch replacement tubing we chose to use is not only larger diameter through the full length of the tube it also has a thicker wall to make it much stronger than the factory 3.5-inch tubing.

14 The factory axle tubes are pressed into the housing and secured with hardened plugs that were very difficult to try to drill out so we took the housing to Twin City Auto Machine where Jack Sorah used a mill to bore out the complete axle tube.

15 Twin City machinist Steven Hensley machined the spindles and new axle tubes to have a 0.003-inch interference fit.

16 & 17 Fire and dry ice are used to make the machined parts fit together. Michael Powell used a torch to heat the housing while Nelson filled the end of the tube with dry ice purchased from the local grocery store.

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

18 The heat causes the housing to expand while the cold causes the tube to contract, allowing the tube to slip perfectly into the housing.

19 After the other axle tube was installed the housing was moved to a workbench where Nelson installed the alignment jig and polished stainless steel shaft to keep everything aligned and true while welding.

20 & 21 Nelson used the same heat and cold technique to install the spindle into the new axle tube. Thanks to the precision machining from Twin City, everything went together perfectly.

22 Powell fully welded the axle tubes to the housing and spindles to the tubes before welding the Artec Industries axle truss into position. The combination of the larger diameter thicker tubing and the beefy axle truss gives us confidence that the housing should hold up to our abuse even jumping the truck and flying through the desert as we plan to do once we complete the build.

23 After a fresh coat of paint, Powell aligns the housing under the truck and mounts it to the new spring perches he installed and mounts it with upward facing U-bolts for better ground clearance.

24 Looking at the Yukon Grizzly Locker side by side with the factory differential it’s easy to see that the Grizzly is a much stronger unit that shouldn’t have any problem holding up to the power from our Duramax.

25 Nelson hand started all of the G2 ring gear bolts and then cinched them with the impact gun, finally torqueing them to 150 ft=lb with a torque stick on the impact gun. Be sure to use thread locker on the bolts to prevent them from backing out. We used the same WRP Threadlocker that many Baja desert race teams trust for their race trucks and buggies.

26 While Nelson was installing the new G2 bearings and ring gear, Powell used the Yukon spindle bore kit to open up the inside of the spindles enough to accommodate the larger-diameter 38-spline axle shafts.

27 After the housing was thoroughly cleaned to remove any metal shavings and debris, Nelson installed the gears and set the backlash. It’s very important that the gears are properly set up as they can be ruined very quickly if they do not mesh properly. If you do not know how to do it, have a professional take care of it for you.

28 The G2 Brute looks great and has additional fluid capacity, making it a great choice for any truck.

29 We decided to take it a step further and install the G2 Torque diff cover with load bolts to help keep the carrier from trying to walk out the back of the housing under load. After Nelson installed the cover, he filled the axle with Royal Purple MaxGear 75W90 gear oil to make sure everything is well lubricated.

30 & 31 Jereme Miltier installed the Branik Motorsport axles in the housing, then installed the 40-spline drive plates that connects the axle to the hub and spins the wheels.

32 The Branik drive plate cover extends over the axle bolts making it legal to be used in most sled pulling organizations.

33 Miltier also fabricated the shock hoops to relocate the shocks to the outside of the frame for more stability, better handling and improved ground clearance over the factory shock positions further inboard and lower on the axle.

34 Rather than mess with the rusty old brake lines, Powell bent up a new set of lines to flow with the axle truss and keep the lines out of harm’s way when off road.

35 After the rear axle was completely assembled and the brakes were bled, it was time to put the wheels back on the truck. Notice that the additional 3 inches of axle width on each side allows room for the large-diameter Pure Performance shocks to fit between the frame and the tire without rubbing.

36 Not only does the axle look much better, it’s also much stronger with larger tubes and a serious axle truss. Also notice that there’s nothing hanging below the axle to get caught on rocks or other obstacles while off road.

37 To correct the speedometer and maintain the shift points Fleece Performance Engineering tuned into the truck; Miltier used the EFILive V2 module and a laptop to interface with the truck and modify the gear ratio.

SOURCES
Artec Industries
855-278-3299)
www.ArtecIndustries.com

Branik Motorsports
260-467-1808
www.BranikMotorsports.com

EFILive
www.EFILive.com

G2 Axle & Geara
310-900-2687
www.G2Axle.com

Royal Purple
888-382-6300
www.RoyalPurple.com

RPM Offroad
423-573-3300
www.RPMOffroad.com

SoCal Diesel
661-775-5620
www.SocalDiesel.com

Twin City Auto Machine
276-669-1301

Woodward Race Products/WRP Threadlocker
909-627-8286
www.WoodwardRaceProducts.com

Yukon Gear & Axle
888-905-5044
www.YukonGear.com