Part Eight: Button Up That Rear End

If you’re an avid reader of Diesel World magazine, this truck should be pretty familiar as we enter Part 8 in the build on this 2012 2500HD LML Duramax. Purchased used in November of 2017 with 117,000 miles on the clock, this basically bone-stock truck that had no idea what it was in for. GM engineers did a great job with the 2011-2014 trucks and the LML powerplant is a solid performer, with more than 370 hp in stock form. They’ll do just about anything the average truck owner could ask, but let’s be honest—we aren’t average truck owners.

HSP Diesel worked with their own shop trucks to develop a simple, well-engineered, affordable solution to wheel hop and axle wrap in GM truck applications due to the massive amounts of torque the Duramax can generate. Traction bars can prevent axle wrap, aid in traction and increase drivetrain and suspension longevity by maintaining the proper pinion and U-joint angles under the most extreme loads.
For our crew cab shortbed truck, the bars from HSP measure 70 inches long and will mount to the frame directly beneath the cab. Kits are available powdercoated in your choice of color; we opted for Kingsport Grey, which offers a nice metallic look to offer a noticeable yet subtle appearance.
The first part of the install will require you to remove the factory rear axle U-bolt bracket. The four U-bolt nuts and washers just need to be loosened and removed. The factory hardware will be re-used, but the axle bracket will be discarded and replaced by the new HSP traction bar axle bracket.
The new powdercoated HSP bracket simply replaces the stock axle bracket. The slotted holes in the new bracket help make installation a little easier. Be sure to torque the axle U-bolt nuts to factory specification and re-check torque after a couple hundred miles of driving.

We’ve upped the power a little (an extra 127 hp/217 lb-ft to be exact) with the help of the Jammer Stage 2 kit from Edge Products. We’ve improved airflow through the engine with a high-flow intercooler pipe from Deviant Race Parts, and we extended our range between fill-ups with a 57-gallon replacement fuel tank from Titan Tanks. The suspension has been upgraded with Bilstein shocks and a leveling kit from Kryptonite Products. To better fit our needs for heavy towing in the summer, a Curt Double EZ-Lock gooseneck hitch and some airless rear suspension bags from SuperSprings were installed. Mickey Thompson Baja ATZ P3 tires and 20-inch SD-5 wheels offer an aggressive look with great on/off-road performance. With plans of adding more horsepower and torque soon with a bigger injection pump and turbocharger, this month we’ll be giving the rear end a little attention to ensure it can handle and put that power to the pavement safely.
In heavy towing situations the rear axle goes through a lot of strain and usually gets neglected. While most owners are good about changing engine oil and fuel filters regularly, it seems many forget about the differential fluid and taking care of it when they should. All that torque from the engine gets pushed through the trans and sent to the rear axle, which must send the power to the tires. Under heavy load a lot of heat can be generated within the axle, and heat can kill a fluid’s ability to keep things lubricated. Inside the differential you’ve got metal-on-metal gear sets in the ring and pinion, and of course you can expect some wear to occur. Wear over time deposits fine metallic debris in the gear oil, which can affect the longevity of the differential.

An aftermarket differential cover can increase longevity with a little extra fluid capacity and also help reduce fluid temperatures under sustained load. The aFe Power rear differential cover for the GM axle allows an additional quart of gear oil and has been cast with cooling fins to help draw the heat out of the axle housing. The magnetic plug will also trap those small metal particles and keep them from being passed through the gearset. They’ve also engineered an easy-see sight glass on the side so you can monitor fluid level and condition without having to drain the diff or remove the cover.

One negative aspect to all the extra torque we’ve added since the truck was stock (with plenty more to come in the next couple installments) is the strain it creates on the drivetrain. Extra torque puts more load on the driveline and rear axle, and under heavy acceleration it will cause the rear axle to “wrap,” or rotate, under the load, throwing off the U-joints and pinion angles and potentially leading to premature failure. It also can change the rear suspension geometry enough that the tires will lose traction at the ground, creating a shudder and wheel-hop situation when towing, racing or sled pulling.

To overcome these issues, the Duramax specialists at HSP Diesel developed a true bolt-in, budget-friendly traction bar kit that will resolve the above noted issues, while looking great underneath the truck thanks to many powdercoating options. Tractions bars are exactly what the name implies: an additional bar added under the vehicle to help locate the axle and prevent it from wrapping under load, planting the tires and eliminating wheel hop and drivetrain shudder.

You’ll need to make sure that the axle U-bolts and the lower axle bracket line up square with the axle. Tightening each bolt a little at a time will help keep everything in line. That new Kingsport Grey powdercoat is going to look great underneath this truck.
Next comes the toughest part of the job: drilling the four mounting bracket holes in the factory frame rails. By mounting the traction bar to the lower bracket already on the truck, the frame bracket can be loosely aligned to the Heim joint and held in its correct location on the frame so the holes can be marked.
To ensure years of trouble-free service, the traction bar kit includes a fully serviceable, top-of-the-line Heim joint to connect the bars at the frame. These Heims will allow you to fine-tune overall bar length and can be easily rebuilt should they ever need to be.
With the mounting holes drilled and the upper brackets installed at the frame, the traction bars can be assembled and bolted into place using the supplied Grade 8 hardware. Be sure to grease the upper Heim joint so it can move freely as needed.

The rearward axle brackets will slip right into place on your factory axle U-bolt plates using all the factory hardware. The frame-side bracket will require holes drilled in the frame, which will be the most challenging part of this install. The double-walled boxed frame is tough to drill, but patience and a good drill bit goes a long way here. Since the boxed frame won’t allow a bolt to be installed easily, HSP supplies a nifty self-holding stud that will grab the frame from the inside when positioned correctly and allow the brackets to be bolted in place. The bars themselves use a rebuildable and greaseable upper joint for full range of motion, and the threaded attachment point allows the bars to be made longer or shorter by loosening the jam nut and rotating the heim one way or the other to change preload on the axle.

After an initial test drive and a couple wide-open throttle runs, we quickly noticed how much better the transmission seemed to shift as the tires and axle stayed planted and offered a more positive feel through the drivetrain. When towing, we no longer feel that slight shudder when leaving a stoplight and we’re trying to apply all that torque to get the load moving and up to speed quickly. The traction bars made a noticeable difference in driving feel and the added fluid capacity should keep the rear differential and axles happier while towing through the 100+ degree summer days.

For the next parts of the build, we’re planning to remove the cab from the frame and swap in a whole bunch of performance goodies to take this truck to an all new level of performance and drivability. We’ll also be looking into a full transmission build soon, since we’re positive the stock 124,000-mile Allison won’t endure 600+ horsepower for long. We also plan to do a few more cosmetic upgrades with some fender flares, maybe a front bumper and some new side steps. Stay tuned as we continue to take Project Looks. Muscle. Longevity from stock truck to the ultimate daily driver and weekend tow rig.

The newly installed HSP traction bars not only look great under the truck but serve a real purpose. We noticed almost instantly the improvement in the truck’s feel while towing. There is no longer shudder from the drivetrain upon leaving a stop and the tires stay planted regardless of throttle input.
The factory rear axle is an extremely tough piece of the drivetrain, but it’s often the most neglected part of the truck when it comes to maintenance. Not many think about how much heat and abuse the gear oil takes, especially when towing, and a rear end service is easily completed on the 2011+ trucks.
The axle has a built-in drain that makes servicing pretty simple by using just a 3/8” ratchet to remove the plug. Upon draining, it appeared this fluid was in decent condition as it didn’t show any signs of being burnt or neglected for the 125,000 miles on the odometer. The magnetic drain plug didn’t show much metal material either, which was a good sign.
Since we’re already doing a fluid change on the rear differential, it seemed the perfect time to replace the factory cover with something a little better. The cast aluminum cover from aFe Power will allow an additional quart of oil and has been designed to help keep fluid temperatures down under heavy towing conditions. Plus, it looks great.
With the OEM cover removed and sitting beside the new aFe cover, you can see the difference in overall shape and casting. The new cover will hold a little extra fluid and help dissipate heat due to its finned design.
Before installing the new pan, the axle must first be thoroughly cleaned to remove any of the original pan’s gasket to ensure a clean, flat sealing surface. This is also a good time to inspect the ring gear for any abnormal wear marks.
The aFe Power differential cover uses a simple O-ring supplied in the kit to seal the fluid inside instead of the factory-style paper gasket. This O-ring will offer a solid seal and make future axle repairs easier if the cover ever had to come back off for some reason.
Using the supplied stainless Allen head bolts, the cover can be installed and snugged into place. The bolts need to be torqued to specs listed in the aFe Power instruction manual. Be sure to torque in a star pattern around the differential to make sure the O-ring makes the proper seal without binding up.
On this GM application, refilling the axle with fresh Synthetic 75W-90 gear oil is simple. The included fill port allows easy access to pour fluid in, and the sight glass on the side of the cover lets you know when it’s full. Our truck took exactly 5 quarts. Note the pre-drilled and tapped 1/8” port that could be used for a temperature sensor should you ever want to install one.



HSP Diesel

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