The rear axle on all H3 Hummers are originally equipped with a solid AAM 860 axle. This axle is commonly referred to as the GM 8.6. This GM 8.6 axle is very well suited for off-road applications and hold up quite well. However, the axle does have one particular flaw. The differential cover that seals the ring and pinion off from dirt and debris is not capable of supporting the weight of the vehicle. Unlikely, but situations do occur when the weight of the H3 end up on the rear diff cover. Underneath that differential-cover are some of the most important parts of any vehicle. The ring & pinion, the spider gears, bearings, and also axle shafts. Those parts turn the torque from the motor 90° and transferred onto the tires. This process of turning the torque 90° requires a lot of precision to work properly. Being off even just a 1/16 of an inch can cause major problems. That is why the AAM 860 axle needs to be protected with an aftermarket heavy-duty differential cover.
Being a fan of company Solid Axle Industries, I decided to go with their solution for protecting my AAM 860 solid axle. Even though the company Solid Axle doesn’t sell a heavy-duty differential cover for AAM 860, they do have a product for the GM corporate 10 bolt cover which is also another name for the AAM 860. The heavy-duty corporate 10 bolt cover is made from cast iron 3/8 inch thick and reinforced with thick nodular bracing, literally making the AAM 860 bulletproof.
Installing the Solid Axle differential cover on the H3 Hummer is, for the most part, straightforward, but I do have some recommendations before starting this project. While swapping the diff covers absolutely no dirt can get inside the axle housing, this is very important. The bolts that Solid Axle Industries are shipped in the box are not long enough and forced me to buy 10 new bolts 5/8 course thread bolts with 1 inch of threads, this is approximately 1/4 longer than the bolts supplied. The owner’s manual mentions that the 3 litters of 75w-90 oil need to meet the GM specification 9986115, however, the local NAPA auto parts said that any modern-day off the shelf synthetic 75W-90 with or without the limited-slip additive will work just fine. The mounting bracket for the brake line uses one of the ten bolts holding on the differential cover on and because of the extra thickness of the diff cover it will not bolt up. I used my grinder to shave off approximately a 1/4-inch of metal from the outside to make it fit with no problems, which I show in one of the images above. Finally, pouring the 75W-90 oil into the differential is rather difficult because of where the sway-bar is located. Some ingenuity, with the combination of jacking the Hummer up and a hose I was able to pour the oil into the axle.
Awesome write up and I completed this upgrade today on my 2006 Hummer H3 with 194k miles. I added the Lube Locker gasket for added sealing. I noticed with the Solid Axle differential cover, I used (4) quarts of Royal Purple and the internal fluid level did not reach the fill hole. Do you recommend adding more or staying with the OEM fluid specs?