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Project Over/Under: Part 8 - Gearing Up with Nitro Gear & Axle

Project Over/Under: Part 8

Mar 11, 2019
Photographers: Jeremy Cook
As we near completion of our ’03 Chevy Silverado project, we have just a few more items to cover to make the truck function as great as it looks. With the addition of a 7-inch Bulletproof/Fox/Atlas lift and 35-inch General ATX tires, the most important remaining mod for this build was to regear the rear end. When GM originally built this truck, the rearend gear ratio was selected by taking into account the available horsepower and torque curve, transmission gearing, and factory tire size, then deciding what would provide the best highway mileage while having power to pull off the line. So when we add lifts and big tires, we throw all that off, adding extra stress on the engine and transmission. Because the first way many of us try to compensate for the loss of power with big tires is with a heavy foot! Another way people try to fix the problem is bolt-on horsepower. Intakes, tuners, headers, and exhaust are great and will help mask the fact that you need gears, but you are not solving the problem.
There seems to be some big mystery surrounding rear ends and gear swaps. Now, you do need some specialized tools and you need to press some parts in and out, but you don’t need to be a scientist to set up a differential with new gears—or to pick the correct gear ratio in the first place. While there is an equation for determining the ideal gear ratio for your application, the pros have already figured it out for you. Most of the modern automatics have a reasonably similar high gear, and there are only so many available combinations of ring and pinion combinations, so for most of the trucks we’re working on, it goes as follows. If we settled on 33-inch tires, the recommended gear ratio was 4.10:1; 35-inch tires: 4.56:1 gears; 37s: 4.88:1. Of course, if your truck only sees dirt or only does highway driving, you may have different ideas, but these numbers will most often get your engine and transmission working similar to when you had stock tires.
Photo 2/34   |   One thing we really like about Nitro Gear & Axle is it’s a one-stop shop for everything we needed to do the job, right down to the gear oil. Included in our package was the 4.56:1 ring and pinion gearset, Nitro’s limited-slip differential that uses Eaton carbon-fiber clutches and 400-pound springs, axle bearings and seals, a master overhaul kit, and everything else needed to do the job.
Our ’03 Silverado work truck came with a 3.42:1 ratio from the factory and tires around 29 to 30 inches. We initially had the idea to find a limited-slip 2500 rear end from an ’11-and-up GM HD truck, but Carl Montoya from Nitro Gear & Axle urged us to stick with our stock 10-bolt. Although the HD rear end would be bulletproof in this truck, we would also be dragging around more than a couple hundred pounds of extra baggage—even when we didn’t need the added strength. Carl set us up with the parts to give us a limited-slip rear end, the 4.56:1 ring and pinion we needed, and all the new bearing sets and other parts recommended to do the job right. We even picked up a custom Nitro finned aluminum diff cover and Nitro’s own gear oil to finish the job.
Carl also mentioned a name that might help with the install that was a blast from the past—Martin Barraza has been a mobile gear installer for many years around town. He actually did our C10 rearend for us many moons ago, but we had since lost touch. As luck would have it, he still spends a couple days a week at Rebel Off Road in Laguna Hills, California, regearing all the insane Jeep builds done over there, so we made an appointment with him. To say Martin and his assistant are quick would be a gross misrepresentation—we could barely keep up with the camera. And at times when we might accuse a rookie installer of cutting corners, Martin knocks out steps like setting pinion depth, bearing preload, and setting the backlash in seconds like it isn’t a concern how the parts go in, but the truth is he has probably done more custom gearsets than anyone else around—ever! He will tell you by feel exactly where the specs are, and when we secretly checked his work he was spot-on! He makes it look so easy we were considering doing the next one ourselves…but we’ll probably call Martin. Follow along as we rip through the install, do a little cleanup, install the Hellwig rear sway bar, and test to see what the rpm looks like at highway speeds.
Photo 3/34   |   When we arrived at Rebel Off Road, Martin wasted no time getting Project Over/Under on a lift and pulling the wheels. Things only moved faster from there.
Photo 4/34   |   Just a couple of minutes later, the cover was removed and what was most likely the factory gear oil was drained. The driveshaft was also unbolted from the pinion flange at this time.
Photo 5/34   |   In order for the C-clips and axles to be removed, the cross pin bolt and cross pin were removed.
Photo 6/34   |   Then the calipers were unbolted and hung out of the way, and the axles were pushed in slightly, allowing the C-clips to be removed. Then the axles were slid out of the housing. We’re going to reuse our axles, but just about everything else coming out will be replaced.
Photo 7/34   |   The bearing caps were buzzed off with an impact and the carrier was removed from the housing. They need to be marked to go back in the same place from which they were removed.
Photo 8/34   |   Moving to the pinion gear, the nut was removed and an air hammer was used to separate the flange from the shaft.
Photo 9/34   |   With the pinion gear out, the inner and outer bearing races were persuaded out next. The axle bearing races required a long pry bar and dead-blow hammer.
Photo 10/34   |   The inside of the housing was bare, so it got a healthy spray down with brake cleaner. Rags were pushed through the axle tubes, and the cover mounting surface was cleaned up with a sanding disc. Now we’re ready to rebuild!
Photo 11/34   |   Again, our man Martin makes it look too easy. He has the right tool for the job off his cart and doing work in seconds. First the inner pinion bearing race is pressed (tapped) into place.
Photo 12/34   |   The outer race followed suit. Here it is pressed into place.
Photo 13/34   |   A long solid rod with a driver on one end and a hammer on the other is what it takes to get the axle bearing races in place.
Photo 14/34   |   Next, the carrier bearings were pressed on each end of our limited-slip case.
Photo 15/34   |   The ring gear was then bolted up to the carrier. As per the rules, we used thread locker and torqued the bolts to 65 lb-ft.
Photo 16/34   |   Moving to the new pinion gear, the bearing was pressed on. You can also see the bearing spacer Nitro recommends to replace the problematic factory crush sleeve.
Photo 17/34   |   Getting the backlash correct, as shown by having the correct contact pattern on the gear, is as simple as shimming each side of the carrier correctly—which is often not simple at all. But Martin, armed with only the measurements of the shims that came out and thousands of rearends worth of experience, grabbed a few shims, checked them with the dial calipers, did a dry run by installing the carrier with the pinion not yet in place, and said “these will be the ones.”
Photo 18/34   |   Then the pinion was installed, with Martin having determined the correct pinion depth. Things really started to move fast from here.
Photo 19/34   |   The pinion seal was pressed in place next.
Photo 20/34   |   Then the flange, washer, and nut went on the pinion shaft. Martin set the pinion strictly by feel, and he knew exactly when he was dead center of the 14 to 19 inch pounds that were called for.
Photo 21/34   |   The carrier and Martin’s shim selection were slapped in and the bearing caps were buzzed on. It was finally the moment of truth.
Photo 22/34   |   The proof is in the paint! But first Martin checked the backlash between the ring and pinion. It needed to be between 6 and 10 thousandths, and Martin checked it so fast we didn’t get a photo—he just smirked. We’re guessing it was .008. Next, we painted up the gears and gave them a spin. Another smirk from Martin: We were dead on, meaning the pattern is dead center on the teeth. If the contact area looked like it was hangimg off the top or bottom (face or flank) of the teeth, the pinion depth would need to be increased or decreased, respectively. It’s actually not necessary that the contact point is dead center between the inside and outside (toe and heel) of the teeth, but it certainly doesn’t hurt!
Photo 23/34   |   With everything within spec, we torqued the bearing caps to 60 ft-lb, then slid the axles back into place. The C-clips slipped back into place, followed by the cross pin.
Photo 24/34   |   Finally, the cross pin was tightened up and we were ready to button our regeared, limited-slip rear end up.
Photo 25/34   |   We gave the assembly a few more spins, as sometimes there can be clearance issues when adding a bunch of aftermarket parts, but we were golden!
Photo 26/34   |   The Nitro cast-aluminum diff cover is a nice piece. And the gasket, silicone, and stainless Allen bolts came with the kit.
Photo 27/34   |   We snugged the cover bolts down in a nice cross pattern and quit while we were ahead. People love to overtighten diff covers!
Photo 28/34   |   Nitro included its synthetic 80W-140 gear lube with the kit, so in it went, using our new filler hole. Soon we were back on the road and headed home. Mobile Gear Installers and Rebel Off Road really came through.
Photo 29/34   |   Back at the shop, we had a few more loose ends to tie up. For starters, we hadn’t cleaned up and painted the housing yet, so that happened.
Photo 30/34   |   We also skipped the Hellwig rear sway bar when we were installing the rear suspension, knowing we would be back here soon. We’ve used this kit before, so we knew it would bolt up without issue.
Photo 31/34   |   U-bolts and brackets, along with the correct bushings and grease hold the sway bar in place on the axlehousing.
Photo 32/34   |   Another set of U-bolts was slid over the factory crossmember to hold the end link mounts.
Photo 33/34   |   Then the end links are bolted in place and adjusted so the sway bar sits as level with the ground as possible. Then the length is locked in by the nuts.
Photo 34/34   |   Our final step was to jump in the cab with our Hypertech programmer and punch in our new gear ratio. Otherwise, the speedometer will be way off. At this point, we were really looking good. And the truck pulled coming off the line like it never had before—even with the 35s. But how was highway driving you ask. Before we were running a sad 1,700 rpm at 75 mph. Now we’re at a powerful XXmph and the truck is no longer trying to downshift when we don’t need it to. We have certainly enhanced the driving experience of Project Over/Under—and we haven’t even hit the dirt yet!

Sources

Nitro Gear & Axle
Sacramento, CA
916-673-6464
www.nitro-gear.com
Rebel Off-Road
Lake Forest, CA 92630
1-866-900-8841
www.rebeloffroad.com
Hellwig
800-435-5944
www.hellwigproducts.com
Mobile Gear Installers
949-306-2805
mobilegearinstallers.com

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