It can be argued that there's no bigger bang-for-the-buck performance upgrade than new axle gears. Auto manufacturers are tasked with building a vehicle that is optimized for both highway efficiency, comfort, and performance. Because of this, there's going to be a trade-off of some sort. All too often, manufacturers lean toward efficiency over performance. Add to the mix the fact that most enthusiasts add larger tires at some point, which effectively raises the gear ratio, and it's easy to see where changing the gearset can make a big difference.
Using our 2002 Chevrolet Silverado
2500HD project as an example, the truck came from the factory with 3.73:1 axle gears. When we increased the tire size from factory to 37-inch tires, we effectively raised the gear ratio (numerically lower) by about 20 percent. It would be as if we installed 3.23:1 axle gears. The result of this is more difficult acceleration but lower engine rpm at highway speeds.
| Changing axle gears is one of the quickest and least expensive ways to add measurable performance to a pickup.
To get the engine back in its proper powerband and regain the lost acceleration, we needed to change the axle gears to a lower (numerically higher) gearset. To get the truck back to relative normal, we would have needed to install 4.30:1 gears. However, because we did a six-speed transmission conversion, and to get a big of added performance, we opted to go even lower to 4.56:1 gears.
For this project we turned to the gear and axle experts at Yukon Gear. We ordered up a ring and pinion set, master install kit, the company's Dura Grip limited-slip differential, and 4340 chromoly axle shafts. We opted for the Dura Grip limited-slip over a locking differential for its improved highway manners. The Dura Grip is fully rebuildable and features 4320 chromoly spider gears, Raybestos composite clutches, and a strong four-pinion design.
Even though the GM 14-bolt axle is one of the easiest to set up gears in, we opted to leave this important job to the professionals and headed over to South Bay Truck & 4x4 in Hawthorn, California. Owner Frank Gilliland has installed thousands of gearsets during his career, leaving us no doubt that he'd get ours done right the first time. The job requires special tools and knowledge to avoid a frustrating situation and potential damage. Frank has the rear axle set up in just a few short hours. The front differential of these IFS Chevy
HD trucks is another project altogether and one that we'll address in the next installment.
| After getting the truck up on the rack and removing the rear tires, Frank Gilliland set to work by first removing the truck's rear driveshaft from the pinion yoke. The shaft was then strapped up and out of the way for the duration of the gear install, although it could just as easily have been fully removed.
| Next up, the rear differential cover is removed and the fluid drained. Be sure to leave at least one of the top bolts partially threaded as this helps control the flow of gear oil out of the differential.
| The GM 11.5-inch axle, commonly referred to as either a 14-bolt or corporate 14-bolt, is a full-floating axle. Removing the axle shafts requires unscrewing the eight retaining bolts at the drive hubs.
| With the bolts removed, the axle shaft is free to slide out of the housing. Keep a rag handy as excess gear lube is likely to leak out of the axle housing.
| The carrier bearing caps inside the housing are mated to their respective sides. Because of this, they need to be returned to the correct side and in the same orientation that they are removed. One of the tricks that Frank uses is marking both the cap and the housing with a chisel. This ensures that there's no possible way for the bearing caps to get mixed up.
| After marking the bearing caps, they can then be removed. A high-quality impact wrench is a lifesaver in this process, as the bearing cap bolts are torqued to 200 lb-ft.
| The GM 11.5-inch axle features built-in preload adjusters for the carrier. Once the bearing caps are removed, the adjusters can be backed off and the carrier and ring gear removed. Extra care should be taken when removing the carrier as dropping it can be disastrous for your toes.
| Once the carrier is removed the pinion gear can be, as well. To remove the pinion, first the pinion retaining nut needs to be removed. With the nut removed, the yoke can be disengaged from the pinion splines by using a special puller.
| A special driver adapter and an air hammer are used to remove the pinion gear from the case. The crush washer used to set the bearing preload makes the gear difficult to remove otherwise.
| To make assembly of the carrier just a touch easier, Frank placed the Yukon Dura Grip differential into a freezer and the ring gear into an oven. While microscopic, cooling the carrier causes it to contract and heating the ring gear causes expansion. This tiny change makes a noticeable difference in installing the ring gear on the carrier.
| While the temperatures normalized on the ring gear and carrier, Frank installed the new carrier bearings included with the Yukon master install kit. Care must be exercised to ensure that the bearings are installed square.
| New ring gear bolts are also provided with the Yukon master install kit as these bolts aren't reusable. With the carrier supported in a vice, the new ring gear bolts get torqued to 120 lb-ft.
| While neither the factory pinion gear nor the pinion bearing is reused, Frank likes to start each install with the previously used pinion shim. This helps get the gear pattern set quicker in most cases. However, it requires a press to remove the pinion bearing to access the shim.
| Before the new parts are installed in the axle housing, James set to work cleaning out all of the old gunk and smoothing the cover gasket mating surface with a polishing disc. This needs to be done before installing the differential as to not contaminate the new parts.
| With the housing clean, the new pinion bearing races can be installed. Driving them into the housing requires a steady hand and a special punch.
| The new outer pinion gear bearing race is installed in the same fashion. It's important to replace the races and bearings as a set, as reusing old races with new bearings will cause failure to occur.
| It pays to have a second set of hands available when the time comes to install the new differential. Because of its size, one person can hold the differential in place while the other sets the bearing preload and installs the bearing caps.
| Even for a pro like Frank, who has set up thousands of gearsets, it's important to accurately measure the backlash. Simply put, this measurement indicates how tightly meshed the ring gear and pinion gear are. To increase the backlash, you'll tighten the right adjuster nut and loosen the left (which moves the ring gear way from the pinion), and to decrease the backlash you'll do the opposite.
| The pinion gear uses a crush sleeve to set its bearing preload. The amount of preload is measured in inch-pounds and is tested with the special tool, shown here. To increase the amount of preload, slowly tighten the pinion nut and recheck. If you've gone too far, the assembly will need to be removed and a new crush sleeve installed.
| After the backlash and bearing preload is set, both being good indicators that the gears are set up correctly, the gearset's contact pattern can be tested with yellow marking paste.
| If everything is set up correctly, you'll end up with a pattern that looks like the one pictured here. You want the paste to show that the ring and pinion gears are engaging each other in the center of the gear tooth and with a nice broad contact patch.
| Much like the high-quality impact wrench needed to remove the bearing cap bolts, an even better torque wrench is crucial to reinstalling them. Torqueing the bolts to the needed 200 lb-ft is a tough task for even the strongest shop help.
| While the factory axle shafts are adequate for normal use, our larger tires and more power warranted upgrading. We chose to use Yukon's cut-to-length 4340 chromemoly axle shafts. To get the new shafts to the correct length Frank busted out the trusty abrasive chop saw.
| After cutting the axle shafts, Frank used a belt sander to deburr the splines for a better engagement with the differential.
| A quick application of gasket maker on the hub flange helps to ensure leak-free service for years to come. Next, the new axle shaft can be inserted back into the axle housing and the eight retaining bolts tightened to 115 lb-ft.
| As part of the gear install, we also chose to try out one of the most talked-about new accessories of the past year, a Banks differential cover. We haven't had issues with any other type or brand of cover, this is just the life a project truck lives.
| Because the Yukon Dura Grip differential is a clutch-type limited-slip it requires the use of a friction modifier to reduce clutch chatter. Because of how aggressive this limited-slip is, Amsoil recommended the use of three tubes of the company's Slip Lock limited-slip additive.
| Before heading out on the first test drive, we filled the axle with four quarts of Amsoil 75W-90 Severe Gear oil. Packed in the company's new Easy Pack sack, the lube is easily squeezed into the differential's fill hole.