Adding bigger tires to a 4x4 is a rite of passage that we all go through in our early days of four-wheeling. Most of us of are of the mentality that you can never have too much of a good thing, like ice cream, bacon, and of course, big gnarly tires! The more rubber between your wheels and the trail, the more places your rig can conquer off the beaten path. While it sounds like a great idea, it's not without some consequences, the biggest of which will be having enough power to make your rig move out of its own way with the parasitic loss of larger tires. The quickest way to gain that lost power back is to upgrade the gears on your rig to a lower ratio (numerically higher) gear-set.
Your new gears should be chosen based on the size of the tires you'll be using, as well as the type of wheeling you'll be encountering and whether or not you will trailer your 4x4 to the trailhead or daily drive it on the highway. Once you have done your homework on what the proper gear ratio your 4x4 will need, you'll need to decide whether you want to try your hand at setting up your own gears or taking it to a knowledgeable installer to make sure the job is done right.
Installing gears isn't quite as easy as most people might think. It's the little details that can mean the difference between a long healthy gear life and a trip home from the trail on a tow truck. We've decided to show you some of the finer points of a proper gear installation for a stronger and longer-lived gear-set in a high-torque application that routinely sees gnarly off-road conditions, along with brutal daily driving duties in heavy traffic conditions.
Our 2003 Ford F-350 Super Duty is a long-term project truck from our old sister publication, 4-Wheel & Off-Road magazine. This 1-ton, 6.0L Power Stroke diesel truck has seen more than its fair share of abuse over the years, but after our last desert outing, old Big and Purple decided to develop a slight vibration in the rear end while flat towing a Baja Bug out to the desert. We knew something was definitely wrong when we pulled the magnetic dipstick on our rear Mag-Hytec differential cover and were greeted by a chunk of ring gear tooth. Of course we didn't let it spoil our weekend, so we pushed on the rest of the way and drove it back home in front wheel drive after pulling the rear axle shafts and driveshaft.
We decided that the funky combination of 5.13 gears meant for a Sterling 10.25 in our Sterling 10.5 might have been a little more than we needed for a truck with 40-inch tires and gobs of low-end torque, so we ordered up a brand-new set of Yukon Gear and Axle's 4.88 gears and master overhaul kits for both the front Dana 60 and the Sterling 10.5 rear in our high-mileage Super Duty.
When it comes to properly installing gears on our trucks, we always go to the experts. South Bay Truck and 4x4 has done countless gear installs for us in the past that are holding up strong to this day, so we couldn't think of anyone better than them to share with us the secrets to properly setting up gears that will last for a long time in a high-powered, highly abused 4x4. Check out all the preload, pinion depth, and backlash wisdom that they shared with us to make your next set of gears stay in it for the long haul.
Frank Gilliland and his brother Charlie of South Bay Truck and 4x4 had our Super Duty up on the rack in no time at all, and within minutes they had pulled our axleshafts and rear diff cover to determine the culprit of our gear disaster.
With the differential cover pulled on our Sterling 10.5 rear end, we could clearly see a couple of broken teeth on the ring gear. These gears were just over three years old when they gave up the ghost, but Frank and Charlie quickly set us straight on how this happened. The last time the gears were done on this truck, the carrier preload was not adjusted properly and ended up resulting in a catastrophic failure after the carrier bearings spun inside the housing. The loose carrier sent our backlash out of spec, and while going down the highway, the structural integrity of the gear mesh pattern was compromised. Long story short? Gear teeth go bang.
While the Dana 60 front axle in our Super Duty didn’t fail catastrophically, it did howl quite a bit while limping the truck back home for 160 miles in front-wheel drive. Once Charlie pulled the differential cover off the front axle, it was clear that the gears were starting to exhibit a little knifing due to an improper carrier preload. Do you see a pattern?
The Yukon Gear and Axle’s Dana 60 and Sterling 10.5 4.88 gears and Master Overhaul’s kits should be the long-term solution to our gear issues once properly installed. While you can install a set of 5.13 gears in a Sterling 10.5 by using a Sterling 10.25 ring-and-pinion with a taller pinion bearing, we decided on using 4.88 gears instead since we do a lot of highway driving with this truck to get to the trailhead and, with all the low-end torque of the 6.0L Power Stroke, we really weren’t hurting for power.
The new Yukon Gear and Axle ring gear was dropped onto our ARB Air Locker, and once we had the ring gear properly positioned on the carrier, we gave each of the new ring gear bolts a few drops of Loctite before torqueing them down to the specified 95 ft-lb of torque. Make sure you use a brake parts cleaner to remove any protective coating or oily residue that the ring and pinion might have on its surface.
Frank and Charlie tell us that it’s a good idea to make a note of the shim stack taken out with the old gearset. Make sure you save your old pinion nut so you don’t have to use the new pinion nut, which really should only be used during final assembly. It’s important to note that the crush sleeve should also be omitted until final assembly so you don’t run the risk of accidentally ruining it.
Here you can see the relationship between the ring-and-pinion gear and how the pinion depth can be changed by either adding more shims to the pinion to set the depth deeper or removing shims to set it shallower. Keep in mind that on a Ford 9-inch axle, adding shims to the pinion support will result in a shallow pinion depth, while removing shims will move it in deeper. The first thing to do is set up your pinion depth so you can set the pinion preload. Typically the factory will call for a pinion depth of 0.025-inch, so it’s a good rule of thumb to use that as your starting point. Next you can install the pinion in the housing and tighten the old pinion nut. Frank recommends installing the pinion without the crush sleeve and tightening the old pinion nut until you feel a bit of drag on the pinion. You should be able to turn it by hand but not enough to where the pinion is wobbling in the housing. The key thing to remember is that a snug fit is good. What this will do is allow you to get 20-35 in-lb of preload
Once the proper preload has been achieved, the carrier is ready to be installed in the housing. Since our Super Duty was already equipped with ARB Air Lockers, we installed new seals in the seal housing and installed it onto our ARB Air Locker.
This is a crucial step in the setup process, and if you’re not careful, you could be in for a catastrophic failure down the road. You want to make sure you have some amount of backlash so that the gears aren’t forcing against one another but have it close enough that you can achieve the proper setting. The carrier should not drop in with a loose fit. It should take a little bit of effort with a brass drift and a hammer to achieve a snug fit. This doesn’t mean you should hammer down on it like there’s no tomorrow. A snug fit is what you’re after. Once you get the carrier in, you can install the carrier caps and torque them down to 80 ft-lb of torque.
The next step is to install a dial indicator tool so you can accurately check the amount of backlash between the ring-and-pinion. Rotate the ring gear until you meet a bit of resistance and then go ahead and zero out your dial indicator so you can check the amount of backlash.
You want to aim for a backlash of 0.006-0.010 inches and then apply the marking compound to the ring gear in a few spots so you can check the mesh pattern. You want the pattern to be smooth and even on both the drive and coast side, preferably in the center of the tooth. The outside edge of the ring gear is known as the heel, while the inside edge is known as the toe. A pattern that is biased toward the heel means your ring gear needs to be moved closer to the pinion, while a toe-biased pattern means the ring gear needs to be moved away from the pinion. A high contact pattern means your pinion depth needs to be increased while a low to light contact pattern means you need to move the pinion away from the ring gear by decreasing the shim thickness.
Once you have achieved the proper 0.006-0.010-inch of backlash, as well as an acceptable gear mesh pattern, you are ready to pull everything back apart and install your crush sleeve and new pinion nut. In our case, our backlash came in at 0.008-inch, which is ideal for a strong healthy gear mesh. It’s a good idea to add a few drops of Loctite onto the threads of the pinion, as well as the new pinion nut. Make sure you use a reliable 1/2-inch pneumatic impact gun and stop to check the pinion preload often or you’ll end up going too far and ruin the crush sleeve. The pinion preload should be set at 0.020-0.035 in-lb of torque on new bearings such as the ones we used. Frank recommends aiming for 0.020-0.025 for a long lasting gear life.
Frank then installed our Mag-Hytec finned aluminum differential cover. We’ve come to really love this differential cover since it offers a larger capacity for oil to help dissipate any heat buildup from hardcore use, whether wheeling or towing.
We topped off both the front and rear differentials with synthetic gear oil that helps keep the bearings working at cooler temperatures, even in demanding environments.
One of the added benefits of the Mag-Hytec finned aluminum differential cover is that it gives us both a drain plug and a magnetic filler dipstick that allows us to quickly check the oil without spilling a drop.
We’ve now logged just over 3,000 miles on our new gears and we’re glad to say they’re working smooth and quietly. We even drove our project Super Duty all the way out to Moab earlier this year for the 2015 Easter Jeep Safari, where we got a chance to wheel the truck on the Fullsize Invasion run. We still have plenty of torque at low rpms, but highway cruising is a lot more civil now thanks to the fact we’re not revving our 6.0L Power Stroke to the moon while doing 80 mph down the highway.