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Building a rearend and/or setting up a ring & pinion is one of those things that a lot of hot rodders leave to the professionals. Getting the pinion depth and gear pattern correct with all the preload adjustments, shims and what not, can be tricky, and in our case it’s always just been easier to pay someone that knows what they’re doing to accomplish it, and then stick it in the car after the fact. Think of it like painting—if you’ve tried to paint a car then you know what a tremendous pain in the neck it can be, and oftentimes the finished product turns out to be less than expected, even after all that work.
We went to GearFX’s shop in Mooresvile, North Carolina—right in the hot seat of NASCAR country—and got the full scoop on how they build a rearend. Shop Operations Manager Jeff “Fuzzy” Horton ran us through the entire process and said, “If it’s a manual transmission or a rear gear, we’ll tackle it” but also pointed out that the Ford 9-inch rear is the company’s bread and butter, representing a big chunk of the business at GearFX. “We’ve built 8.8s, rears for C5 and C6 Corvettes, Tremec transmissions, and did the rear gears for the Mazda Cup race cars, Spec Miatas primarily.” He also pointed out a few custom NASCAR 4-speed boxes used in various top levels of the game.
Horton said, “Building a 9-inch gear really isn’t all that difficult and a lot of guys can do it, but there are things that we’ve learned over the years of building championship-winning NASCAR drivelines that we apply to all of our street gears as well. We strive to ensure that anything that leaves here is top-notch and you won’t have to deal with any noise. We’re essentially blueprinting the gears.”
For this installation, we had GearFX set up a 9-inch center section with a Tru-Trac differential. Follow along below and let’s take a look at the process.
This nodular iron gear case is very popular when using a 9-inch rearend. GearFX uses new cases that come from original Ford molds.
The first step is pressing on the pinon’s pilot bushing. All Timken bearings are used in a GearFX setup, as well as ARP hardware. All good stuff right there, and if you’ve ever played with ARP fasteners, you know they’re like jewelry.
The pinion bearing gets a light coating of engine oil on the bearing seat, and then the bearing is pressed into place as seen here.
With the pinion and gear assembled, attention turns to the ring gear, which is a tight fit that needs to be pressed on. The ring gear is usually heated up in the steam cleaner to increase the clearances, and then pressed on using two or three dowel pins made of ring gear bolts that have had their heads removed. See those little studs? Those are the headless bolts. They make it easier to line the gear up, and are replaced with ARP bolts during final assembly.
Again, GearFX only uses ARP fasteners in their rearend setups, including the ring gear bolts.
Shop Foreman Rob Knipp torquing the ring gear bolts to spec.
The ring gear bolts are also safety-wired in place. This is typical race car stuff that most street guys don’t do, another advantage to having GearFX set a rear up for you.
With the ring gear, bearings, and safety-wired ARP bolts in place, this is a complete Truetrac rear differential setup ready for assembly into the whole package.
This is the pinion seal that also holds an oil slinger. It’s a thin metal washer that is used to “sling” the oil and keep it off the back of the seal.
The pinion seal gets a thin coating of Hylomar, which is similar to RTV sealant but has better adhesion, then is pressed into place.
The pinion bearing gets a light coal of oil before being pressed into place.
The pinion bearing is installed into place with a hydraulic press.
Here is the fully assembled pinion gear and ring gear with differential ready to go. Now they are ready to be mated together.
This handy bench-mounted tool holds the pinion yoke in place so that the nut can be tightened into place.
Red Loctite is used on the pinion nut with a little grease on the face of the nut to ensure the torque wrench gets a correct reading, and after it’s tightened, an inch-pound wrench is used to measure the rolling torque of the pinion.
Installing the pinion retainer.
U joint runout is an issue that is not often talked about. "We’ve had production yokes where when production tolerances stack up the wrong way, you can get .035-inch of runout, and that’s way too much," said Fuzzy. We try to keep it down to .004-inch runout with our gears.” This tool fits right where the U-joint rides and measures runout and is something you’ll only find with a GearFX setup.
This dial indicator is used to measure pinion depth, which is set to the manufacturer’s specs.
Metal shims are used to help set pinion depth.
The pinion yoke is always a NASCAR 1350 series billet unit.
With the pinion in place, the ring gear and diff are placed into the housing to set the gear pattern. Pattern is important and is a major factor in how much noise (and wear) you get with the overall gear setup.
To check the gear pattern, some yellow marking “paste” is used, and GearFX uses a handle to turn the setup that uses a 31-spline axle stub and turns the diff to see how the gears mesh together.
Final assembly includes a combination of red and blue (on the pinion retainer bolts) Loctite. A Daytona pinion support is used. Note the serial number located on the pinion support—every GearFX rear gets a serial number right here so that it can be quickly referenced in the computer.
The differential retainer bolts are torqued in place and then the backlash is checked to make sure it’s in spec.
With the center section ready to go, the last step is to run it on GearFX’s run-in dyno, which most of their competitors don’t have. The dyno runs the gears in and checks for noise and other issues, and every gear setup is run on the dyno before it ever leaves the door.
Every GearFX rear gets this build certificate that lists off the specs and clearances, as well as the people responsible for assembling it. This is handy reference material to have in your toolbox that provides a little more faith that it was all done correctly and by talented people.
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