If you've been following the build of our 1998 Cobra, you have an idea of what we've been trying to accomplish. It's been driven all across the state of Florida, ringing in over 26,000 miles this past year, and is used everyday. Our Snake has visited the likes of Bradenton Motorsports Park for some quarter-mile action, and graced the asphalt of Sebring International Raceway's famous road-course. To sum it up, we've been building a Stang that's capable of handling just about anything.
This past November, after returning from Palm Beach International Raceway and its road course for some suspension tuning ("Snake Handler," Feb. '14), we noticed a wretched whining sound coming from the almost-stock 8.8-inch rearend. Our Cobra featured 4.30 gears, installed prior to purchasing, and the odometer had just rolled over 100,000 miles.
After removing the differential cover, it was clear what was wrong. Several carrier shims had backed out and were damaged, leaving the differential vulnerable to exploding. With our Cobra disabled, we decided to trailer it to Blow-by Racing (BBR) for an overhaul.
Pictured here are all the parts needed to upgrade our 8.8-inch rearend, including a 31-spline Yukon Dura Grip differential, Moser axles, UPR Products differential cover, and FRPP 4.30 gear set and installation kit.
We decided to stick with a Traction-Lok–style differential. We needed something stronger than stock that could handle high-rpm launches and aggressive cornering, while also offering smooth driveability. BBR owner Chris Jones suggested a full 8.8 rebuild, starting with a 31-spline Yukon Dura Grip differential (PN YDGF-88-31-1; $449.95).
"The Yukon Dura Grip is a differential strong enough for track use, yet drives like stock around town," Jones explained. "It features a nodular case with composite clutches. Each unit is also completely rebuildable and comes with a one-year warranty."
From '79-'04, Ford equipped Mustangs with 28-spline axles, excluding the '03-'04 Cobras. Since our Cobra visits the 1,320 several times a year, as well as the road course, we wanted something stronger than stock. BBR set us up with a pair of Moser Engineering 31-spline axles (PN A883153; $275). Each axle is made from forged steel and is non-tapered, featuring hardened and ground bearing seats, and machined C-clip grooves. Replacement C-clip axles are available in lengths up to 37 inches. If you plan to narrow your rearend in the future, these forged axles can also be shortened and re-splined.
With our Ford Racing Performance Parts 8.8 installation kit (PN M-4210-C3) in hand, BBR Technician Rob Vargo got to work. Now, follow along as we show you how to beef up your 8.8-inch rearend and make it track ready.
1. To begin, BBR technician Rob Vargo removed the rear wheels, followed by the rear brakes.
2. He then drained the fluid from the rearend.
3. After removing the differential cover, several carrier shims had backed out and were broken. We could rock the differential back and forth with ease. This caused a horrible whining sound and was not safe to drive.
4. To remove the differential, Vargo removed the lock screw from the pinion shaft. Next, he removed the C-clips holding each axle in place.
5. BBR owner Chris Jones then removed both axles.
6. (Editor’s note: Before removing the differential, it’s important to check backlash. Backlash is the clearance between a pinion tooth and the adjacent ring gear teeth. It is the total movement allowed between contacting teeth. Ford recommends setting backlash between 0.008 to 0.012 inches. Also, record the carrier shim thickness relative to each side. This will give you a good starting point once you install your new differential. Our differential was moving around, so we skipped this step since we would be starting from scratch. Rob removed the bearing caps to remove the differential.
7. Here you can see our damaged carrier shims.
8. Vargo then removed the pinion flange and nut, followed by the pinion.
9.He installed the new Ford Racing Performance Parts pinion and axle bearings and seals.
10. Here’s a look at the new Moser Engineering 31-spline axles compared to the stock pieces.
11. Vargo used a Ford pinion-depth tool to set proper pinion depth.
12. Our ’98 Cobra previously had a 4.30 gear set installed. Due to the damage from the misaligned differential, our old set was trash. We had an FRPP 4.30 gear set sitting on our parts shelf, so we decided to install it. (Editor’s note: FRPP no longer sells a 4.30 gear set.)
13. Using a machine press, Vargo installed the pinion bearing onto the new pinion.
14. The Yukon Dura Grip differential (PN YDGF-88-31-1, $449.95) features a nodular case and stronger internals with composite clutches. Each unit is completely rebuildable and comes with a one-year warranty. According to Jones, this differential is perfect for the street, and the occasional dragstrip and road course enthusiast.
15. Here, Vargo pressed on the new carrier bearings.
16. He then installed the new ring gear onto the differential.
17. If you’re reinstalling your differential, now is a good time to reuse the shims from when you removed it. If you’re starting from scratch like us, we had to measure and shim accordingly based on trial and error. Pictured here, Vargo measured the shims and installed the new differential, along with the shims and new crush sleeves.
18. Using a dial indicator, Vargo then checked the ring-gear backlash. According to FRPP’s instructions, to correct for high or low backlash, increase the thickness of one differential bearing shim and decrease the thickness of the other differential bearing shim by the same amount.
19. The FRPP kit includes a marking compound. Complete at least two ring gear revolutions to test for an appropriate wear pattern. A desirable ring-gear-tooth pattern should have a well-centered pattern on both the drive and coast pattern side, have clearance between the pattern and the top of the tooth, and no hard lines where the pressure is high.
20. Vargo then re-installed the axles, followed by the brakes and wheels.
21. UPR supplied us with a new billet differential cover (PN 3024-07, $139.99), which Rob installed.
22. To wrap it up, Vargo filled our freshened 8.8 rearend with 3 quarts of Amsoil 75W-140 gear oil and Ford Racing friction modifier. Our Snake is now back on the road and anxious to revisit the track. Stay tuned.