2002 Ford Mustang GT 8.8 Rearend Rebuild - Rearend Revival
Sometimes Things That Go Clunk Can't Be Right
From the April, 2011 issue of 5.0 Mustang & Super Fords
By KJ Jones
Photography by KJ Jones
GTR's Chris Balster sprays...
GTR's Chris Balster sprays a blast of parts wash through our '02 GT's 8.8 rearend housing. Thoroughly cleaning the housing is a mandatory prep move for differential and gear swaps, as any debris in the housing could potentially damage a new differential or, if big enough, become lodged in the gears. Using fine-grit sandpaper and solvent to remove metal particles from seal surfaces is also a good idea.
While a 'Stangbanger's ultimate dream to have a super-modified car that remains absolutely perfect and free of bad noises forever, the truth is, horsepower and torque have a unique ability of finding the weak link in a Mustang. There's always a chance that a bad-noise-emitting problem could arise at some point, especially as more performance modifications are made.
Noises go hand-in-hand with the Mustang hobby. There are beautiful noises, such as the sounds that come from fine-tuned 'Stang engines at full scream on the chassis dyno or dragstrip. Then there are funny noises, like belt squeals, chassis creaks, dash rattles, and such. Finally, there are bad noises that are usually signs of serious problems with a Pony's engine or drivetrain, brought about by a lack of maintenance or an abundance of power.
Now before we go further with this report, we're not saying this to scare you or deter you from making moves that will put your Pony on the next plateau of performance. Rather, we encourage you to make those moves. As many of you know, making big steam is a favorite pastime for your tech editor and the crew at 5.0 Mustang & Super Fords. However, each time we increase the power output of the Ponies we work with, we're doing so with the understanding that we're taking the cars beyond the safe haven of stock.
Notice the aftermarket differential...
Notice the aftermarket differential cover that Chris is removing from the 'Stang's rear? The cover topped off an installation of 3.90 gears and a Detroit Truetrac limited-slip differential, which was done in 2008.
A loud clunking noise is our...
A loud clunking noise is our reason for taking this new look inside the rear.
Here is the carnage-two broken...
Here is the carnage-two broken teeth on the 3.90's ring gear. Without performing ultra-scientific metallurgic tests on the gearset, and without being aware of any abnormal stresses or loads being put on the rearend, it's difficult to place blame on anything for this particular type of failure. Sometimes the saying "it is what it is" works well when it comes to problems we encounter. This is one of those times.
With that said, bad noises are inevitable when you push closer and closer to the edge of the high-performance envelope. We've heard the sounds of catastrophic failure a few times now. We also understand that when parts do fail and you start hearing bad noises, sometimes you just have to charge the bad fortune to the game, identify the problem, make necessary repairs, and move on.
That's what we're doing with this tech effort-getting to the bottom of a disturbing hollow-sounding, clunk-type noise that's coming from the 8.8 rearend in our '02 Mustang GT. We first encountered the racket immediately after making our final pass at a dragstrip test session. Clunking and a severe vibration in the drivetrain (that increased and decreased with wheel speed) made their presence known during our drive back to our pit.
With the problem properly...
With the problem properly diagnosed, we decided to step up the GT's rearend game a bit and install a set of Ford Racing Performance Parts 4.30 gears (PN M-4209-G430M; $199.99), foregoing the Truetrac for a full-on, 31-spline Detroit Locker unit (PN 187C-145A DL FORD 8.8; 649.95). While 3.90s definitely are ideal from a best-of-both-worlds standpoint-and served our supercharged Pony well on the street, freeway/Mojave Mile and dragstrip-the lower ratio will bring about a tremendous improvement in the car's First-through-Fifth-gear acceleration. Royal Purple's Max-Gear 75W-140 gear oil rounds out the simple collection of parts.
Here is the FRPP ring-and-pinion...
Here is the FRPP ring-and-pinion install kit (PN M-4210-B; $90) that is needed for this type of project. The kit includes pinion and carrier shims, a crush sleeve, pinion seal and nut, new ring-gear bolts, carrier bearings, a pinion-gear bearing and a fresh gasket for the diff cover.
A rearend upgrade can be performed...
A rearend upgrade can be performed by do-it-yourselfers who have patience, good attention to details and are confident in their ability to take precise measurements. A robust work area also is helpful, as such tools a hydraulic press, dial indicator, digital micrometer, lb-ft torque wrenches, and in-lb torque wrenches are required. For anyone taking on this challenge, we strongly suggest you test fit axles in a limited-slip differential, locker, or spool before going any further with the job, to ensure that splines on both are a perfect match.
Right away, checks were made to isolate where the noise originated. After securing the 'Stang on four jackstands, we inspected the undercarriage to see if the noise actually was the result of the driveshaft making contact with the floorpan, torque arm or exhaust. After confirming that the shaft was OK, we checked out the differential (by engaging the transmission in First gear and turning the drivetrain) and found the noise was clearly coming from inside the 8.8.
As a general rule, if axles aren't broken and you're experiencing a bad noise from inside the 'Stang's rearend (but the car is still driveable, like ours), a broken ring or pinion gear, or a broken component in the differential usually is the source of the noise. With the malady requiring a more extensive investigation, we delivered the Mustang to GTR High Performance in Rancho Cucamonga, California, where technician Chris Balster found the source of our distressed diff's problem and performed the surgery that will make things right.
Photos of the search-and-rebuild process are telling, so we suggest you stay with this one and see what we've done to ward off future situations for our supercharged 'Stang's rearend.
New bearings are pressed on...
New bearings are pressed on each end of the locker, before Chris trial fits the unit in the 8.8 housing. A press facilitates this task, as does using one of the old bearings (between the new one and the press), to serve as a buffer and prevent any damage from happening to the new bearings.
The pinion-bearing race is...
The pinion-bearing race is a simple part, but it requires the correct tool and lots of care for proper installation. You don't want to damage this piece and banging it in with a hammer can prevent the race from bonding with the bore. Depending on their diameter, depth shims for the pinion gear are placed either below the race or on the pinion shaft itself (under the bearing).
is secured to the ring gear,...
is secured to the ring gear, to ensure the gear remains tight on the locker when it's go time.
To confirm backlash is good,...
To confirm backlash is good, Chris coats the ring gear with gear-marking paint in two separate areas, and then rotates the ring past the pinion a few times to establish a wear pattern. Backlash is the amount of clearance between the pinion and ring gear, and the source of the roaring type of sound that is sometimes heard coming from the rearend during deceleration (too much backlash).
Note the position of the pattern...
Note the position of the pattern between the face and flank of the teeth. Since the pattern is centered from face (the top land of the tooth) to flank (the base of the tooth) on the Drive side, the pinion depth for our rear is good to go. If the contact pattern had been more toward the face of the teeth on the ring gear, the pinion would have to be moved closer toward the ring-gear's centerline by adding pinion shims. Conversely, if we had seen a pattern that was closer to the flanks of the marked teeth, shims would have to be adjusted so the pinion is moved away from the ring gear's centerline.
A great deal of trial-and-error...
A great deal of trial-and-error goes into building and dialing-in a fresh 8.8. Our Detroit Locker requires side shims before it can be final-installed. Determining correct shim sizes (thickness) takes a while because the locker must be installed and removed several times. However, nailing the perfect shim combination allows Chris to determine the amount of backlash and preload is necessary to absorb the carrier's side-to-side movement on the ring gear. A good, tight fit in the housing is important for any differential.
Setting pinion-gear depth...
Setting pinion-gear depth is another test-fit (using a test bearing), adjust, and refit exercise that culminates with a new rear pinion bearing being pressed onto the pinion shaft. Chris initially tries shims of various diameters, before settling on 0.030 inch as the correct size for proper pinion depth.
This is the front pinion bearing...
This is the front pinion bearing and seal package. A shim is placed between these pieces, and serves as a slinger, of sorts, that meters the amount of gear oil that actually reaches the seal. Chris recommends using gear oil on all of the bearings and grease for the seals in a freshly built rearend.
Each bearing cap is marked...
Each bearing cap is marked with a set of arrows that must point toward the outside of the rearend housing...
...Once caps are installed,...
...Once caps are installed, Chris tightens each fastener with 60 lb-ft of torque.
Pinion-bearing preload is...
Pinion-bearing preload is a measure of the pinion bearings' rolling resistance, once the pinion nut is fully torqued. Chris sets preload carefully at 15 in-lb.
Having the correct amount...
Having the correct amount of preload on the carrier bearings prevents the locker from moving-and thus increasing backlash on the gears-when our 'Stang is flogged. Before calling this project done, Chris checks backlash by mounting a dial indicator on the rear and setting the plunger perpendicular to the teeth on the ring gear, then rotating the ring gear back and forth for measurements. As a rule of thumb, backlash changes roughly 0.007 inch for each 0.010 inch that a differential is moved, either closer to (decreases) or farther away from (increases) the pinion gear's centerline. Chris says from 0.008 to 0.012 inch is OK, but new gears like our 4.30s should be set up more on the tight side, so the backlash will still be within that range after a 100-mile break-in period.
Stock axles just don't cut...
Stock axles just don't cut it when you start making real steam with your Mustang. Our Pony wears Strange Engineering's SS Series C-Clip (31-spline) street axles, which, despite actually having C-clips, are the same induction-hardened, forged axles as other axles in Strange's S-series. We selected these axles to maintain our 'Stang's OEM-style rear braking hardware.
Chris pours Royal Purple 75W-140...
Chris pours Royal Purple 75W-140 gear oil into a gallon-size feeder container, from which the fluid is pumped into the differential. Royal Purple is plenty slick, helps keep things quiet in the housing, and its Synslide additive protects the internal pieces of clutch- and cone-style differentials, if you're using one of those instead of a locker.
It don't mean a thing if it...
It don't mean a thing if it ain't got that bling, right? After installing a new gasket (which is supplied), Chris put UPR's beautiful, new billet differential cover over our Pony's updated gearset and locker. The stout, polished lid features bearing supports that keep caps in place, and help maintain pinion depth and backlash.
Truetrac VS. Locker Q&A
We've used a limited-slip Detroit Truetrac differential in our ProCharger F-1C-supercharged, '02 Mustang GT, and now have a freshly installed Detroit Locker in the 'Stang's 8.8 rearend. However, before we ever installed either unit, a lot of thought was put into the short- and long-term performance goals that we had for the project car. Remember-the car is street driven, and despite the array of race superchargers we have installed, our intent is to maintain good street manners and driveability.
This project has brought about several questions about Detroit's Truetrac and Locker differentials that we think many of you are asking. Here is a quick Q&A that hopefully will give you good insights about both units, and help you make the right selection for your Pony's rearend when it's time to upgrade.
Q: When does the Truetrac lock up?
The Detroit Truetrac is a...
The Detroit Truetrac is a fully automatic, limited-slip differential, which basically performs as an "open" differential until it is needed. The Truetrac's helical-gear design eliminates the need for wearable parts and makes this unit durable, yet smooth and relatively quiet as well.
The Truetrac is a helical-gear, limited-slip differential and never "locks up." The Truetrac operates by transferring power from the spinning wheel to the wheel with the most traction. If one tire breaks traction, the amount of rotation (or spin) is controlled. The torque is then sent to the other tire that still has traction.
Q: I use my car on the street and on the strip. Will the Truetrac hold up?
A: On the street, the operation of the Truetrac is transparent. You will not know it is there until you need it. In racing situations the unit will apply torque to both tires and consistently launch straight.
Q: How much horsepower and torque will the Truetrac take?
A: There are many factors to consider when determining if a specific differential will last in your 'Stang. Horsepower and torque are important factors. However, the weight of the car, tire height/width, the terrain, and suspension are all important things to take into consideration.
Q: When does a Detroit Locker lock?
In contrast to the Truetrac,...
In contrast to the Truetrac, the Detroit Locker is always locked and ready to deliver every available lb-ft of torque to the rear wheels. The locker is designed to keep both wheels in a constant drive mode, but it does automatically allow for wheel-speed variances that occur when turning.
The Detroit Locker is fully locked up when driving in a straight line or if the tires are spinning. The unit unlocks in a turn for the wheel that is turning the fastest.
Q: When I step on and off the gas pedal, I hear the unit make a clunking noise. Why?
A: Detroit Lockers have backlash between the drive and driven teeth, and you will hear clunking usually when going through corners and when going from drive to coast mode. Also, with the 'Stang on the ground and the transmission in neutral you will have 1/4 to 1/3 of a turn of lash in the driveshaft. This is completely normal.
Q: Can I run a Detroit Locker on the street?
A: Many people do use the Detroit Locker during everyday street use. The Detroit Locker is an aggressive differential, so you will hear it and feel it every day on the street.
Q: How much horsepower and torque will the Detroit Locker take?
A: There are many factors to consider when determining if a specific differential will last in your vehicle. Horsepower and torque are important factors. However, the weight of the car, tire height/width, the terrain and suspension are all important things to take into consideration. For example, a setup that uses street tires will live longer because both tires will break loose and spin before any real strain is put on the unit. But as the tire gets wider, the stress on the unit increases because the traction is greater.