Introduction: Upgrading Your Mustang GT with a Detroit Locker Differential

The Ford Mustang GT is a legendary pony car known for its potent V8 power and rear-wheel-drive layout. However, even the most powerful Mustang can struggle for grip when one tire loses traction, especially during hard launches or aggressive cornering. The open differential that comes standard on many GTs allows power to go to the wheel with the least resistance, meaning your engine’s torque is wasted on a spinning tire instead of pushing the car forward. A limited-slip or locking differential solves this by transferring torque to the wheel with more grip. Among the most rugged solutions is the Detroit Locker Differential, a mechanical automatic locker that provides 100% lock-up when torque is applied, delivering maximum traction for drag racing, autocross, or spirited driving on paved roads. This guide walks you through the entire installation process step-by-step, from removal of the factory differential to final test drive, ensuring your Mustang GT gets the traction upgrade it deserves.

Tools and Materials Needed

Before you begin, gathering the right tools and parts will save you time and prevent mid-project trips to the auto parts store. While the Detroit Locker kit includes the locker unit and some hardware, you’ll need additional items for a complete and safe installation.

  • Detroit Locker Differential Kit – Choose the correct spline count and gear ratio for your Ford Mustang GT (typically 28 or 31 spline for 8.8-inch axles; confirm with your vehicle year).
  • Socket and Ratchet Set – Metric and SAE sockets; 1/2-inch drive for large bolts.
  • Torque Wrench – A beam or click-type wrench that reads in ft-lbs (range 10–250 ft-lbs).
  • Jack and Jack Stands – A hydraulic jack (3-ton rating recommended) and two rear jack stands (minimum 6-ton capacity for safety).
  • Brake Parts Cleaner – Non-chlorinated aerosol for degreasing the housing.
  • High-Quality Gear Oil – GL-5 rated, 75W-90 or 75W-140 synthetic. For limited-slip usage, add the appropriate friction modifier unless the oil is already formulated for limited-slip (Detroit Locker does not require friction modifier, but many gear oils already contain it).
  • Shop Towels or Rags – For cleanup and leak detection.
  • Safety Glasses and Gloves – Always protect your eyes and hands when dealing with gear oil and sharp metal parts.
  • Pry Bar or Slide Hammer (optional) – For removing stubborn axle shafts.
  • Differential Cover Gasket – Often included with the locker kit; verify fitment for Ford 8.8-inch rear end.
  • Pneumatic or Electric Impact Wrench (optional) – Speeds up removal of stubborn bolts.
  • Dial Indicator with Magnetic Base (for setting backlash, if adjusting ring gear position).

Preparation and Safety

Proper preparation ensures a clean, safe workspace and prevents accidental damage to your Mustang. Follow these steps before lifting the vehicle:

  1. Park on a Level Surface – Concrete or asphalt is ideal. Avoid gravel or dirt which can shift under jack stands.
  2. Engage the Parking Brake – Pull the parking brake lever firmly. Chock the front wheels to prevent any movement.
  3. Disconnect the Battery – Remove the negative terminal. This prevents the SRS (airbag) system from accidentally deploying and protects the ECU from electrical spikes during welding or heavy wrenching.
  4. Allow the Vehicle to Cool – The exhaust and differential housing can be hot after driving. Let the car cool for at least an hour.
  5. Drain the Differential Fluid – If possible, drain the gear oil before lifting. Removing the drain plug (located at the bottom of the cover) allows gravity to empty the housing while the car is still low.

Removing the Old Differential

This section assumes you are working on a Ford Mustang GT with an 8.8-inch rear axle (common from 1986–2014). Later models may have independent rear suspension (IRS), which requires a different procedure. This guide focuses on the solid-axle version. Always refer to your vehicle-specific service manual for additional steps.

Lifting and Access

  1. Loosen the rear wheel lug nuts while the car is still on the ground.
  2. Jack up the rear of the Mustang using the rear differential or a dedicated reinforced frame rail. Place jack stands under the factory lift points (rear frame rails on each side). Lower the jack until the stands support the weight.
  3. Remove the rear wheels to gain clear access to the differential cover and axle shafts.

Removing the Axle Shafts

  1. Remove the brake caliper and rotor: Unbolt the two caliper guide pins, hang the caliper out of the way using a zip tie or bungee cord (do not let it hang by the brake hose). Slide the rotor off the axle studs.
  2. Unbolt the four retaining bolts from the axle shaft flange (inside the differential housing). These bolts secure the axle bearing retainer.
  3. Pull the axle shaft straight out. Use a pry bar or a slide hammer if necessary. Set the axles aside in a clean area.

Removing the Driveshaft

  1. Mark the orientation of the driveshaft to the differential flange (some are balanced to the yoke). Use a paint pen or punch.
  2. Unbolt the four nuts or bolts from the driveshaft to the differential pinion flange. Keep the hardware together.
  3. Slide the driveshaft forward toward the transmission. You may need to lower the tail shaft slightly. Support the driveshaft with a strap or wire so it doesn't hang heavily on the transmission output shaft.
  4. Place the driveshaft out of the way.

Removing the Differential Cover and Carrier

  1. Place a drain pan under the differential. Remove all cover bolts. Carefully pry the cover loose; residual oil will spill. Remove the cover and gasket.
  2. Inspect the interior. Take note of any shims or spacers that are present (often on the left and right side bearing caps).
  3. Remove the four bearing cap bolts (two per side) using a large socket or breaker bar. These bolts are torqued to 85–100 ft-lbs. Mark the caps' orientation (driver/passenger, left/right) so they go back in the same position.
  4. Using a differential spreader (or careful prying), lift the carrier (housing with ring gear and pinion) out of the axle housing. Avoid damaging the shims.
  5. Remove the ring gear bolts (typically 8 or 10 bolts) and separate the ring gear from the old differential. You will reuse the ring gear on the Detroit Locker.
  6. Clean the ring gear mating surface and inspect it for wear, cracks, or damaged teeth. Replace if necessary.

Installing the Detroit Locker Differential

Now the fun begins. The Detroit Locker is a drop-in replacement for the open or limited-slip differential, but attention to detail is critical for long life and quiet operation.

Preparing the Ring Gear

  1. Thoroughly clean the ring gear and the mounting surface on the Detroit Locker using brake cleaner and a lint-free cloth.
  2. Apply a small amount of thread locker (Loctite 242 or equivalent) to the ring gear bolts. Torque to 70–90 ft-lbs (check the bolt specifications; generally 7/16-inch bolts require around 85 ft-lbs). Tighten in a cross pattern.
  3. If your Detroit Locker includes a side shim pack, compare the overall width of the new assembly to the old one. You may need to reuse the original side shims to maintain the correct backlash. Measure the old carrier width and shim thicknesses, then transfer to the new locker.

Setting Backlash and Bearing Preload

This step is omitted in many basic guides, but it is essential for proper ring-and-pinion mesh. If you are not comfortable with this, have a professional shop set up the gear backlash.

  1. Install the bearing cups onto the Detroit Locker's side bearings (they often come pre-installed). Apply a thin layer of gear lube to the bearings.
  2. Place the carrier assembly (with ring gear attached) into the axle housing. Install the bearing caps loosely.
  3. Using a dial indicator placed against a ring gear tooth, check the backlash. Specification for Ford 8.8-inch is typically 0.008–0.012 inches. Adjust by swapping side shims (if adjustable) or by choosing different shims from the kit.
  4. Once backlash is within spec, torque the bearing caps to the manufacturer’s specification (typically 85–100 ft-lbs). Recheck backlash after torquing.
  5. Check the contact pattern by painting five or six ring gear teeth with marking compound. Rotate the pinion gear and inspect the pattern. Adjust accordingly.

Final Installation Steps

  1. Clean the differential housing sealing surface with brake cleaner. Apply a thin bead of RTV silicone gasket maker (if not using a pre-formed gasket) around the cover.
  2. Install the differential cover. Torque the bolts in a crisscross pattern to 25–35 ft-lbs.
  3. Reinstall the axle shafts: Slide each axle into the housing, align the splines, and secure with the four retaining bolts. Torque to 30–40 ft-lbs.
  4. Reattach the brakes: Slide the rotor over the studs, install the caliper, and torque the guide pins to specification (usually 25–30 ft-lbs).
  5. Reinstall the driveshaft: Align the marks you made earlier, install the bolts, and torque to 35–45 ft-lbs (check your Mustang’s service manual for exact value).
  6. Refill the differential with gear oil through the fill hole on the cover. The 8.8-inch rear end holds approximately 1.5–1.8 quarts. Fill until oil begins to weep out of the fill hole. Do not use friction modifier; the Detroit Locker is a mechanical locker and does not require it.
  7. Reinstall the fill plug and tighten it.
  8. Reinstall the rear wheels and lower the vehicle.

Final Checks and Test Drive

After the physical installation is complete, perform these checks before starting the engine:

  1. Reconnect the battery – Attach the negative terminal and tighten it.
  2. Inspect for leaks – Look around the differential cover, axle tubes, and pinion seal. If you see oil, check torque of cover bolts and fill plug.
  3. Check wheel rotation – With the vehicle on jack stands, rotate both rear wheels by hand. They should turn smoothly but with slight resistance (from the locker's internal springs). Ensure they both rotate in the same direction when turning the driveshaft.
  4. Start the engine – Let it idle for a few minutes. Shift into 1st gear and gradually apply throttle in a controlled environment. Listen for unusual whining, grinding, or clunking from the rear end.
  5. Test drive – Take the Mustang for a short drive at low speeds (under 30 mph). Perform gentle turns and listen for clicking or chattering. The Detroit Locker will make a slight clicking sound in tight, slow turns; that is normal. If you hear severe grinding or constant noise, stop and recheck gear mesh and bolt torque.
  6. Re-torque after 500 miles – After a break-in period, it’s wise to drain the gear oil, inspect for metal particles, and re-torque the ring gear bolts and bearing caps.

Maintenance and Performance Tips

  • Oil Changes – Change the differential fluid every 30,000 miles or after any track event. The Detroit Locker’s internal components rely on clean lubricant to operate smoothly.
  • Driving Behavior – Remember that the locker is always engaged under power. In rain or on slippery surfaces, the rear end may feel unpredictable. Use caution until you adapt to the traction characteristics.
  • Upgrade Your Axle Shafts – If you plan on drag racing with sticky tires, consider upgrading to 31-spline or 35-spline aftermarket axles to handle the increased torque.
  • Check Your Pinion Preload – If you reused the original pinion, ensure the pinion nut is torqued to 160–220 ft-lbs and the preload is correct (10–20 in-lbs for new bearings, 5–10 for used). A loose pinion will cause noise and premature failure.

Conclusion

Installing a Detroit Locker Differential in your Ford Mustang GT is one of the most effective upgrades for improving traction, whether you’re launching at the drag strip or carving back roads. The mechanical lock-up eliminates wheel spin and delivers both rear wheels equally, dramatically reducing 60-foot times and improving corner exit speed. While the installation requires mechanical skill and attention to detail—especially in setting gear backlash and preload—following this guide will help you achieve a professional result in your home garage. For further support, consult the Detroit Locker official website for torque specifications and technical bulletins, or join a dedicated Mustang forum like SVTPerformance for real-world installation tips. With the locker installed and properly set up, your Mustang GT will be ready to put power down like never before.