Introduction

Upgrading the flywheel on a Mustang GT is one of the most effective ways to sharpen throttle response, improve acceleration, and reduce rotational inertia. The Southbend Performance Flywheel is engineered specifically for high-torque applications, offering a robust design that stands up to aggressive driving, track days, and forced induction setups. While the installation is a moderate-to-advanced job, it can be completed by a capable DIY mechanic with the right tools and patience. This guide provides a detailed, step-by-step process to remove the old flywheel, install the Southbend unit, and get your Mustang GT back on the road with noticeably improved performance.

Why Upgrade to a Southbend Performance Flywheel?

The factory flywheel on many Mustang GTs is heavy, designed for smooth engagement and fuel economy rather than performance. A lighter flywheel from Southbend reduces rotational mass, allowing the engine to rev up and down faster. This translates to quicker throttle blips, easier rev-matching, and a more connected driving feel. Southbend uses billet steel or chromoly materials with precision machining to ensure perfect balance and durability. Their flywheels are often paired with Southbend clutch kits for a complete driveline upgrade.

Before you start, understand that installing a lightweight flywheel may slightly change driving characteristics – the engine may stall more easily if you aren’t used to the faster rev drop. However, the performance benefits are undeniable. For more details on the specific flywheel options, visit the Southbend Clutch official site.

Tools and Materials Needed

Having everything on hand before you begin saves time and prevents frustration. Below is a comprehensive list:

  • Southbend Performance Flywheel (make sure it matches your Mustang GT model year and engine – 2V, 3V, Coyote, etc.)
  • Socket set: metric and SAE (3/8” and 1/2” drive) with extensions and universal joints
  • Torque wrench (1/2” drive, capable of at least 150 ft-lbs)
  • Jack and heavy-duty jack stands (or a lift)
  • Transmission jack or a floor jack with a padded block
  • Flywheel alignment tool (usually included with the flywheel or clutch kit)
  • Flathead screwdriver, pry bar, and brake cleaner
  • Thread locker (medium strength, e.g., Loctite 242)
  • New flywheel bolts (Southbend recommends replacing bolts – do not reuse old ones)
  • Torque spec reference: typically 65–85 ft-lbs for flywheel bolts (verify for your specific engine)
  • Transmission fluid (if you need to top off after reinstall)
  • Safety glasses, mechanic’s gloves, and a fire extinguisher

Safety Precautions

Working under a vehicle and handling heavy driveline components carries risk. Follow these safety guidelines:

  • Disconnect the battery negative terminal to prevent accidental starter engagement.
  • Never work under a car supported only by a jack – always use jack stands rated for the vehicle weight.
  • Use a transmission jack or have a helper if removing the transmission manually – it weighs 90–120 lbs.
  • Wear safety glasses when using solvents or when debris may fly.
  • Ensure the work area is well-lit and ventilated.

Preparation: Getting Your Mustang GT Ready

Start by parking the car on a level surface. Chock the rear wheels to prevent rolling. Raise the front of the car high enough to allow access underneath and place jack stands under the frame rails or designated lift points. Remove the negative battery cable and secure it away from the terminal.

If your Mustang has underbody panels (common on 2005+ models), remove them to expose the transmission and bellhousing area. You may also need to remove the exhaust mid-pipe or catalytic converter for clearance – refer to a factory service manual for your specific year. A good resource is the Mustang6G forum, where many DIY threads detail exact steps for each generation.

Step 1: Removing the Transmission

With the vehicle safely raised, drain the transmission fluid if you prefer less mess (optional if you’re careful). Then:

  1. Remove the shifter assembly from inside the cabin (for manual transmissions). This usually involves prying off the boot and unscrewing the shifter handle.
  2. Disconnect the reverse lockout solenoid and any electrical connectors on the transmission.
  3. Remove the starter motor (usually two bolts) and set it aside.
  4. Support the transmission with a jack. Remove the crossmember that holds the tail of the transmission.
  5. Unbolt the bellhousing from the engine block – there are typically 6–8 bolts. Lower the transmission slightly and pull it rearward until the input shaft clears the clutch disc.
  6. Carefully lower the transmission and slide it out from under the car.

Note: If you have a dual-mass flywheel from the factory, the old flywheel may be heavier – handle with care when removing.

Step 2: Removing the Old Flywheel

With the transmission out, you have clear access to the flywheel. Lock the flywheel in place using a flywheel holding tool or by wedging a large screwdriver into the ring gear teeth (against the starter mount). Then:

  1. Remove the flywheel bolts in a crisscross pattern to avoid warping the flywheel.
  2. Pull the flywheel straight off the crankshaft. It may be stuck; tap it gently with a rubber mallet if needed.
  3. Inspect the crankshaft flange for burrs or debris – clean with brake cleaner.

Step 3: Inspecting the Pilot Bearing and Crankshaft

Before installing the new flywheel, check the pilot bearing (or bushing) in the center of the crankshaft. If it shows wear, replace it now. A worn pilot bearing can cause vibration and premature clutch failure. Use a puller to remove the old bearing and tap in a new one – ensure it is seated flush. Also, inspect the crankshaft rear main seal for leaks. If you see oil, replace the seal before proceeding; it’s cheaper than doing the job twice.

Clean the flywheel mounting surface on the crankshaft with a lint-free rag and brake cleaner. It must be free of oil and grease.

Step 4: Installing the Southbend Performance Flywheel

Now for the core of the job. Follow these steps precisely:

  1. Place the new Southbend flywheel onto the crankshaft. It should slide onto the dowel pins (if equipped) or center hub. Do not force it.
  2. Insert the new flywheel bolts. Apply a small drop of medium-strength thread locker to each bolt. Hand-tighten them in a crisscross pattern.
  3. Use a torque wrench to tighten the bolts in stages. The final torque for most Mustang GT flywheels is 65–85 ft-lbs, but you must verify the exact specification for your engine from Southbend or the factory service manual. A common value is 75 ft-lbs for 4.6L 3V engines. Under-torquing can cause bolts to loosen; over-torquing can strip threads.
  4. After torquing, use the flywheel alignment tool to hold it in place. This tool centers the flywheel (and later the clutch disc) – you can leave it in place for now.
  5. Rotate the flywheel by hand to ensure it spins freely and does not wobble. Any wobble indicates a misalignment or a bent flywheel – check fitment.

Important: The Southbend Performance Flywheel is designed to work with both OEM and performance clutches. If you are replacing the clutch at the same time (highly recommended), follow the clutch kit instructions for disc alignment.

Step 5: Reinstalling the Clutch and Transmission

If you have a clutch kit, install the new clutch disc and pressure plate before reattaching the transmission. Use the alignment tool provided with the Southbend flywheel or clutch kit to center the disc. Torque the pressure plate bolts to spec (often 35–45 ft-lbs) in a star pattern. Remove the alignment tool.

To reinstall the transmission:

  1. Lift the transmission with a jack and align the input shaft with the clutch disc splines. You may need to rotate the output shaft or input shaft slightly to engage.
  2. Slide the transmission forward until the bellhousing mates with the engine block. Never use bolts to pull the transmission together – you can damage the pilot bearing or clutch disc. If it doesn’t go, check alignment.
  3. Once fully seated, install and torque the bellhousing bolts.
  4. Reinstall the starter motor, crossmember, shifter assembly, and any electrical connectors.
  5. Reconnect the exhaust if it was removed.
  6. Refill the transmission with the correct fluid (check your owner’s manual for grade and capacity).

Step 6: Final Checks and Test Drive

Before lowering the car, double-check all bolts, connectors, and fluid levels. Reconnect the battery. Start the engine and listen for any unusual noises – a slight rattle may be normal for a lightweight flywheel, but a grinding sound indicates a problem. Press the clutch pedal several times to ensure proper clutch release.

Take the car for a test drive in a safe area. Pay attention to:

  • Smooth engagement from a stop
  • No vibration at idle or under load
  • Proper shifting feel (no grinding or difficulty)
  • Any signs of fluid leaks

If everything feels good, enjoy the improved performance. For more detailed troubleshooting, the Southbend tech support page offers valuable information.

Break-In Procedure for the Flywheel

While the flywheel itself doesn’t have a break-in period like clutch friction material, it’s wise to follow a gentle break-in for the entire driveline:

  • Drive gently for the first 200–300 miles.
  • Avoid hard launches or high-rpm shifts.
  • Vary engine speeds and load conditions to allow the clutch disc to mate properly.
  • After break-in, re-check the flywheel bolts for torque (if accessible) – some builders recommend a re-torque after a few heat cycles.

Common Mistakes to Avoid

Even experienced mechanics can make errors during a flywheel swap. Watch out for these pitfalls:

  • Reusing old flywheel bolts: They can stretch and cause failure. Always use new bolts.
  • Skipping the alignment tool: The clutch disc must be centered precisely, or the transmission will not slide on.
  • Improper torque: Either under or over torquing can lead to catastrophic failure. Use a calibrated torque wrench.
  • Oil or grease on the flywheel surface: Contaminants cause clutch chatter and premature wear. Clean thoroughly.
  • Forgetting the pilot bearing: A worn or missing pilot bearing will cause vibration and noise.
  • Rushing the transmission installation: Forcing the transmission can damage the input shaft or clutch. Patience is key.

Conclusion

Installing a Southbend Performance Flywheel on your Mustang GT is a rewarding project that delivers noticeable performance improvements. With careful preparation, the right tools, and attention to detail, you can complete the installation in a weekend. The result is a more responsive drivetrain that makes every drive more engaging. Whether you’re building a track car or just want to sharpen your daily driver, this upgrade is a solid investment. Remember to consult your vehicle’s service manual for model-specific details and always prioritize safety. After the job, you’ll appreciate the crisp throttle response and the added excitement every time you hit the gas.