Clutch Upgrade Guide: Replacing a Clutch in a 300‑hp Subaru WRX

Upgrading the clutch in a 300‑hp Subaru WRX is one of the most rewarding modifications you can make. The factory clutch is designed to handle stock power levels comfortably, but once you push beyond that threshold—whether with a tune, turbo upgrade, or supporting mods—the OE unit begins to slip, chatter, or wear prematurely. A purpose‑built performance clutch not only handles the extra torque but also delivers crisper shifts, better pedal feel, and improved durability. In this guide, we will walk you through the entire process: workspace preparation, removing the old clutch, inspecting the flywheel, installing the new clutch and pressure plate, reinstalling the transmission, and a proper break‑in procedure. Whether you are a seasoned DIYer or tackling this job for the first time, these steps will help you complete the upgrade safely and efficiently.

Tools and Materials Needed

Before you begin, gather all necessary tools and components. Working on a Subaru WRX’s transmission and clutch assembly requires specific equipment. A missing tool mid‑job can cause frustration and delays.

  • Socket set – Metric sockets (10mm, 12mm, 14mm, 17mm, 19mm) with extensions and a universal joint for hard‑to‑reach bolts.
  • Torque wrench – A quality click‑type torque wrench capable of reading in ft‑lbs (e.g., 10–150 ft‑lb range).
  • Clutch alignment tool – Most performance clutch kits include one; if not, purchase a universal alignment tool or a sleeve that matches the WRX’s input shaft.
  • Jack and jack stands – A low‑profile hydraulic jack and at least four stands rated above your vehicle’s weight.
  • Transmission fluid – OEM or high‑quality GL‑5 75W‑90 gear oil (quantity depends on transmission model; typically 3.5–4 quarts).
  • New clutch kit – Includes clutch disc, pressure plate, throwout bearing, pilot bearing (if applicable), and alignment tool. Choose a kit rated for your power level (e.g., Stage 2 or 3 for 300 hp).
  • Flywheel (optional) – If your current flywheel is glazed, warped, or you want a lighter unit, now is the time to swap it. Many enthusiasts upgrade to a lightweight billet flywheel for quicker revs.
  • Safety glasses and mechanic’s gloves – Protect your eyes from debris and your hands from sharp edges and hot surfaces.
  • Additional items – Brake cleaner, rags, anti‑seize compound, thread locker (medium strength), floor dolly (for transmission), and a helper is strongly recommended.

Preparation Steps

Proper preparation minimizes errors and ensures you have enough working clearance. Follow these steps to set up your workspace and vehicle:

  • Park the Subaru WRX on a flat, solid surface. Engage the parking brake and chock the rear wheels.
  • Disconnect the negative battery terminal. This prevents accidental electrical shorts and protects the ECU.
  • Raise the vehicle using a jack at the front crossmember or frame rail, then place jack stands under the frame points. Never rely on the jack alone.
  • Remove the front wheels. This gives you additional room to access the transmission side bolting and axle nuts.
  • Drain the transmission oil (if you prefer to avoid a mess) or have a drain pan ready for when you pull the axles.
  • Remove the starter motor. It is bolted to the transmission bellhousing; disconnect the wiring and set it aside.
  • Disconnect the shifter linkage and any electrical connectors attached to the transmission (reverse light switch, speed sensor).

Tip: Take photos or mark locations of brackets and harness clips before removal. This will save hours during reassembly.

Removing the Transmission and Old Clutch

With the vehicle lifted and components disconnected, you can begin removing the transmission. This step requires patience and careful maneuvering.

Support and Unbolt the Transmission

  • Place a transmission jack or a floor jack with a block of wood under the transmission pan. Secure the transmission with a strap if possible.
  • Remove the crossmember that supports the rear of the transmission. Usually held by four bolts.
  • Unbolt the driveshaft from the rear differential (on a WRX the driveshaft is two‑piece; unbolt the carrier bearing as well). Slide the driveshaft back slightly and tape over the output yoke to keep grease in.
  • Remove the front axles. Use a pry bar to pop them out of the transmission stubs. Be careful not to damage the axle seals.
  • Remove the bellhousing bolts. There are typically six to eight bolts; one or two may be accessed from the top or side. Use an extension and universal joint to reach tricky ones.
  • Once all bolts are out, carefully slide the transmission away from the engine. Wiggle it slightly and pull straight back. The transmission is heavy (around 100‑120 lb); a helper or a transmission jack is essential.

Remove the Old Clutch Assembly

  • With the transmission removed, you can see the clutch pressure plate and disc secured to the flywheel.
  • Loosen the pressure plate bolts in a star pattern, a little at a time, to release spring pressure evenly. Then remove the bolts and take off the pressure plate and disc.
  • Inspect the flywheel surface. Look for blue discoloration (indicating heat), deep grooves, cracks, or glazing. If the flywheel is in good condition, you can resurface it; otherwise, replace it.
  • Remove the pilot bearing (if so equipped) from the crankshaft pilot hole using a puller. Install a new pilot bearing (often included in the clutch kit).
  • Remove the old throwout bearing from the clutch fork. It may be a separate sleeve with a clip; replace it with the new one.

Inspecting and Prepping the Flywheel

The flywheel is a critical interface for the new clutch. A warped or glazed flywheel will cause premature wear or chatter.

  • Place a straightedge across the flywheel surface; if you see more than 0.002” gap, it needs machining or replacement.
  • If the flywheel is original, have it resurfaced by a machine shop that specializes in Subaru flywheels. The surface must be clean and flat.
  • When installing a new flywheel (upgrade or OEM), clean off any protective oil with brake cleaner. Never use solvents that leave residue.
  • Apply a small amount of anti‑seize to the flywheel bolts (or use new bolts if recommended by the manufacturer). Torque the flywheel bolts to specification (typically 65–70 ft‑lb with medium thread locker).

Installing the New Clutch

Now that the flywheel is ready, you can install the new clutch kit.

Position the Clutch Disc and Alignment

  • Ensure your hands are clean and free of grease. Oil contamination on the disc will cause future slippage.
  • Insert the clutch alignment tool through the disc and into the pilot bearing. The disc must be centered; the tool holds it in place.
  • Place the new pressure plate over the disc. Align the dowel pins (if present) or match the bolt holes.
  • Hand‑tighten the pressure plate bolts. Then, torque them in a crisscross pattern to the kit manufacturer’s specification. Common torque is around 15–20 ft‑lb for the pressure plate bolts, but verify with your kit.
  • Remove the alignment tool. Check that the disc moves freely within the splines.

Install Throwout Bearing and Pilot Bearing

  • Push the new throwout bearing onto the clutch fork sleeve. Ensure it clips securely.
  • Lightly grease the input shaft splines (a very thin film) to ease installation onto the disc splines. Excess grease can contaminate the clutch.

Reinstalling the Transmission

Reinstalling the transmission is the most physically demanding part of the job. Patience and attention to alignment are key.

  • Clean the mating surface on the engine block. Remove any old gasket material or debris.
  • Apply a thin bead of RTV silicone to the bellhousing face (some Subaru owners use a gasket; check your service manual).
  • With the transmission on a jack, align the input shaft with the clutch disc splines. Rotate the output shaft slightly to help engagement.
  • Slide the transmission forward gradually. Do not force it; if it resists, check alignment. Tapping lightly with a rubber mallet can help, but avoid bending the pilot bearing.
  • Once the bellhousing seats fully against the engine block, install and hand‑tighten the bellhousing bolts. Torque them to factory specification (usually 40–50 ft‑lb).
  • Reattach the crossmember, driveshaft, axles, starter motor, shifter linkage, and all electrical connectors.
  • Fill the transmission with fresh fluid through the fill plug until it runs out (engine off).

Bleeding the Hydraulic Clutch System

If you disconnected the clutch slave cylinder or removed the master cylinder line (or if the system was opened), you need to bleed the clutch hydraulics.

  • Locate the bleeder screw on the slave cylinder (mounted on the transmission bellhousing).
  • Attach a clear hose to the bleeder, leading into a container with some brake fluid.
  • Have a helper pump the clutch pedal slowly several times and hold it down.
  • Open the bleeder screw; fluid and air will rush out. Close the screw, then release the pedal.
  • Repeat until no air bubbles appear in the line. Top off the master cylinder reservoir frequently.
  • Test the clutch pedal: it should feel firm with a consistent engagement point.

Break‑In Procedure for the New Clutch

A performance clutch must be broken in properly to ensure longevity and even wear. Improper break‑in can cause glazing or chatter.

  • Drive the car gently for the first 300–500 miles. Avoid full‑throttle launches, aggressive clutch dumps, and hard shifts.
  • During the break‑in, vary your driving speed and do not drive at the same RPM for extended periods.
  • After the break‑in, perform a few moderate‑load accelerations (e.g., 3rd gear from 2000 to 4000 rpm) to seat the surfaces.
  • Check for any leaks around the slave cylinder, transmission plugs, and axles after the first few drives.

Common Mistakes and Troubleshooting

  • Clutch drag or poor shifting – Most often caused by an incorrectly aligned disc or a faulty hydraulic system. Double‑check that the disc slides freely on the input shaft.
  • Slipping under load – Often due to an overtightened pressure plate or an incompatible clutch for your power level. Verify torque specs and kit rating.
  • Noises (chatter, squealing) – Lightweight flywheels can make more gear rattle; this is normal. Persistent noise may indicate a worn pilot bearing or misalignment.
  • Hard pedal – Some performance clutches have a stiffer pedal; ensure the hydraulic system is bled properly and there are no binding in the pedal linkage.

Conclusion

Replacing the clutch in a 300‑hp Subaru WRX is a challenging but hugely rewarding project. A well‑chosen performance clutch will transform how your car drives—giving you the confidence to put the power down without worrying about premature slip. By following this guide, taking your time with each step, and respecting the break‑in period, you will enjoy many miles of aggressive yet reliable driving. Always consult your specific clutch kit instructions and the factory service manual for exact torque values and fastener requirements. For additional resources, check out Clutch Masters for performance clutch options, IWSTI’s clutch installation community, or NASIOC’s clutch FAQ for Subaru‑specific tips. Happy wrenching!