Clutch Masters Honda K20/K24 To Subaru EJ Single Disc Flywheel (5 or 6-Speed)

Putting a Honda K20 or K24 in front of a Subaru EJ gearbox is one of the more involved swaps out there, and the flywheel is the part that makes the two halves talk to each other. Clutch Masters FW-K721-SF is the single-disc steel flywheel built for their K-series to Subaru adapter plate. It bolts to the Honda crank and gives you a friction face and clutch pattern that accepts a Subaru STI 6-speed style clutch, so you are not stuck hunting for a one-off disc.

What this flywheel does in a K-swap Subaru

In a factory car the flywheel just carries the clutch and the ring gear. In this swap it also has to bridge two manufacturers. Clutch Masters makes the K721-EAP engine adapter plate 1 inch thick, and that spacing is what lets this flywheel position a single disc correctly against the Subaru input shaft. Clutch Masters states the flywheel works with any of their FX series single-disc kits or any other OE-style Subaru STI 6-speed clutch kit. The adapter kit keeps the factory Honda starter and sets the K engine at its factory 11-degree lean toward the passenger side.

One important detail: the listing title says 5 or 6-speed, but Clutch Masters describes FW-K721-SF as the flywheel for the 6-speed Subaru transmission. They sell a separate flywheel, FW-K915-SF, for the 5-speed box. The adapter plate itself covers both. If your build uses a Subaru 5-speed, tell us before you order.

Signs you need this flywheel (or a fresh one)

• You are building a K20 or K24 into a Subaru chassis or an AWD project that uses an EJ 6-speed
• An earlier flywheel on the swap shows heat checks, blue spots or cracks on the friction face
• Clutch chatter or judder on takeoff after a hard launch or heat soak
• A new clutch slips or engages unevenly because it was fitted to a glazed or warped surface
• Starter grind from chipped ring gear teeth

How to confirm before you buy

Confirm three things: you are running the Clutch Masters K721 adapter plate, the gearbox is the Subaru 6-speed, and you want a single-disc clutch rather than a twin. Clutch Masters also makes a twin-disc flywheel for their 850 Series twin kits if the build is aimed at high horsepower. If you are chasing a slip on an existing swap, rule out oil contamination and hydraulic problems first; our slipping diagnosis guide walks through how to tell a worn clutch from other causes, and this repair cost breakdown helps when you are budgeting the clutch side of the job.

Fitment

This part is for Honda K20 and K24 engines mated to a Subaru EJ-family 6-speed manual using the Clutch Masters K-series to Subaru adapter plate. It is not a direct fit for any factory vehicle, and it will not work with other adapter plates unless their spacing matches. Clutch Masters part number FW-K721-SF is the number to match. If you are mixing parts from more than one swap supplier, send us your engine code, transmission and adapter details and we will check it before it ships.

Install notes

• Pick the clutch first. Any STI 6-speed style single kit should fit, for example the Exedy Stage 1 organic STI kit or the XClutch XKSU24001-1A Stage 1. Higher-torque K builds can step up to a Clutch Masters FX200 STI kit.
• Clean the crank flange and flywheel mating surface, use new flywheel bolts and torque them to the value in the Clutch Masters instructions.
• Clutch Masters offers an optional Flywheel Friction Lock shim (about 0.010 to 0.012 in thick) to stop fretting between the crank and flywheel. It is worth considering on a high-power or launch-heavy build.
• Wipe the friction face with brake cleaner before fitting the clutch and center the disc with an alignment tool.
• Check that the Subaru release fork, bearing and hydraulics are in good shape. A swap is the time to sort the master cylinder as well, since pedal feel depends on it.

Does FW-K721-SF work with a Subaru 5-speed?

Clutch Masters lists FW-K721-SF as the 6-speed flywheel. They make a separate flywheel, FW-K915-SF, for the 5-speed transmission. The adapter plate covers both boxes, but the flywheel does not.

What clutch do I use with it?

Clutch Masters says it accepts any of their FX series single-disc kits or any OE-style Subaru STI 6-speed clutch kit. Choose the clutch by how much torque the K engine makes and how the car is driven.

Do I need the Clutch Masters adapter plate?

Yes. The flywheel is designed around their 1 inch thick K721-EAP adapter plate. A different adapter with different spacing will change the clutch position, so match the plate before buying.

Which starter does the swap use?

The Clutch Masters adapter kit uses the factory Honda starter and includes a plate to cover the starter hole on the Subaru transmission.

Is there a twin-disc option?

Clutch Masters offers a twin-disc flywheel for their 850 Series twin kits. FW-K721-SF is the single-disc version.

• How to Diagnose Transmission Slipping
• Automatic vs. Manual Transmission Repair Cost
• Shift Flare vs. Slip: How to Tell Them Apart