XClutch 15-21 Subaru WRX STi Base 2.5L 9in Twin Sprung Ceramic Clutch Kit

Once an EJ257 STI gets a larger turbo, E85 and a serious tune, the factory clutch and most single disc upgrades start slipping in the upper gears. The XClutch XKSU23531-2B is a Stage 5 pull-type twin disc for the 2015 to 2021 Subaru WRX STI 2.5L that pairs two 9 inch sprung ceramic discs with a matching flywheel. XClutch rates it at 1550 ft-lb of torque capacity, including its safety factor, and classes it for high power street use.

What this twin disc does in a WRX STI

The STI drives all four wheels through the six-speed, so every hard launch loads the clutch, center differential and gear train at once. A twin disc puts two friction discs and an intermediate plate inside a package close to stock diameter, which doubles the friction area and gives the very high holding power in the space the original clutch used.

The important detail on this version is the sprung hub. XClutch designed the twin sprung disc for cars that keep the standard transmission in motorsport use. The springs in each hub soak up some of the shock of a hard engagement before it reaches the gears and the driveline, which is exactly what a factory STI six-speed needs on a high-traction launch. The ceramic friction handles heat on track days and repeated launches. The kit is pull type, like the factory STI clutch, and uses a 1 inch, 24 spline input hub.

Symptoms that call for a twin disc

• A single disc upgrade that slips in fourth or fifth when full boost arrives
• Revs flaring on launch or on the 2-3 shift
• Clutch engagement point moving around after a few hard pulls as heat builds
• Burnt smell or glazing on a fairly new disc
• A power goal above the rating of your current clutch

How to confirm before you buy

Slip should be confirmed in a high gear at moderate rpm under full boost: if engine speed rises faster than road speed, the clutch is giving up. Before blaming the discs, check the release side. A worn fork, pivot or pull-type bearing, a weak slave cylinder or air in the line all affect pedal feel and can keep the clutch from releasing or clamping correctly. XClutch tells buyers to check these actuation parts whenever the clutch is replaced. If your STI is a moderate street build, a less aggressive kit such as the ACT Mod-Twin HD street kit may suit it better, while track-only cars can look at the ACT Mod-Twin XT race kit. For overall job cost, read our manual versus automatic repair cost guide.

Fitment

Our listing covers the 2015 to 2021 Subaru WRX STI with the 2.5L EJ257 and six-speed manual, including Base and Limited trims. XClutch's own compatibility chart also lists this part for the special editions such as the Type RA and S209, and for earlier STI models back to 2004, plus JDM EJ207 cars. It does not fit the regular WRX with the FA20 engine. If your car has a swapped gearbox or you are not sure, send us the year and last 7 of the VIN before ordering. Other clutch parts are in our rebuild kits department.

Install notes

• The discs come assembled in the correct order. XClutch's TSB-171 says to keep track of that order if you take the pack apart: bottom disc against the flywheel, intermediate plate indexed to its drive tabs, then the top disc with its outer ring facing up to mate with the bottom disc.
• Fit the pilot bearing, center the discs with an alignment tool, and run the pressure plate bolts down slowly and evenly to a final pass of 32 Nm per TSB-171.
• On a pull-type clutch, never clip the release bearing onto the diaphragm before the gearbox goes in. XClutch's TSB-PULL01 warns that doing so makes it impossible to engage the bearing with the fork. Fit the bearing to the fork and input sleeve, then push the fork by hand after mating until the bearing clips in.
• Inspect the clutch fork contact areas and replace it if worn. Support the gearbox so it never hangs on the input shaft.
• Before final assembly, check the engagement point with the car in gear while a helper turns the axles to confirm the clutch releases fully. Bleed with fresh brake fluid; a pneumatic bleeder makes it quicker.

Does this fit the regular WRX?

No. It is for the WRX STI with the 2.5L EJ257 and six-speed. The non-STI WRX uses a different engine, gearbox and clutch.

Will it work in my 2008 to 2014 STI?

XClutch's compatibility chart lists this part for STI models from 2004 through 2021, but our listing is written for 2015 to 2021. Send the year and last 7 of the VIN and we will confirm before you order.

Is it streetable?

XClutch classes it as high power street use. The sprung hubs help, but a twin disc still engages more abruptly and can make more noise than stock. XClutch notes cabin NVH changes are normal with this kind of upgrade.

How much torque does it hold?

XClutch rates it at 1550 ft-lb of torque capacity, including its factor of safety.

Is it push or pull type?

Pull type, the same as the factory STI clutch. Follow the pull-type bearing procedure: the bearing clips into the diaphragm only after the transmission is bolted up.

What is the service pack part number?

XClutch lists the replacement disc service pack as XMS230SU01-2B-XC for this kit.

• Automatic vs Manual Transmission Repair Cost