ACT XACT K-Series Streetlite and Prolite Flywheels

ACT XACT K-Series Flywheels: 600125 Prolite and 600295 Streetlite

You've got the gearbox out for a clutch change on your RSX, Civic Si or first-generation TSX. It's a good time to choose the flywheel you'll actually want to drive with. ACT's XACT Streetlite 600295 weighs 11.4 lb; Prolite 600125 brings that down to 7.8 lb. Both are forged chromoly steel flywheels for the Type-S-compatible clutch arrangement, so check that pairing before ordering.

Streetlite or Prolite

Streetlite is the sensible starting point if your car still has a commute. It isn't ACT's lightest flywheel for this application, and that's part of its appeal: you don't have to choose the most aggressive option just because the transmission's already out. ACT recommends 600295 for street and race use, with street or drag racing as its preference. ACT describes its gear rattle as OE to mild.

Prolite takes another 3.6 lb out. That's worth considering when you're building around engine response for road racing or rally, which are ACT's preferred uses for 600125. It comes with reduced driveability and mild to moderate gear rattle in ACT's description. If stop-and-go traffic is a regular part of your week, don't dismiss those compromises just to get the lower number on the scale.

There's less stored rotational energy in a lighter flywheel, so engine speed can change more quickly and you've got less of that energy available as you pull away. Your clutch disc matters here too. Streetlite with a street disc and Prolite with a ceramic race disc aren't the same driving proposition. Pick the combination for the car's job; the lightest flywheel and most aggressive disc aren't automatically the best pair for your build.

Why the steel construction matters

These aren't aluminum flywheels with replaceable friction inserts. ACT machines each from a heat-treated chromoly steel forging, then induction-hardens the ring gear. The friction face and ring gear belong to the same one-piece construction. When it's time for a clutch change, you can have the steel face resurfaced within ACT's limits instead of buying a separate friction insert.

ACT CNC machines and dynamically balances both versions, and lists SFI 1.1 for each. Both have a 119-tooth ring gear. Choosing Prolite doesn't change that tooth count or turn this into an aluminum flywheel; you're choosing a lighter version of ACT's steel construction.

Fit the clutch and flywheel as a pair

Take a 2004 base RSX with its manual gearbox. It's listed for these flywheels, but that doesn't mean you should bolt one behind an unmatched base-model clutch. You're using the Type-S-compatible arrangement. If you've already picked the clutch, confirm its part number before ordering the flywheel. The EP3 Civic Si and first-generation TSX conversion need the same care.

Don't order from the words K-series alone. A 2012-2015 Civic Si with a K24Z7 isn't listed for these flywheels, and neither is the later TSX K24Z3 application. A swapped chassis needs a check of the actual engine, gearbox and clutch combination. Automatic cars aren't covered.

Installation and later servicing

You get the selected flywheel; the clutch isn't included. Check the pairing before fitting it, then follow ACT's installation instructions and the fastener specifications for your application. The flywheel's step sets the relationship between its friction face and pressure-plate mounting face. That dimension needs to stay correct when you service the part too.

At a later clutch change, ACT calls for a flywheel grinder and no more than 0.040 in / 1.02 mm of material removal. Have the machinist take off only what's needed to clean up the face while keeping the proper step. That maximum isn't an allowance to remove another 0.040 in every time you change the clutch.

ACT covers the flywheel with its 12-month limited warranty under the published terms. Keep your receipt: installation labor isn't covered. Both 600125 and 600295 carry a California Proposition 65 warning.