ACT 2004 Mazda RX-8 HD/Race Rigid 4 Pad Clutch Kit

The ACT ZM8-HDR4 is a race-only clutch for the 2004-2011 Mazda RX-8: ACT's Heavy Duty pressure plate clamped onto a rigid, four-pad ceramic disc. It is the kit for an RX-8 that lives at the track or the drag strip, whether that is a naturally aspirated Renesis turning to redline every lap or a boosted car that has already burned through a street disc. If your RX-8 is a weekend race car and you want the lightest, fastest-shifting disc ACT offers for it, this is that kit.

How the rigid 4-pad clutch works in an RX-8

The RX-8 uses a single-plate dry clutch between the 1.3L rotary and the manual gearbox. A rotary makes its power high in the rev range, so on track the clutch is engaged at high rpm many times per session and has to shed a lot of heat. The 4240008 disc uses four ceramic pads on a heat-treated chromoly hub with no torsional springs. Removing the springs saves weight and rotating mass, so the gearbox input slows quickly and shifts go through faster. ACT notes that the four-pad layout suits smaller disc sizes like the RX-8's 240 mm (9.4 in) disc with a 1 in input shaft.

The MZ010 Heavy Duty pressure plate uses ACT's heat-treated diaphragm to raise clamp load and limit deflection. ACT rates the complete kit at 420 ft-lb at the crank, 72 percent over the factory clutch, and lists it at 14.1 lb. Compared with the sprung six-pad ZM8-HDG6, this kit trades away damping and pad area for lower weight. ACT describes the feel as light to moderate pedal effort, aggressive engagement and increased gear rattle, and warns that a rigid disc may increase spline wear. That is why it is sold for race use only.

Symptoms that call for a race clutch

• High-rpm slip in third or fourth late in a session as an organic disc heats up
• Fade and a soft, rising engagement point after back-to-back hard launches
• Glazed, blue-spotted flywheel and pressure plate faces at teardown
• A worn-out street clutch on a car that now runs boost or a race tune
• Release bearing noise with the pedal down, which this kit's RB110 bearing replaces

How to confirm before you buy

Make sure the problem is the clutch. A leaking or air-bound hydraulic system can leave the clutch partly engaged and feel like slip, or fail to release and grind. Bleed it and check the fluid first. Then test under load: a hard pull in a high gear from mid rpm will show the tach jumping ahead of road speed if the disc is slipping. Our slipping diagnosis guide explains the test, and the engagement point guide explains what a changing pedal height means. Be honest about use, too. If the car is driven daily, the chatter and rattle of a rigid ceramic disc will wear thin quickly.

Fitment

Our listing names the 2004 RX-8. ACT's table for ZM8-HDR4 covers the 2004 Base, the 2005 Base and Shinka, the 2006 and 2007 Base, the 2008 40th Anniversary, GS, GT, Grand Touring, Sport and Touring, the 2009 GS, GT, Grand Touring, R3, Sport and Touring, and the 2010 and 2011 GT, Grand Touring, R3 and Sport, all with the 1.3L rotary. Race cars often run swapped gearboxes or engines, so compare your old disc and pressure plate before ordering, or send us the year and VIN. More RX-8 parts are in our rebuild kits collection.

Install notes

• In the box: MZ010 pressure plate, 4240008 disc, RB110 release bearing, PB1013 pilot bearing and AT03 alignment tool. No flywheel or counterweight is included.
• ACT requires its counterweight whenever an ACT flywheel goes on the RX-8. For a race build, the 600140-03 Prolite kit with CW03 is the matching lightweight setup, or keep the stock flywheel and have it resurfaced.
• Center the disc with the AT03 tool and torque the pressure plate bolts evenly in a star pattern to the factory value.
• Bleed the clutch hydraulics with the fluid shown on the reservoir cap.
• ACT says race discs only need a few hard slips to lap in. Do not cook the clutch while it beds in.

Want ceramic grip with some damping for occasional street miles? The ZM8-HDG6 sprung 6-pad kit uses the same plate and torque rating.

Which RX-8 models does the ZM8-HDR4 fit?

ACT lists it for 2004-2011 Mazda RX-8 trims with the 1.3L rotary, from the 2004 Base through the 2011 R3. Send us your year and VIN if the car has a swapped gearbox.

What is the difference between ZM8-HDR4 and ZM8-HDG6?

Both use the MZ010 Heavy Duty plate and are rated at 420 ft-lb. The HDR4 has a rigid four-pad disc with no hub springs for lighter weight and faster shifts; the HDG6 has a sprung six-pad disc with more damping.

Can I daily drive it?

ACT recommends it for race use only. Expect aggressive engagement, more gear rattle and possible extra spline wear from the rigid hub.

Is a flywheel included?

No. Resurface the stock flywheel, or use an ACT flywheel kit with the CW03 counterweight, which ACT requires with its RX-8 flywheels.

How heavy is the pedal?

ACT describes pedal effort for this kit as light to moderate and expects 15 to 40 percent more effort from its Heavy Duty plates in general, depending on the car.

How do I break it in?

ACT says its race discs need only a few hard slips to lap in the surfaces. Avoid overheating the clutch during that first drive.

• Transmission Slipping Diagnosis Guide
• Shift Flare vs. Slip: How to Tell the Difference
• Automatic vs. Manual Transmission Repair Cost