2015-2021 Subaru WRX Rotor Replacement: Resurface or Replace

By Core Brake Parts Tech Team — Circleville, OH warehouse

On a 2015-2021 Subaru WRX, a rotor can be resurfaced only if it is uncracked and will still be above the minimum thickness stamped on it after the lathe takes its cut. If it is already near that limit, has deep grooves, heat cracks, or a lip you can catch a fingernail on, replace it. Because the WRX rear rotor is a thin solid disc, the rears are usually a replace job, not a machining job.

What brakes the 2015-2021 WRX uses

The VA-chassis WRX (the 2.0L turbo car, not the STI) runs ventilated rotors up front and solid rotors in the rear. Replacement rotors built to the original dimensions list the front as 316 mm in diameter and 30 mm thick with internal cooling vanes, and the rear as 286 mm in diameter and 10 mm thick, solid. Subaru part numbers commonly cross-referenced for the front rotor include 26300AL010, 26300AL04A and 26300XA00A, which tells you the WRX front rotor is shared with other Subaru turbo platforms such as the Forester XT and Levorg.

The STI is a different brake package and is not covered by these rotor fitments, so if you own an STI, ignore the sizes in this article and check your own part numbers. Base, Premium and Limited WRX trims for 2015-2021 all share the same standard rotor fitment.

The minimum thickness for your exact rotor is cast or stamped on the rotor itself, usually on the hat or the edge. Read that number off your rotor, not off a chart online.

The resurface-or-replace decision, step by step

Work through these checks in order. The first one that fails decides it.

1. Look for cracks. A cracked rotor is replaced no matter how thick it is. Check the friction face and the edges, especially around any drilled holes or slots on aftermarket rotors.
2. Measure thickness with a micrometer. Measure in several spots around the rotor, in the middle of the pad track where it wears thinnest. If any reading is at or below the stamped minimum, it is done.
3. Estimate the cut. Resurfacing always removes metal. If the rotor would drop below minimum once the lathe cleans up the grooves and high spots, replace it. A rotor that just barely clears minimum after machining will reach the limit on the next set of pads, so you only delay the purchase.
4. Check groove depth. Deep scoring from metal-to-metal contact often needs more material removed than the rotor has left to give.
5. Check runout and the hub. If the complaint is a pulsing pedal, measure lateral runout with a dial indicator. Light runout and light glazing are exactly what resurfacing fixes, but clean the hub face first: rust or dirt between the hub and rotor causes runout on its own, and no amount of machining fixes a dirty hub.

If the rotor passes all five, resurfacing is a legitimate option. If it fails any of them, buy new.

Why the WRX front and rear rotors behave differently

The front rotor is a thick vented disc. That 30 mm of cast iron with vanes in the middle gives a machine shop room to work, and it is the rotor that takes most of the braking heat. On a WRX that sees a lot of spirited driving, the front is where you see blue heat discoloration, glazed pad deposits and pedal pulsation. A clean cut on a lathe can restore a flat, true surface if the rotor has thickness to spare.

The rear rotor is a 10 mm solid disc. There is simply not much metal there to begin with, and the gap between new thickness and the stamped minimum is small. Many shops will not machine rear rotors this thin at all. Rear rotors on all-wheel-drive Subarus also tend to rust on the inner face and around the outer edge, which leaves a lip and pitting that eat into usable thickness. In practice, worn WRX rears get replaced.

Symptoms that point to the rotors

• Pulsing brake pedal or steering wheel shake when stopping from speed - usually lateral runout or uneven pad deposits on the front rotors.
• Grinding - pads worn to the backing plate, which almost always scores the rotor past saving.
• A raised lip on the outer edge - normal wear, but a tall lip means a lot of thickness is gone.
• Blue or dark heat spots and a shiny glazed face - overheated pads; light glazing can be machined off.
• Visible cracks or heavy rust pitting - replace.

Not every complaint needs rotors. Light surface rust after rain, cosmetic discoloration, or a squeal by itself is not a reason to cut or replace rotors. Fix the actual cause first.

What to replace together

New or resurfaced rotors always get new pads. Old pads are shaped to the old surface and will not seat properly on a fresh face. Replace rotors in pairs on the same axle so both sides brake evenly. While the calipers are off, replace or clean and lubricate the pad hardware and slide pins, and inspect the rubber brake hoses for cracking. Brake fluid that is dark or past its service interval is worth flushing at the same visit.

If you are doing both axles, a complete kit saves you from mismatched parts. Our 2015-2021 Subaru WRX (non-STI) front and rear brake kit includes rotors and pads for both ends of the car. For one axle at a time, browse brake rotors and matching ceramic brake pads, and pick up a brake hardware kit while you are in there.

How hard is the job on a WRX?

Rotor replacement on this car is a straightforward driveway job for someone with basic tools. The front caliper bracket comes off to free the rotor, and rotors that have been on for years often seize to the hub and need penetrating oil and a few firm hits with a rubber mallet on the hat. Clean the hub face to bare metal before the new rotor goes on. The rears are similar; make sure the parking brake is fully released before you pull the rear rotor, and if it still drags, check the factory service manual for the parking brake procedure rather than forcing it.

Torque the caliper bracket bolts and lug nuts to the values in the factory service manual, and tighten lug nuts in a star pattern. Over-tightened or uneven lug nuts are a common cause of runout that shows up as pulsation a few hundred miles later. Finish with a proper bed-in procedure so the new pads lay an even transfer layer on the rotors.

Frequently Asked Questions

Can I put new pads on my old WRX rotors without resurfacing?

Yes, if the rotors are above minimum thickness, flat, free of cracks and deep grooves, and the pedal does not pulse. Smooth, in-spec rotors do not need to be cut just because the pads are being changed.

Are WRX front rotors the same as the STI?

No. The STI uses a different, larger brake package. The 2015-2021 WRX Base, Premium and Limited share their own front rotor, which is also used on some other Subaru turbo models.

Where is the minimum thickness on a WRX rotor?

It is cast or stamped on the rotor, usually on the hat section or the outer edge. Use that number and measure with a micrometer, not a tape measure.

Why do my rear WRX rotors rust so badly?

The rears do less braking work, so the pads scrub the surface less often, and the inner face and edges collect moisture and road salt. Rust pitting and a heavy lip reduce usable thickness, which is why worn rears are usually replaced.

Is it worth resurfacing instead of buying new rotors?

Only when the rotor has plenty of thickness left. If the cut takes it close to minimum, the rotor will be done by the next pad change, and buying new now saves a second job.

Sources

1. Jalopnik - When to resurface vs replace brake rotors - minimum thickness, cracks, deep grooves, runout, hub cleaning and lug torque guidance
2. MSB Garage - WRX VA 2015-2021 front rotor listing - 316 mm x 30 mm front rotor and Subaru OEM cross-reference numbers
3. UroTuning - 2015-2021 WRX front rotor 316x30 vented - front rotor size and vented construction
4. UroTuning - 2015-2021 WRX rear rotor 286x10 solid - rear rotor size and solid construction
5. Subispeed - DBA 2015-2021 WRX front rotor - fitment for WRX Base, Premium and Limited; STI not listed