Wheel studs can wear down due to a number of reasons: improperly tightened lug nuts, dirt and road salt in the threads, or they can weaken over time from repeated expansion and contraction. When worn down, the lugs will not engage the studs correctly, and the lugs cannot securely attach the wheel to the vehicle. A critical part of the vehicle, wheel studs can be replaced easily and inexpensively.
Jack up the WRX and put it on jack stands. Chock the wheels at the opposite end of the car. If replacing a stud in one of the rear wheels, disengage the emergency brake. Remove the wheel.
Remove the brake caliper. Hang the caliper onto the axle or place a support underneath it so that the brake cable is not under stress. Remove the rotor. If the rotor cannot be pulled off by hand, thread an 8mm x 1.25 bolt into each of the two holes near the centre of the rotor to push the rotor off of the hub. If replacing a wheel stud on one of the rear wheels, remove the five screws that hold the ABS sensor ring onto the hub and move the ring out of the way. Remove the bolt on the ABS sensor then move the sensor out of the way.
Look behind the hub. Line up the wheel stud that needs to be removed with an open area behind the hub.
Hit the stud with a dead blow hammer to force it out. Wheel studs are held onto the hub by a press fit so one strong blow with the hammer should suffice. Tapping on the stud will not force it out of the hub.
Insert the new wheel stud. Push the stud in from the back of the hub by hand.
Stack spacers onto the stud and place a wheel bolt at the end of the spacers. Tighten the bolt with the ½-inch drive wrench. Tightening the bolt will pull the wheel stud through the hub. Continue to tighten the bolt until the stud is seated firmly in the hub. Visually verify that the stud is seated by comparing it to the other studs; there should be no splines visible when looking at the back of the hub.
Remove the wheel bolt and spacers. Reinstall the rotor, caliper and wheel using the correct torque specifications.