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How to Measure the Size of the Brakes and Rotors

by Justin Cupler

Brake pads and rotors are some of the more common items to replace on a vehicle. Brake pads need replacement every 25,000 to 35,000 miles, while rotors typically last 50,000 miles. These intervals may become shorter, depending on driving styles, heavy hauling or stop-and-go traffic, all of which can impact the intervals greatly. It is recommended to check the brake pads and rotors at every oil change interval. Many times, inexperienced mechanics will gauge the brakes by simply looking at them, but the proper way to check is to remove and measure the pads and rotors.

Step 1

Loosen, but do not remove, the lug nuts from the wheels whose pads and rotors you are checking, using the ratchet and socket.

Step 2

Raise the front or rear of the vehicle --- depending on the pads and rotors that need to be measured -- and secure the vehicle with the jack stands.

Step 3

Remove the lug nuts and pull the wheels from the vehicle.

Step 4

Look on the rear of the brake caliper and locate the two caliper bolts: one upper and one lower.

Step 5

Loosen, but do not remove, the upper caliper bolt, and loosen and remove the lower caliper bolt, using the ratchet and socket.

Step 6

Pivot the caliper upwards, exposing the brake pads.

Step 7

Grasp the inner and outer brake pads by hand, and pull them from the brake assembly. Place the brake pads on the ground (back side down), leaving the pad material facing upward.

Step 8

Close the jaws of the electronic caliber and press the "Zero" or "Reset" button. This recalibrates the caliber so that the measurement is accurate.

Step 9

Open the caliber's jaws and look for a pin-like object that protrudes from the bottom of the caliber.

Step 10

Place the pin-like object on the brake pad so that it contacts the back of the brake pad. Press the caliber downward, until it contacts the brake pad material. Observe as the pin-like object goes back into the caliber -- this is measuring the thickness of the pad's material or lining.

Step 11

Read the measurement on the LCD screen -- brake pads are measured in thousandths of an inch.

Step 12

Compare this measurement to the "brake lining thickness" specification in the repair manual. Replace the brake pads if they fall below this specification.

Step 13

Open the caliber's jaw and place the open jaws over the disc portion of the rotor -- the flat metal surface that the pads were contacting.

Step 14

Close the caliber over the disc and take the reading from the LCD screen on the caliper (the rotor measures in thousandths of an inch).

Step 15

Spin the rotor approximately 180-degrees, then repeat steps 13 and 14.

Step 16

Compare the lowest of the two measurements to the specifications -- "minimal machining thickness" and "discard thickness" -- listed in the repair manual.

Repeat steps 4 through 16 to measure the brake pads and rotors on the other side of the vehicle.

Tips

  • If the rotor is above the minimal machining thickness, it can either be resurfaced or simply reused without resurfacing, depending on the rotors condition.
  • If the brake pads on one side of the vehicle fall below the brake lining thickness specification, both sides must be replaced.

Warning

  • Never drive with a rotor that is below discard thickness -- this may cause damage to the braking system.

Items you will need

  • Floor jack
  • Jack stands
  • Ratchet
  • Socket set
  • Electronic brake caliber
  • Repair manual (Haynes or Chilton's)

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