When Replacing Disc Brakes Do Rotors Need to Be Turned?

The quick answer to whether rotors need to be turned (also known as machined or resurfaced) is no. Turning the rotor removes it from the vehicle and brings it to a lathe or employs an on-car lathe, which will true to the rotor to the hub of the vehicle. When replacing pads nowadays, however, many brake installation and repair facilities will want to turn or replace the rotors. The reasoning is twofold: It will provide a clean surface for the pads to break in on as well as minimize customer complaints.

Replacing Disc Brakes

The term "disc brakes" refers directly to the rotor, despite the friction material coming from the pads. When replacing pads, inspect the rotors thoroughly to look for grooves, rust pits, hard or heat spots (discolored deviations in the surface of the rotor plates) or visual cracks. Measuring the thickness of the rotor, employing a micrometer, will ensure that the rotor has enough material to allow turning it if any of the above symptoms are present (except cracks, which would require replacement). Every make and model vehicle features different-size rotors in length and diameter. As the friction material of the pads rubs against the rotor, the rotor thins down, although not as much as the pads. However, rotors feature what is called discard. This means they're only allowed to get so thin in width. Once they reach a certain width, they no longer have the capacity to maintain the intense heat they must endure during the braking process.

Inspect pad wear to help determine whether you should turn a rotor. Pads stuck in the caliper carrier or stuck caliper pistons that cause premature pad wear or angular wear will create heat spots on the rotors that should--but may or may not--be able to be turned out on a lathe.

In addition, a warped rotor that causes a noticeable vibration when braking will most likely not adapt to a resurfacing, because of excessive runout. Once a rotor is warped, most often caused by cheap material or excessive heat, machining the warping out of the rotors will be nominal and temporary at best.

Identifying the Rotor

Once you have removed the caliper, in addition to the pads and carrier if applicable, inspect the rotors. As mentioned, any visual rust pits, surface cracks or grooves caused by the metal backing plates of the pads may be compromised. Hard or heat spots on the rotor also may not take well to the resurfacing bits on the brake lathe. In the case of grooves in the rotor, the micrometer measurement requires several readings in and out of the grooves to determine whether you can turn down the rotor on the lathe to create a smooth surface at the lowest measurement. Visible lines in the rotor, called a record effect (much like the lines on the old LP albums) can be turned out as long as the rotor is within measurement specifications.

Smooth surfacing on the brake rotor gives the new pads a fresh surface to brake into. Without the rotor being smooth, the flat pads will end up wearing away to the surface of the rotor and cause adverse braking symptoms.

Rotors that show no signs of visual deviations, or that were not vibrating from excessive runout before the pads were replaced, may not need to be serviced.

Rotors of Today

Most rotors for cars nowadays are imported from other countries. While this has dropped the pricing down significantly, it has also compromised the quality of the rotors. Turning rotors isn't as financially viable as replacing them with new ones. Before deciding whether to slap on pads, machine the rotors or replace them, compare how the pads are wearing, what the condition of the rotor is and whether it's cheaper or even just a little more expensive to replace them rather than turning them.