Most of the time, replacing the rotor is unnecessary with a brake pad change, but it's important to reface it if the surface is damaged or scratchy. If this happens, the rotor can be refaced using a hub-mounted on-car brake lathe or replaced. A hub-mounted on-car brake lathe is preferred as it guarantees the rotor is correctly aligned with the vehicle's hub and bearing, reducing runout. All rotors include minimum thickness information on the un-machined portion, showing how little they may be ground. Machining the rotor at a thickness below this value is not allowed. Clean off any chips, rust or dirt from the rotor face and the hub adapters before you start installation. Make sure to follow all the manufacturer's directions when using the brake lathe and turn both sides of the rotor at once to prevent uneven cuts. The TIR and thickness must be maintained within acceptable limits during refacing, so great attention is needed in the way the rotor turning machines are operated.