Replacing the disc isn't required for every pad change, but you should do so when the surface is damaged, shows warping or causes brake noise or steering feel problems. In this situation, either reface the disc with a hub-mounted on-car brake lathe or replace it. Rather than other methods, the hub-mounted brake lathe is better, as it keeps unnecessary runout from truing the brake disc against the vehicle's hub and bearing. You will find the minimum thickness of the disc, as cast, marked on an un-machined surface of the disc. Never run the machine on the disc if it drops below the given gauge. It is essential to clean the surface of the disc and all the mounting adapters before you begin the installation. While working on a brake lathe, stick to the manufacturer's instructions and machine both sides of the brake disc together to ensure they are even. The Total Indicator Reading (TIR) and thickness change rules must be followed during refacing which requires a lot of careful operation of the disc turning machines.