Many people like to blame the friction material when a brake job goes awry, but when a rotor with a rough surface is installed during a new brake job, it will likely result in increased pedal effort, poor stopping performance and increased stopping distances until the friction stabilizes after several stops. So, here is a step-by-step guide on how to properly machine a rotor from the Bendix Brakes Answerman team.
1. Measure the rotor’s thickness to make sure that machining will not reduce it beyond the minimum thickness required by the manufacturer.
2. Thoroughly clean the inner and outer hat area so that it is free of rust and corrosion. A rotor cannot be mounted correctly in the lathe if there is rust/corrosion present.
3. After mounting the rotor, it may take one or two fast cuts to clean up the surface and correct any runout. The last cut should be made with the slowest possible lathe speed. This ensures that the resulting finish will be as smooth as possible.
4. For a non-directional finish, use a brake rotor hone. This tool mounts in a drill or die grinder and is pressed against the surface of the rotor while it is turning on the brake lathe. It is designed to smooth out the peaks and valley. Sanding blocks with 180-200 grit paper can also be used.
5. The best way to accurately check for a smooth finish is to use a hand held tool that is designed specifically to measure a rotor’s roughness average. The Bendix team suggests a finish of approximately 50 microinches for acceptable performance. A rotor finish of up to 80 microinches is still considered by many to be satisfactory, but any higher and a tech could be asking for trouble.
For more brake tips, visit www.bendixbrakes.com.