Brake rotor finish — How to apply the proper finish

The right brake rotor finish; How smooth does the brake rotor finish have to be?

Brake rotor finish, or its surface “smoothness” is rated by its Roughness Average (RA). The RA should be below 80. The proper RA is critical to achieving the proper transfer layer for adherent style brake organic and ceramic pads as well as abrasive semi-metallic brake pads.

If the surface is too smooth, it’s impossible to form a transfer layer onto the rotor face. If it’s too rough, the brakes will make noise, wear faster and even lose stopping power.

Non-directional finish

A non-directional finish is also critical to proper bedding and noise-free operation. When a brake rotor is machined on a lathe, the cutting tool can leave spiral “record grooves” if the machining is done too quickly. If left that way, the grooves wear off too much friction material.

A properly machined rotor should have a non-directional finish applied after using the cutting tool.

How to apply a non-direction finish to brake rotors

Brake rotor hone

Although it’s best to apply a non-directional finish with the rotor on a brake lathe, you can sometimes do it yourself using a drill and hone or a random orbital sander.

Keep in mind that neither of these techniques are designed to remove grooves. You need a brake lathe for that. All you’re trying to accomplish here is to apply a very light finish

Using a brake hone

Clean the rotor face and load the hone into the drill chuck. Using very light pressure, work the hone around the rotor in a clockwise direction. Then, set the drill in reverse and go back over the same area, keeping the same pressure and speed.

Using a random orbital sander or DA sander

Load 120-grit sandpaper onto your sander. Set it to random pattern and using very slight pressure, run it around the disc face clockwise. The do the same thing in a counter-clockwise pattern.

©, 2022 Rick Muscoplat