DISC BRAKE SURFACE REFINISH - SEMI-METALLIC BRAKE LININGS

Semimetallic brake linings have been selected for many new vehicles based upon brake system performance requirements. In order to maintain this performance, semimetallic linings are also specified for service replacement.

Good contact is important between the rubbing surfaces of any brake system. But this is more important with semimetallic lining because they are less tolerant to a rough refinish surface which can temporarily reduce brake effectiveness.

The original equipment new disc brake rotors have an initial rubbing surface finish of 10 to 50 microinches and all have a nondirectional swirl pattern.

SERVICE RECOMMENDATIONS

Routine replacement of disc brake shoe and linings does not require rotor refinish.

Should rotor refinish be required because of parallelism, runout, or because of deep scoring, the semimetallic brakes lining requires a microinch surface refinish like the new vehicle rotor specification.

The recommended procedure for obtaining this finish is as follows:

Rough Cut Finish Cut

Spindle speed 200 RPM 200 RPM Depth of cut per side .005" .002" Tool cross feed per rev. .006"-.010" .002" Max. Vibration dampener Yes Yes Swirl pattern - 120 GRIT No Optional*

*Duplication of new rotor and smoother finish for best initial brake effectiveness.

When installing new rotors from service stock "do not" refinish the surface as these parts are to recommended finish. It also is not required to refinish a rotor on a vehicle which has a smooth finish.

When refinishing brake rotors for semimetallic linings, the following is important:

The brake lathe must have the capability to produce the intended surface finish.

Use correct tool feed and arbor speeds. Too fast a speed or too deep a cut can result in a rough finish.

Cutting tools must be sharp.

Adapters must be clean and free of nicks.

Lathe finish cuts should be further improved and made non-directional by dressing rotor surface with a sanding disc power tool (AMMCO Model 8350 Safe Swirl Disc Rotor Grinder or equivalent).

Rotor surfaces are to be refinished to 10 to 50 microinch.

To become familiar with the required surface finish, drag a penny over the surface of a new rotor from parts stock or one on a new vehicle. If your brake equipment cannot produce an equivalent finish when correctly used, contact the equipment manufacturer for corrective instructions.

General Motors bulletins are intended for use by professional technicians, not a "do-it-yourselfer". They are written to inform those technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions and know-how to do a job properly and safely. If a condition is described, do not assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See a General Motors dealer servicing your brand of General Motors vehicle for information on whether your vehicle may benefit from the information.