BRAKE DISC (ROTOR)

GENERAL INFORMATION

Any servicing of the braking disc requires extreme care to maintain the braking disc within service tolerances to ensure proper brake action.

CAUTION: If the braking disk (rotor) needs to be replaced with a new part. The protective coating on the braking surfaces of the rotor MUST BE REMOVED with an appropriate solvent, to avoid contamination of the brake shoe linings.

When replacing a rotor with a new part do NOT reface the new rotor. Rotor already has the required micro finish when manufactured, only remove the protective coating.

SERVICE PROCEDURES

CHECKING BRAKING DISC FOR RUNOUT AND THICKNESS

On vehicle, braking disc (rotor) runout is the combination of the individual runout of the hub face and the runout of the disc. (The hub and disc are separable). To measure runout on the vehicle, remove the wheel and reinstall the lug nuts tightening the disc to the hub. Mount Dial Indicator, Special Tool C-3339 with Mounting Adaptor, Special Tool SP-1910 on steering arm. Dial indicator plunger should contact disc (braking surface) approximately one inch from edge of disc (See Fig. 1). Check lateral runout (both sides of disc) runout should not exceed 0.13 mm (0.005 inch).

INSPECTION DIAGNOSIS

Before refinishing or refacing a braking disc, the disc should be checked and inspected for the following conditions:

Braking surface scoring, rust, impregnation of lining material and worn ridges.

Excessive lateral rotor runout or wobble. Thickness variation (Parallelism). Dishing or distortion (Flatness).

If a vehicle has not been driven for a period of time. The discs will rust in the area not covered by the brake lining and cause noise and chatter when the brakes are applied.

Excessive wear and scoring of the disc can cause temporary improper lining contact if ridges are not removed before installation of new brake shoe assemblies.

Some discoloration or wear of the disc surface is normal and does not require resurfacing when linings are replaced.

Excessive runout or wobble in a disc can increase pedal travel due to piston knock back. This will increase guide pin bushing wear due to tendency of caliper to follow disc wobble.

Thickness variation in a disc can also result in pedal pulsation, chatter and surge due to variation in brake output. This can also be caused by excessive runout in braking disc or hub.

Dishing or distortion can be caused by extreme heat and abuse of the brakes.

Fig. 1 Checking Braking Disc for Runout

If runout is in excess of the specification, check the lateral runout of the hub face. Before removing disc from hub, make a chalk mark across both the disc and one wheel stud on the high side of runout. So you'll know exactly how the disc and hub was originally mounted (Fig. 2). Remove disc from hub.

Install Dial Indicator, Special Tool C-3339 and Mounting Adaptor, Special Tool SP-1910 on steering

Fig. 2 Marking Braking Disc and Wheel Stud

knuckle. Position stem so it contacts hub face near outer diameter. Care must be taken to position stem outside the stud circle but inside the chamfer on the hub rim. Clean hub surface before checking. (See Fig. 3)

Fig. 3 Checking Hub for Runout

Runout should not exceed 0.08 mm (0.003 inch). If runout exceeds this specification, hub must be replaced. See Suspension Group 2. If hub runout does not exceed this specification, install disc on hub with chalk marks two wheel studs apart (Fig. 4). Tighten nuts in the proper sequence and torque to specifications. Finally, check runout of disc to see if runout is now within specifications.

Fig. 4 Index Braking Disc and Wheel Stud

If runout is not within specifications. Install a new braking disc or reface disc, being careful to remove as little as possible from each side of disc. Remove equal amounts from each side of disc. Do not reduce thickness below minimum thickness cast into the unmachined surface of the rotor.

Thickness variation measurements of disc should be made in conjunction with runout. Measure thickness of disc at 12 equal points with a micrometer at a radius approximately 25.4 mm (1 inch) from edge of disc (Fig. 5). If thickness measurements vary by more than 0.013 mm (0.0005 inch) disc should be removed and resurfaced (Figs. 6 and 7), or a new disc installed. If cracks or burned spots are evident in the disc, disc must be replaced.

Fig. 5 Checking Disc for Thickness