Operation CHARM: Car repair manuals for everyone.

Brake Rotor/Disc: Testing and Inspection

GENERAL INFORMATION
Any servicing of the rotor requires extreme care to maintain the rotor to within service tolerances to ensure proper brake action.

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

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

Excessive rotor lateral runout or wobble.

Thickness variation in braking surface of the rotor (Parallelism).

Dishing or distortion in braking surface of the rotor (Flatness).

If a vehicle has not been driven for a period of time, the rotors 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 rotor can cause temporary improper lining contact if ridges are not removed from braking surface of rotor before installation of new brake shoe assemblies.

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

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

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

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

CHECKING ROTOR FOR RUNOUT AND THICKNESS

NOTE: The procedure for checking rotor runout and thickness is the same for the front and rear rotor. If there is a specification difference between the front and rear rotor it will be designated as such in the specifications of the following procedure.

Checking Brake Rotor For Runout:





On-vehicle rotor runout is the combination of the individual runout of the hub face and the runout of the rotor. (The hub and rotor run-outs are separable). To measure runout on the vehicle, remove the wheel and reinstall the lug nuts tightening the rotor to the hub. Mount Dial Indicator, Special Tool C-3339 with Mounting Adaptor, Special Tool SP-1910 or equivalent on steering arm. Dial indicator plunger should contact braking surface of rotor approximately 10 mm (0.393 inch) from outer edge of rotor. Check lateral runout on both sides of rotor. Lateral runout of the rotor should not exceed 0.13 mm (0.005 inch).

Marking Rotor And Wheel Stud:





If lateral runout is in excess of the specification, check the lateral runout of the hub face. Before removing rotor from hub, make a chalk mark across both the rotor and one wheel stud on the high side of the runout. This will ensure that the original location of the rotor in relation to the hub can be retained. Remove rotor from hub.

Checking Disc Brake Rotor For Runout:





Thoroughly clean the front surface of the front hub. Then install Dial Indicator, Special Tool C-3339 and Mounting Adaptor, Special Tool SP-1910 or equivalent on steering 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 face. Clean hub surface before checking.

Indexing Rotor And Wheel Stud:





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

Minimum Rotor Thickness Markings:





If lateral runout is not within specification, install a new rotor or reface rotor, being careful to remove as little rotor material as possible from each side of rotor. Remove equal amounts from each side of rotor. Do not reduce thickness below minimum thickness marking cast into the un-machined surface of the rotor.

Checking Rotor For Thickness Variation:





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

Light scoring and/or wear is acceptable. If heavy scoring or warping is evident, the rotor must be refinished or replaced (See Refinishing/Refacing Rotor). If cracks are evident in the rotor, replace the rotor.