Brake Rotor/Disc: Testing and Inspection

NOTE: If a vehicle has not been driven for a period of time, the discs will rust in the area not covered by the lining and cause noise and chatter.

NOTE: Some discoloration or wear of the disc surface is normal and does not require re-surfacing when linings are replaced.

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

BEFORE REFINISHING OR REFACING A BRAKING DISC INSPECT FOR:
- Scoring, rust, impregnation of lining material and worn ridges.
- Excessive wear and scoring of the disc

CAUTION: 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.

- Runout or wobble

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

- Thickness variation (Parallelism).

NOTE: Thickness variation in a disc can also result in pedal pulsation, chatter and surge due to variation in brake output when disc section is uneven.

- Dishing or distortion (Flatness).

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

SPECIFICATION - Specifications

Fig 1 Checking Total Indicated Runout (TIR):

MEASURING TOTAL DISC RUNOUT ON VEHICLE:

NOTE: 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.

NOTE: Before checking the runout, wheel bearings should be adjusted.

1. Remove the wheel and reinstall the lug nuts to tighten the disc to the hub.
2. Mount dial indicator with adapter on steering arm with plunger contacting disc braking surface approximately one inch from edge of disc.
3. Check lateral runout (both sides of disc).
- Runout should not exceed 0.13mm (0.005 inch).
- If total runout is in exceeds specification check the lateral runout of the hub face.

Fig 2 Marking Brake Disc And Wheel Stud For Reassembly:

MEASURING LATERAL RUNOUT OF THE HUB FACE:
1. Before removing disc, 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.
2. Remove disc from hub.

Fig 3 Checking Hub Runout:

3. Install dial indicator with adapter on steering arm with stem contacting hub face near outer diameter (care must be taken to position dial indicator stem outside the stud circle but inside the chamfer on the hub rim. Runout should not exceed 0.05mm (0.002 inch).
- If runout exceeds this specification, hub must be replaced.

Fig 4 Indexing Disc And Wheel Stud:

- If hub runout does not exceed this specification, install disc on hub with chalk marks 180 degrees apart.
4. Finally, recheck runout of disc to see if runout is now within specifications.
- If runout is in excess of specifications, install a new 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 the recommended minimum thickness.

NOTE: Thickness variation measurements of disc should be made in conjunction with runout.

Fig 5 Checking Disc Thickness:

THICKNESS VARIATION MEASUREMENTS
Measure thickness of disc at twelve (12) equal points with a micrometer at a radius approximately 25.4 mm (one (1) inch) from edge of disc.
- If thickness measurements vary by more than 13 mm (0.0005 inch) disc should be removed and resurfaced, or a new disc installed.
- If cracks or burned spots are evident in the disc, disc must be replaced.
- For additional information on how to use a micrometer, see Fundamentals and Basics.Fundamentals and Basics

NOTE: Light scoring and/or wear Is acceptable:
- If heavy scoring or warping is evident, the disc must be refinished or replaced.
- If cracks are evident in the disc, replace the disc.