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Any servicing of the braking disc requires extreme care to maintain the braking disc within service tolerances to ensure proper brake action. This includes wheel bearing adjustment (front) after lubrication as well as any refinishing operations.
Before refinishing or refacing a braking disc, the disc should be checked and inspected for the following conditions:
(1) Scoring, rust, impregnation of lining material and worn ridges.
(2) Runout or wobble.
(3) Thickness variation (Parallelism).
(4) 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 lining and cause noise and chatter. Excessive wear and scoring of the disc can cause temporary improper lining contact if ridges are not removed before installation of new shoe assemblies.
Some discoloration or wear of the disc surface is normal and does not require re-surfacing when linings are replaced.
Excessive runout or wobble in a disc can increase pedal travel due to piston knockback and increase piston seal wear due to necessity 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 when disc section is uneven. Dishing or distortion can be caused by extreme heat and abuse of the brakes.
CHECKING BRAKING DISC FOR RUNOUT AND THICKNESS
(1) With wheel bearings adjusted to zero end play, check each side of braking disc for lateral runout. Mount dial indicator C-3339 on steering arm having plunger contacting braking disc face approximately one inch from edge (Fig.
1). If runout exceeds .004 inch on either side, reface disc or replace braking disc and hub assembly.
(2) Check thickness variation of braking disc in conjunction with the runout check. Twelve equally spaced circumferential measurements should be made approximately one inch from edge of disc, using a micrometer (Fig. 1). If measurements vary more than .0005 of an inch remove and resurface the disc or replace the hub and disc assembly (Fig. 1). Readjust wheel bearings after check is completed.
(3) Reface the braking disc if heavy scoring or warping is evident. If machining does not correct the defects, or cracks in the disc are evident, replace the hub and disc assembly.
(1) Raise vehicle on hoist or jackstands. Remove wheel cover and wheel and tire assembly.
(2) Remove caliper assembly, as described under �Removing Caliper� this Group, (but do not disconnect brake line). Suspend caliper from wire hook or loop to avoid strain on flexible hose.
(3) Remove grease cap, cotter pin, nut lock, nut, thrust washer and outer wheel bearing.
(4) Remove from wheel spindle.
(1) Slide braking disc and hub assembly on spindle.
(2) Install outer bearing, thrust washer and nut.
(3) Tighten wheel bearing adjusting nut to 240-300 in. lbs. (27-34 N�m) while rotating disc and hub assembly. Recheck braking disc runout as described previously.
(4) Back off adjusting nut to release all preload, then retighten adjusting nut finger tight.
(5) Position lock on nut with one pair of slots in line with cotter pin hole. Install cotter pin.
(6) Clean grease cap, coating inside with wheel grease (do not fill cap) and install cap. Clean both sides of braking disc with alcohol or suitable solvent.
(7) Install caliper assembly, as described in �Installing Caliper� paragraph.
Resurfacing Braking Disc
This operation can be used when the braking disc surface is rusty or has lining deposits. A sanding disc attachment will remove surface contamination without removing much material. It will generally follow variations in thickness which are in the braking disc.
Refacing Braking Disc
Rotors do not have to be refaced whenever a new shoe and lining set is installed. If the rotor surface is deeply scored or warped or there is a complaint of brake roughness the rotor should be refaced (Fig. 2). Remove the least amount of metal necessary to correct the condition and remove an equal amount from each side.
When refacing a disc brake rotor the required 0.004 TIR (Total Indicator Reading) and 0.0005 thickness variation limits MUST BE MAINTAINED. Extreme care in the operation of rotor turning equipment is required. The collets, shafts and adapters on the lathe and the bearing cups in the rotor MUST be clean and free from any chips or contamination. When mounting the disc on the lathe strict attention to the manufacturer�s instructions is required. If the disc is not mounted properly the run-out will be worse after refacing than before refacing.
The use of a double straddle cutter which machines both sides of the disc at the same time is highly recommended.
The following chart and (Fig. 3) shows the location of measurements and specifications when servicing the braking disc:
All rotors (disc) will show markings of minimum allowable thickness cast on an un-machined surface (Fig. 4). This surface may be as shown in Fig. 4 or on an alternate surface. This marking includes .030 inch allowable rotor wear beyond the recommended .030 inch of disc refacing.
Minimum Allowable Thickness .940
Allow for Wear .030
Do Not Reface Beyond .970
|Braking Disc||Thickness||Minimum Thickness||Thickness Variation||Runout *||Finish Micro Inches|
|All Models||1.000 - 1.010||.940||.0005||.004||15-80|
|* T.I.R. - Total Indicator Reading (Measured on vehicle)|