Note: Please read the warning at the beginning before you begin paragraph 4 about the dangers of asbestos dust.
Examination
Note: If at least one of the discs requires replacement, BOTH discs must be replaced at the same time to ensure equal braking torque on both wheels. All four brake pads must be replaced along with the discs.
1. Apply the parking brake firmly, then jack up the front of the vehicle and support it on stands (see "Lifting the car and placing it on supports"). Remove the corresponding front wheel.
2. Slowly rotate the brake disc so that you can inspect the entire disc from both sides. To improve the viewing area of the inner surface of the disc, remove the brake pads. Minor scratches are allowed on the surface under the pads, but if the disc is significantly scratched or cracked, it must be replaced.
3. It is considered acceptable to have a rusty step around the perimeter of the disc, which can be removed if desired. However, if the step is due to wear caused by the pads, the thickness of the disc should be checked with a micrometer (see fig. 6.3). Measure at various points on the disc surface, on the inner and outer sides of the surface under the pads. If the wear in at least one place exceeds the maximum permissible technical data, the disc must be replaced.
Fig. 6.3. Measuring the thickness of the disc using a micrometer
4. Check the axial runout of the brake disc (see fig. 6.4). However, first, rule out axial play in the hub bearing as a possible source of the problem (see Chapter 10).
Fig. 6.4. Checking the disc runout using a micrometer with a circular scale (rear disc shown - the procedure is similar for the front disc)
5. To check the wheel runout, place large flat washers under the heads of the two wheel mounting bolts and screw them into the hub through the wheel. Position the bolts diagonally so that the wheel is installed evenly, then tighten the bolts. You can use a dial indicator attached to a fixed support or use flat feeler gauges to measure the clearance (at some points on the entire surface of the disk) between the disc and a fixed surface, such as a caliper mounting bracket. If the values obtained are outside the limits specified in the technical data, the disc should be replaced.
6. Check the disc for cracks, especially around the bolt holes. If cracks are found, as well as wear or other damage, the disc should be replaced.
Replacement
Note: Installation will require new caliper mounting bolts and disc mounting screws.
7. Remove the two bolts securing the brake caliper bracket to the steering knuckle, then slide the caliper off the rotor. Tie the caliper to the front suspension spring to avoid stretching the hydraulic hoses or wires from the caliper. Discard the bolts; new ones must be used for installation (see fig. 6.7).
Fig. 6.7. Loosen the bolts and remove the caliper from the steering knuckle...
8. Loosen and remove the screws securing the brake disc to the hub (see fig. 6.8). Discard the screws as new ones must be used during installation.
Fig. 6.8. ...then unscrew the mounting screws and remove the brake disc
9. Remove the disc from the hub, remembering the correct position of its mounting pins. If it is tight, lightly tap it on the back side with a wooden hammer.
Installation
10. Before installation, remove all traces of the old locking compound from the threads of the hole for the caliper mounting bolts by going through the holes with a tap of the appropriate size. Similarly, clean the threads of the hub holes for the disc mounting screws.
Tip: If you don't have the required tap, go through the threads using an old bolt with a split groove on the thread.
11. Check that the mating surfaces of the disc and hub are clean and flat and that the dowel pins are in place. If a new disc is being fitted, use a suitable solvent to remove the protective film from the disc. Apply a thin coat of high temperature grease to the mating surfaces of the hub, but ensure that the disc surface is not contaminated.
12. Install the disc onto the hub, positioning it correctly on the mounting pins. Install a new mounting screw and tighten it to the required torque (see fig. 6.12).
Fig. 6.12. When installing, secure the disk with a new mounting screw
13. Slide the caliper over the top of the disc, making sure the pads pass on each side of the disc.
14. Install new caliper bracket mounting bolts and tighten them to the specified torque.
15. Install the wheels, then lower the car to the ground and tighten the wheel bolts to the required torque. Press the brake pedal several times to press the pads against the brake disc and restore normal pedal pressure. Check the brakes thoroughly before driving on the road.