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Front brake disc - inspection, removal and refitting
Braking system / Front brake disc - inspection, removal and refitting


Note: To prevent uneven braking, BOTH front brake discs should be renewed or reground at the same time.

Inspection
1 Apply the handbrake. Loosen the relevant wheel nuts, jack up the front of the vehicle and support it on axle stands. Remove the wheel.

2 Remove the front brake caliper and carrier bracket with reference to Section 3, but do not disconnect the flexible hose. Support the caliper on an axle stand, or suspend it out of the way with a piece of wire, taking care to avoid straining the flexible hose.

3 Temporarily refit two of the wheel nuts to diagonally-opposite studs, with the flat sides of the nuts against the disc. Tighten the nuts progressively, to hold the disc firmly.

4 Scrape any corrosion from the disc. Rotate the disc, and examine it for deep scoring, grooving or cracks. Using a micrometer, measure the thickness of the disc in several places. The minimum thickness is stamped on the disc hub (see illustrations). Light wear and scoring is normal, but if excessive, the disc should be removed, and either reground by a specialist, or renewed. If regrinding is undertaken, the minimum thickness must be maintained. Obviously, if the disc is cracked, it must be renewed.

4.4A Using a micrometer to measure the thickness of the front brake disc
4.4A Using a micrometer to measure the thickness of the front brake disc

4.4B Disc minimum thickness marking
4.4B Disc minimum thickness marking

5 Using a dial gauge or a flat metal block and feeler gauges, check that the disc run-out 10 mm from the outer edge does not exceed the limit given in the Specifications. To do this, fix the measuring equipment, and rotate the disc, noting the variation in measurement as the disc is rotated (see illustration). The difference between the minimum and maximum measurements recorded is the disc run-out.

4.5 Measuring the disc run-out with a dial gauge
4.5 Measuring the disc run-out with a dial gauge

6 If the run-out is greater than the specified amount, check for variations of the disc thickness as follows. Mark the disc at eight positions 45° apart, then using a micrometer, measure the disc thickness at the eight positions, 15 mm in from the outer edge. If the variation between the minimum and maximum readings is greater than the specified amount, the disc should be renewed.

7 The hub face run-out can also be checked in a similar way. First remove the disc as described later in this Section, fix the measuring equipment, then slowly rotate the hub, and check that the run-out does not exceed the amount given in the Specifications. If the hub face run-out is excessive, this should be corrected (by renewing the hub bearings - see Chapter 10) before rechecking the disc run-out.

Removal
8 With the wheel and caliper removed, remove the wheel nuts which were temporarily refitted in paragraph 3.

9 Mark the disc in relation to the hub, if it is to be refitted.

10 Remove the two special washers (where fitted), and withdraw the disc over the wheel studs (see illustrations).

4.10A Remove the special washers . . .
4.10A Remove the special washers . . .

4.10B . . . and withdraw the disc
4.10B . . . and withdraw the disc

Refitting
11 Make sure that the disc and hub mating surfaces are clean, then locate the disc on the wheel studs. Align the previously-made marks if the original disc is being refitted.

12 Refit the two special washers, where fitted.

13 Refit the brake caliper and carrier bracket with reference to Section 3.

14 Refit the wheel, and lower the vehicle to the ground.

15 Test the brakes carefully before returning the vehicle to normal service.


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