Use a micrometer to measure the disc thickness
6.4 To check disc run-out, mount a dial indicator as shown and rotate the disc
6.6b Suspend the caliper from the coil spring with a cable tie or string; don't let it hang by the brake hose alone
6.6a Undo the two caliper mounting bracket bolts (arrowed)
6.6b Suspend the caliper from the coil spring with a cable tie or string; don't let it hang by the brake hose alone and vehicle support). Remove the appropriate front roadwheel.
2 Slowly rotate the brake disc so that the full area of both sides can be checked; remove the brake pads if better access is required to the inboard surface. Light scoring is normal in the area swept by the brake pads, but if heavy scoring or cracks are found, the disc must be renewed.
3 It is normal to find a lip of rust and brake dust around the disc's perimeter; this can be scraped off if required. If, however, a lip has formed due to excessive wear of the brake pad swept area, then the disc's thickness must be measured using a micrometer. Take measurements at several places around the disc, at the inside and outside of the pad swept area; if the disc has worn at any point to the specified minimum thickness or less, the disc must be renewed (see illustration).
4 If the disc is thought to be warped, it can be checked for run-out. Either use a dial gauge mounted on any convenient fixed point, while the disc is slowly rotated, or use feeler blades to measure (at several points all around the disc) the clearance between the disc and a fixed point, such as the caliper mounting bracket (see illustration). If the measurements obtained are at the specified maximum or beyond, the disc is excessively warped and must be renewed; however, it is worth checking first that the hub bearing is in good condition (Chapter 1). Also try the effect of removing the disc and turning it through 180°, to reposition it on the hub; if the run-out is still excessive, the disc must be renewed.
5 Check the disc for cracks, especially around the wheel bolt holes, and any other wear or damage, and renew if necessary.
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