Each time the disc pads require renewal, it is important to examine the surfaces of the discs. Light scoring is normal, but deep grooves indicate that the disc must be renewed or machined, provided that the thickness is not reduced below 0.51 in (13.4 mm) for the front discs and 0.331 in (8.4 mm) for the rear. Disc run-out must not exceed 0.004 in (0.1 mm) and should be measured by turning the disc against the plunger of a dial gauge; if this instrument is unavailable, feeler blades can provide an approximate indication by measuring the gap between the disc and a fixed point. To remove a front disc, unbolt the caliper and tie it securely to one side to prevent strain on the flexible hoses, then remove the disc/hub securing screws and withdraw the assembly. For a rear disc, unbolt the caliper, remove the clip securing the rigid brake line to the rear axle casing, and pull the caliper far enough away from the disc to tie it up without bending the brake pipe. Insert a screwdriver in the adjuster hole in the handbrake drum to release the shoe adjustment, then unscrew and remove the two retaining screws to withdraw the disc/drum assembly. Refitting is a reversal of removal, but the handbrake shoes on the rear brake should be adjusted so that the drum is stiff to turn, then back off the adjuster by between 4 and 5 teeth.