Abstract: A disc brake comprising a caliper housing having axially aligned first and second cylinder bores; a first piston slidable within the first bore and bearing against one of the friction pads; and a second piston slidable within the second bore and having an internally threaded tubular opening into which a clearance adjuster is screwed. The threads of the adjuster are power transmission threads with a helix angle of about 15 degrees, and there is end-wise clearance between the threads sufficient to allow the first piston to move between its retracted and brakes-applied positions. At the end of the adjuster is a head having a conical clutch facing that engages a corresponding conical seat in the bottom of the first piston, so as to exert a powerful rotational clutch action. A spring urges the adjuster toward the bottom of the first piston, and causes the adjuster to unscrew and extent itself whenever there is any clearance between the clutch facing and seat.