Abstract: Clutch apparatus having a drum having an inner annular clutching face adapted to be secured to the crank shaft of an engine for rotation therewith. A hollow drive shaft centered within the drum carries a plurality of radially movable clutch shoes which are normally urged inwards to a disengaged position by means of an annular tension spring and are moved radially outwards against the tension spring by means by a linearly movable conical wedge which is grooved to accept round-ended stems extending from each of the clutch shoes and which is normally spring-urged into engagement with the shoe stems to move the shoes into clutching engagement with the drum against the action of the coil spring. The conical wedge is operatively connected to a piston which rides in the hollow drive shaft into which a pressurized fluid is injected to operate the piston and move the wedge out of engagement with the shoe stems to enable the tension spring to move the shoes radially inwards out of engagement with the clutch drum.