An over-speed safety device for a pneumatic rotation motor having a cylinder (10), a forward end wall (11), a rear end wall (12) with a pressure air inlet opening (21), a rotor (13), and an actuator (15) co-rotative with the rotor (13) and responsive to centrifugal action, and a valve lid (20) tiltable relative to a pivot axis (C) and shiftable between an open position and a closed position for controlling the airflow though the inlet opening (21), wherein the pivot axis (C) extends along a chord in relation to the rotor centre, and the actuator (18) comprises a cam surface (25) for camming engagement with the valve lid (20) for moving the latter in a radial direction toward the closed position. The valve lid (20) is supported by ridge positions (33, 34) on a seat surface (2) disposed around the inlet opening (21), and a wire spring (26) is arranged to retain and bias the valve lid (20) against the seat surface (32). The ridge portions (33, 34) are arranged to make the valve lid (20) perform an over-centre movement when shifted from open to closed position, thereby obtaining a bi-stable action of the valve lid (20) under action of the wire spring (26).