Lockable motor assembly for use in a well bore

A lockable motor assembly for use in a well bore, comprising a stator;

• a rotor rotatably mounted in the stator;
• a locking member movable axially of a motor of the assembly between a first position in which the locking member is rotationally fast with the stator and is in engagement with the rotor to prevent rotation of the rotor relative to the stator, and a second position in which one of the stator and the rotor is free to rotate relative to the locking member to permit relative rotation between the stator and the rotor;
• holding means for holding the locking member in the first position and selectively releasable to permit the locking member to move to the second position;
• and pressure sensitive means for moving the locking member between the first and second positions;
• wherein the pressure sensitive means comprises a differential area piston provided by the locking member wherein fluid acting on one portion of the piston is sealed from fluid acting on another portion of the piston.