Abstract: A lockable motor assembly for use in a well bore comprises a PDM motor having a rotor and a stator. A locking member has a splined projection which is received within a splined recess of the rotor and external splines which mate with splines provided on a sub connected to the stator. The components are held in this configuration by a shear ring until the fluid pressure within a chamber defined between the locking member and the sub is sufficient to cause the shear ring to shear. The locking member may then move out of the engagement with the rotor to free the motor for operation. The locking member is held in this position by a ratchet mechanism.