Abstract: A device for locking a piston rod of a hydraulic piston and cylinder assembly in an extended condition so that mobile construction equipment or other heavy machinery cannot be driven from a job site. The locking device is placed in ensleeving relation to an extended piston rod between a proximal hydraulic cylinder and a distal bearing point to prevent retraction of the piston rod. The device has a hollow sleeve configuration formed of confronting first and second sleeve parts that are hingedly connected to one another. A first lock secures the first and second sleeve members to one another along a longitudinally extending parting line. A second lock is opposed to the first lock and prevents hinged movement of the first and second sleeve parts relative to one another even if the first lock is defeated. The device is adaptable to a variety of hydraulic piston and cylinder assemblies.