An adjustable length cylinder assembly includes a housing and a piston reciprocably mounted within the housing for movement between an extended position and a retracted position. The piston defines first and second hydraulic fluid chambers of differing cross-sectional area on opposite sides of the piston. An accumulator is housed within one of the housing and the piston. Channels communicate hydraulic fluid the first and second chambers and the accumulator during the movement of the piston. A valve member is concentric with the piston and accumulator and rotatable between a first rotary position in which the channels are open so that fluid can be transferred between the chambers to allow retracting and extending movement of the piston and a second rotary position in which the channels are closed so that fluid cannot be transferred and the piston is locked against retracting and extending movement.