A hydraulic fluid power cylinder assembly 10 includes a cylinder chamber 14, a piston 15, a piston rod 19 and an end cap 21. End cap 21 includes a drift stop chamber 24 and a drift stop blocking surface 25. Chamber 24 is in open fluid pressure communication with chamber 14 under all conditions. Piston 15 includes a piston body portion 16 and a drift stop body portion 29. Drift stop body portion 29 includes a drift stop blocking member 31 and a positioning surface 34 for preventing the blocking member 31 from contacting the walls of cavity 24 and cylinder chamber 14 when disengaged from blocking surface 25. In one position drift stop body portion 29 is received within drift stop cavity 24, and blocking member 31 engages blocking surface 25 to prevent unintended movement of piston 15.