A modular portion (71) of a hydraulic valve actuator (64) comprises a hollow cylinder portion (74), a piston (78), a cylinder head portion (80), and a piston retainer portion (88). The piston (78) is located in the cylinder portion (74), and the piston (78) is adapted to slidably fit in the cylinder portion (74). The cylinder head portion (80) closes a second end (110) of the cylinder portion (74). The piston retainer portion (88) is located at an interior surface (134) of the cylinder portion (74) and at a first end (106) of the cylinder portion (74). Part of the piston retainer portion (88) extends radially inward relative to the interior surface (134) of the cylinder portion (74) farther than a portion of the piston (78) such that the piston (78) cannot slide completely past the piston retainer portion (88). Thus, the piston retainer portion (88) keeps the piston (78) within the cylinder portion (74) when the actuator (64) is disassembled for repair or replacement of seals, and when the modular actuator portion (71) is operably assembled.