Abstract: An actuator comprising a housing having a chamber and a piston slidably received within the chamber. The housing is provided with an inlet port to the chamber for the introduction of an operating fluid to cause the piston to slide within the chamber. The housing is provided with means for releasably attaching the actuator relative to a stem to be actuated. Additionally, the housing includes a plug for sealing an aperture to the chamber and receiving the stem. The plug moves into the chamber to enable the piston to act upon the stem via the plug.