A wax filled actuator includes a piston in direct contact with the thermally responsive wax material. A cup containing the wax is mechanically secured to a guide that receives and controls axial movement of the piston in response to expansion and contraction of the wax. A seal between the cup and guide prevents leakage of the wax. Another seal surrounds the piston and prevents leakage of the wax around the piston. In a disclosed embodiment, the piston has a cylindrical, polished outside surface that aids in preventing adhesion of the wax to the piston. In a disclosed embodiment, the seal surrounding the piston is a radially compressed annular elastomeric member. The seal surrounding the piston may be disposed between flat, annular wipers. The seal and wipers may be axially retained between the guide and a washer trapped between the cup and guide.