Abstract: A thermal actuator of the expansible wax type is disclosed having an actuator rod extending through an enlarged clearance hole in an upper guide casing, a closely fitting hole in a guide washer, and a snugly fitting anti-chafe ring. The washer and ring are located intermediate the upper surface of a flexible diaphragm and a counterbore defined by the guide casing. The clearance between the outer diameter of the guide washer and the counterbore formed by the guide casing is substantially less than the clearance between the guide casing opening and the actuator rod. The guide washer and anti-chafe ring mounting arrangement in relation to the guide casing opening provides improved rod alignment and a self-cleaning action as the rod moves.