Abstract: A mounting apparatus for a proximity switch for sensing the position of a piston within a fluid operated, expandable chamber cylinder. The mounting device comprises a block shaped member with at least a first depressed, arcuate surface, the degree of curvature of which is adapted to fit the surface of the cylinder. By first attaching the mounting apparatus to the proximity switch and then positioning the proximity switch with respect to the fluid cylinder, tightening down of fastening devices provided for the proximity switch will cause the arcuate surface to firmly engage the surface of the cylinder, thereby providing a secure, vibration resistant mounting of the proximity switch to the cylinder.