Abstract: A cap for covering a cylindrical tubular well casing is comprised of a thin planar circular cross-sectioned cover plate which has a diameter slightly larger than the outside diameter of the casing so as to form an overhanging lip thereon. An expandable clamp ring, which secures the cover plate to the casing, includes a semicylindrical outer band, which has a nominal diameter equal to the diameter of the cover plate, and an inwardly extending upper rim, which is joined normally to the top edge of the outer band and overlies the upper surface of the overhanging lip of the cover plate. A gasket, which is attached to the outer band immediately below the lip of the cover plate, is arranged to be tightly compressed between the band and the casing when the ends of the clamp ring are urged together by an adjustable fastener.