Closure plug

In combination, a closure plug, a sealing gasket and a bushing in a container opening, the plug having

• a bottom end portion,
• a circumferentially enlarged rim,
• a cylindrical sidewall surrounding the bottom end portion and terminating in the plug rim, the sidewall having
• an external helical screw thread;
• an annular gasket seat between the plug rim and the screw thread, and
• a gasket retaining flank interposed between the gasket seat and the screw thread, wherein the gasket retaining flank comprises an axially upwardly and radially outwardly facing gasket supporting surface contiguous with the gasket seat and having a radial extension less than a radial extension of a crest of the external helical screw thread, wherein the gasket seat flares radially outwardly and upwardly from the retaining flank to the plug rim,
• the sealing gasket being received on the annular gasket seat so as to be held and supported in face-to-face contact with at least a substantial portion of the gasket supporting surface so as to retain the gasket on the gasket seat as the closure plug is unscrewed;
• the bushing having a cylindrical wall with an internal helical screw thread with which the plug external helical screw thread is engaged, and
• an upwardly extending, smooth gasket sealing area, the sealing gasket being substantially radially compressed between the bushing gasket sealing area and the plug gasket seat and covering the radial extent of the interengaged bushing internal helical screw thread and plug external helical screw thread.