Abstract: A self-locking fastener is disclosed having a load surface adapted to be tightened against a flat mating surface providing self-locking means which include an inner bearing surface having an outer extremity located in a bearing plane and inclined inwardly and back from such plane, radially extending teeth extending outwardly from the inner bearing surface and a skirt at the outer extremity of the teeth. The skirt provides an axial extremity located in the bearing plane and inclined inner and outer walls extending back therefrom. The teeth define recesses into which material from the mating part is displaced to provide the locking of the fastener. The inner bearing surfaces and the skirt cooperate to support the ends of the teeth and to provide a full seal at each extremity thereof.