Abstract: A sealing nut having a shouldered insert of heat stabilized-lubricated nylon with a protruding crown, locked into a recess provided at one end of the nut body by means of a coined lip engaging the shoulder. Sealing is effected when the nut is engaged on a threaded male member and screwed against a bearing surface through which the male member extends by exterior folding of the crown upon itself and confining the resulting fold within the lip, said lip being brought into metal-to-metal engagement with the bearing surface. The folded crown or turtle-neck-like fold may be partially preformed prior to use to enhance reusability.