A plug for sealing the end of a tube including a cylindrical housing member having longitudinal bore that communicates with a threaded portion. An insert member having a threaded end portion is received within the threaded portion of the bore and supports a frustroconical member that deflects at least a portion of the wall of the housing member that is highly ductile. The frustroconical member has a correspondingly large angle of taper to deflect the wall of the housing member outwardly as much as possible without causing fracturing of the wall.