Abstract: An expandable plug for use in effecting a fluid-tight seal at a remote location in a tube includes a compressible sleeve located between a rigid cylindrical member and a rigid sleeve. A bolt extends through the two sleeves and is threaded into the cylindrical member to apply compressive force to the expandable sleeve causing the sleeve to expand radially outwardly into fluid-tight sealing relation with the inner wall of the tube and radially inwardly into fluid-tight sealing relation with the bolt shank thereby protecting the bolt threads from contamination and corrosion. An eccentrically mounted disk is provided at the remote end of the cylindrical member to lock this member against rotation during installation of the plug.