9 so that the components mounted thereon can be slid off and replaced when required outboard of the threaded section 12 is a suitable high pressure connector 13 capable of joining the shaft to a 20 000 psi to 60 000 psi variable pressure fluid supply in the operation of the mandrel assembly 7 for the expansion of the tube plug 1 o ring 14 and a polyurethane backup ring 15 sit in a recess 16 formed in the shaft 8 outboard of the threaded section 12 of the shaft 8 suitable annular grooves 17 a and 17 b are formed in the reduced section of the shaft 11 seated in the grooves 17 a and 17 b respectively are o rings 18 a and 18 b made of a suitably soft material outboard and adjacent to o rings 18 a and 18 b are polyurethane backup rings 19 a and 19 b made of a resilient composition that are considerably harder than the o rings 18 a and 18 b but will deform in a plastic manner when placed under extremely high pressure these backup rings 19 a and 19 b are of a slightly larger diameter than the increased diameter section 9 on the mandrel shaft 8 to better enable them to fill the annular extrusion or expansion gap for the mandrel assembly 7 as defined by the outer surface of the increased diameter section 9 on the mandrel shaft 8 and the inner wall of the tube plug 5.