seated a sealing o ring 66 the upper end joint member 12 threadedly supports an inner sleeve 68 defining a central bore 70 down to its termination adjacent lower joint member 14 see fig1 c the upper en of inner sleeve 70 is formed with an annular end wall 72 extending into an outer cylindrical wall 74 having threads 76 for mating engagement with threads 30 of the joint member 12 inner sleeve 70 then extends into an outer cylindrical wall 78 that extends throughout most of the insulative gap sub assembly 16 to formation of a canted annular shoulder 80 fig1 b and to a cylindrical surface 82 that terminates in a downwardly facing annular shoulder 84 fig1 c and finally to form a cylindrical wall 86a and 86 the portion 86a of the cylindrical wall 86 adjacent to and accommodating gap block 140 is an equi lateral hexagon cross section as will be further described below the lower end of cylindrical wall 86 includes two annular grooves 88 and 90 wherein respective o rings 92 and 94 are seated and the top outer cylindrical wall 74 of inner sleeve 68 is sealed by annular groove 96 with o ring 98 the lower joint member 14 is.