looses shape and flows downwardly through a void 85 and out of the operative position shown in fig2 b of the drawings preferably a thermally insulating sleeve 86 will extend externally and coaxially with strip heater 84 and a thermally insulative disk 88 will extend beneath heater 84 thermally insulating sleeve 86 and disk 88 serve to minimize transfer of heat from heater 84 to sleeve assembly 68 and housing assembly 38 this assures that the heat of strip heater 84 is directed primarily to alloy member 82 collet assembly 78 includes a set of flexible collet fingers 90 positioned around and encircling an upstanding longitudinally moveable probe 91 on operating mandrel 92 probe 91 is positioned so that it will slide downwardly through collet fingers 90 when collet fingers are deflected outwardly collet fingers 90 are equipped with inwardly protruding shoulders 94 located proximate the end of each collet finger 90 collet fingers 90 and shoulders 94 are wedged or blocked from deflecting radially outwardly by alloy member 82 when such member is in its solid state but are released when alloy member 82 is melted an overhanging shoulder 96 located at the upper end of probe 91 blocks probe 91 from passing between shoulders 94 the blocking position defined at this juncture is held as long as the components have the shape illustrated in the drawings some movement of probe 91 is allowed however to facilitate equalization of down hole pressures when alloy member 82 is melted collet fingers 90 are then free to flex radially outwardly thereby allowing shoulder 96 to pass and releasing probe 91 and operating mandrel 92 for downward movement operating mandrel 92 includes an enlargement 100 which supports an annular groove 102 and o ring.