the back head 22 and piston seal cap 36 can be attached to the casing member 14 using any suitable connection means such as threaded bolt connections the same applies to the connection between the retaining ring 80 and air nozzle 74 the reference numeral 90 is used in connection with all of the bolts used it is to be understood however that the present invention is not intended to be limited to a particular type of connection means prior to bolting the chamber 24 is filled with grease not shown and where re greasing is required there is also provided a plug 88 extending through the side wall of casing member 14 fig4 illustrates a second embodiment of the cushion sub assembly 10 in this embodiment there is a single annular shaped piston shock pad 94 as an alternative to three annular shaped piston shock pads 46 48 and 50 illustrated in previous embodiments also in this embodiment the plug 88 not shown on fig4 is disposed towards an upper section of the cushion sub assembly 10 furthermore in this embodiment piston wear strip 84 is disposed below the top surface 52 of the piston member 28 the cushion sub assembly 10 provides a means of significantly reducing vibrations resulting from downhole hammer type drilling which are typically carried up through to the rotary.