member 100 having a diameter slightly smaller than the diameter of threaded hammer assembly end 11 so that member 100 requires stretching for installation and it will thereafter contract into sealing position in undercut portion 90 member 100 has a flexible flap portion 102 that seals orifice 96 and flow groove 98 when percussive fluid is not flowing seal 100 also prevents back flow of debris from hole annulus 95 into hammer assembly 5 when percussive fluid is not flowing during the flow of percussive fluid flap 102 moves out of sealing contact permitting the ejection of water from backhead body 7 the pressure difference that exists between annular passageway gallery 80 and drillhole annulus 95 accelerates heavier material through orifice 96 separator 6 is slip fit into body 7 and pinned from radial rotation by indexing means 110 i prefer indexing means 110 to include a spherical ball bearing member 112 force fit into a cavity 114 in separator sidewall 68 so that at least a portion of member 112 extends out of cavity 114 to slidably fit into one of a plurality of axially extending indexing grooves 116 machined into sidewall 20 of backhead body 7 in assembly the separator 6 is indexed over a groove 116 and slip fit into body 7 by positioning undercut 82 adjacent to different sized orifices fig3 b and c the amount of flow from backhead 7 can be preselected to change the flow one merely re.