31 indicated by the arrow in fig3 5 is counterclockwise when viewed in axial elevation i e a left hand twist see fig1 based on the convention that the drill is rotated in a clockwise direction the surface of pumping chamber 31 when view in axial cross section may define a tear shape or pear shape having a continuously curved perimeter so as to minimize the creation of stress concentration points that might otherwise result in fracture of lip 32 or destruction of the wallcake or mudcake lining the borehole the pumping chamber is characterized by having an undercut portion with respect to the surface of the drillstring component so that lip 32 is formed to overhang the pumping chamber as shown in fig6 in the preferred embodiment configuration the pumping chamber when viewed in axial cross section defines a continuously curved volute having at least two portions with different radii of curvature referring again to fig6 pumping chamber 31 is comprised substantially of two portions having radii of curvature 34 c 34 and 34 d 34 the precise configuration of the pumping chamber axial cross section is not critical provided that the radius of curvature of portion 34 d 34 of the volute is substantially smaller than that for.