as in the same sense of rotation and in opposite senses of rotation optionally the drill means may also have a single rotary or rotary percussion drive acting upon the outer pipe and rotating the outer pipe in the embodiment of fig1 and 2 a generally tubular outer pipe end piece 12 is screwed to the front end of the outer pipe string 10 through a threading 13 this outer pipe end piece 12 has a conical portion 12a in which its wall thickness increases outward and to the front namely to 1 5 times the wall thickness of the outer pipe string 10 the inner width in contrast thereto is equal over the entire length of the end piece 12 to the inner width of the outer pipe string 10 at its front end in the region of its greater wall thickness the outer pipe end piece 12 is provided with tabs 14 projecting axially parallel and respectively defining windows 15 these windows 15 are rectangular apertures in the wall of the outer pipe end piece 12 i e they are portions in which the wall is completely removed the apertures forming the windows 15 terminate freely at the front i e borehole side end of the outer pipe end piece 12 since also the rectangular tabs 14 project freely and are not integrally interconnected at their ends in the present embodiment four tabs 14 are evenly distributed along the circumference.