centralizer 16 includes one or more mud passages 19 in the annular space between the mwd too 14 and the casing 12 to enable free flow of drilling mud during drilling operations fig2 b shows an end view of the centralizer 16 of fig2 a the mud passages 19 may be formed by having the central passage for the mwd tool 14 to be within a center ring 19 a formed from the material used to make the centralizer 16 the center ring 19 a may be disposed inside an outer ring 19 c by ribs 19 b extending from the center ring 19 a to the outer ring 19 c the outer ring 19 c may be threadedly or frictionally retained in position inside the casing 12 the mud passages 19 result from the unfilled space between the ribs 19 b the centralizer 16 may be machined molded or cast from a single component into the form shown in fig2 a and 2b thus minimizing the manufacturing cost one example of the lower centralizer 18 is shown in cross section in fig3 a and in end view in fig3 b the lower centralizer 18 can be substantially similar in material and in structure to the one or more middle centralizers explained above with reference to fig2 a and 2b the lower centralizer 18 preferably includes a key 21 or similar feature disposed in the central passage such that when engaged with a mating feature not shown in fig3 a and 3b on the exterior of the mwd tool 14 in fig1 the longitudinal position of the mwd tool will be fixed and the rotational orientation of the mwd tool with respect to the casing 12 in fig1 will be fixed and known thus it will.