recessed area 130 preferably forms a circumferential recess along the perimeter of mandrel 40 and between packing support areas 131 132 in such manner recessed area 130 will remain fluidly connected with radial passage 190 and inlet 200 of sleeve 150 see fig6 and 7a mandrel 40 takes substantially all of the structural load from drill string 20 in one embodiment the overall length of mandrel 40 is preferably 52 and 5 16 inches 132 87 centimeters mandrel 40 can be machined from a single continuous piece of heat treated steel bar stock nc50 is preferably the api tool joint designation for the box connection 70 and pin connection 80 such tool joint designation is equivalent to and interchangeable with 4 u00bd inch 11 43 centimeter if internally flush 5 inch 12 7 centimeter xh extra hole and 5 u00bd inch 13 97 centimeter dsl double stream line connections additionally it is preferred that the box connection 70 and pin connection 80 meet the requirements of api specifications 7 and 7g for new rotary shouldered tool joint connections having 6 u215d inch 16 83 centimeters outer diameter and a 2 u00be inch 6 99 centimeter inner diameter the strength and design formulas of api 7g u2014 appendix a provides.