is vertical referring now to fig1 a bottom hole assembly 10 comprises a boring device or bit 11 and an entrenching powering device or mud motor 12 having its forward end carrying the bit 11 a drill string 13 is coupled to a trailing end 14 of the mud motor 12 in a conventional fashion the mud motor 12 as shown in fig2 a 2 d includes a hollow cylindrical bearing mandrel 18 having a central through bore 19 the bit 11 is coupled to a bit box 21 formed in the forward end of the bearing mandrel 18 thus the bearing mandrel 18 is enabled to drive the bit 11 in rotation and to transmit thrust from the drill string 13 adjacent its forward end 22 the bearing mandrel 18 is rotationally supported in a lower tubular cylindrical housing 23 by a set of radial bearings 24 a conical shoulder 28 of the bearing mandrel 18 is received in a conical bore 29 of a radial ring 31 a radial face of the ring 31 is arranged to abut an adjacent one of the set of radial bearings 24 male threads 36 of the lower or forward housing 23 couple with female threads 38 in a forward end 39 of an elongated hollow circular outer housing 41 sets of thrust bearings 44 46 are assembled on a carrier nut 47 at.