by means of a threaded socket 2 the pipe string itself being omitted from the drawing an outer tube 4 is rotatably mounted on the stem 1 by means of a bearing assembly 3 this outer tube 4 carrying a core cutter or a core drilling bit 5 at its lower end the stem 1 contains a central passage 6 of inside diameter d 1 and is connected at its loer end to a thin walled flexible sleeve 8 which serves as a connecting element the connecting proper being made via a threaded joint 7 the hollow stator 10 of a fluid driven motor 11 connects to this sleeve via a further threaded joint 9 the rotor 12 of this motor is located on the inside of the outer tube 4 the rotor 12 and the stator 10 are helically profiled generally similar to the profile of a worm gear and remain in continuous engagement with each other forming a working chamber 47 the motor operates on the principle attributed to moineau if in a motor of this type one of the two parts is fixed in a manner such that it cannot move radially the other part executes an eccentric orbit since in the present case the mounting of the outer tube 4 by means of the bearing assembly 3 prevents the rotor 12 from moving radially the stator 10 has to describe this orbit the corresponding radial displacement relative.