A tilted drive sub for a mud motor having a straight outer bearing housing, comprising: an outer bearing housing for a mud motor, the outer bearing housing having a top end and a bottom end, an outer diameter and an internal diameter, with internally threaded surfaces (106, 108) at each of two opposite extents thereof for incorporating the outer bearing housing in the mud motor; wherein the internal diameter of the outer bearing housing is on a predetermined center line and the outer diameter of the outer bearing housing is also on a predetermined centerline, and wherein the center line of the internal diameter is offset at an angle from the center line of the outer diameter of the outer bearing housing, thereby producing a region of the internal diameter of the outer bearing housing which is at an angle to the outer diameter, with the outer diameter of the outer bearing housing remaining straight, wherein the center line of the internal diameter is offset at an angle from the center line of the outer diameter of the outer bearing housing starting at a point near the top end of the housing, the said point being well above the imaginary line which bisects the outer bearing housing longitudinally, such that, from the said point upwardly, the threaded internal surface is co-incident with the outer diameter centreline, wherein the offset internal diameter region includes the said internally threaded surface at the bottom end of the outer bearing housing into which components of the mud motor can be screwed such that the offset angle introduces an offset to a conventional bit to be attached to a downhole end of an attached mud motor. an outer bearing housing for a mud motor, the outer bearing housing having a top end and a bottom end, an outer diameter and an internal diameter, with internally threaded surfaces (106, 108) at each of two opposite extents thereof for incorporating the outer bearing housing in the mud motor; wherein the internal diameter of the outer bearing housing is on a predetermined center line and the outer diameter of the outer bearing housing is also on a predetermined centerline, and wherein the center line of the internal diameter is offset at an angle from the center line of the outer diameter of the outer bearing housing, thereby producing a region of the internal diameter of the outer bearing housing which is at an angle to the outer diameter, with the outer diameter of the outer bearing housing remaining straight, wherein the center line of the internal diameter is offset at an angle from the center line of the outer diameter of the outer bearing housing starting at a point near the top end of the housing, the said point being well above the imaginary line which bisects the outer bearing housing longitudinally, such that, from the said point upwardly, the threaded internal surface is co-incident with the outer diameter centreline, wherein the offset internal diameter region includes the said internally threaded surface at the bottom end of the outer bearing housing into which components of the mud motor can be screwed such that the offset angle introduces an offset to a conventional bit to be attached to a downhole end of an attached mud motor.