Abstract: An assembly, apparatus, and method relate to a downhole motor drive shaft assembly drilling a subterranean wellbore. The assembly includes a drive shaft having a longitudinal rotational axis, a plurality of drive key sockets, an end housing having a longitudinal rotational axis, a plurality of circumferentially spaced axial keyways, and a concave spherical thrust bearing surface, and a drive key operatively connecting each drive key socket with a respective axial keyway for transferring torque from the drive shaft to the end housing. The drive key includes a substantially planar drive face slidably engaging the axial keyway, and a radiused, cylindrical back portion rotatably engaging the drive key socket, the substantially planar drive face and radiused, cylindrical back portion forming a nominal half-circle.