Abstract: A rotor adapter for connection between a drive shaft having eccentric motion and a driven shaft having circular motion within a downhole mud motor. The rotor adapter has two symmetrical ends on either end of a rotor adapter shaft. Each end includes a coupler defining a socket for receiving one end of the rotor adapter shaft. A thrust ball is located between the rotor adapter shaft and the coupler and seated in pockets that have radii larger than the thrust ball and depths shallower than the radius of the thrust ball. Circumferentially spaced pockets around the rotor adapter shaft near the end of the shaft fit steel balls which protrude into complementary pockets on the coupler. The coupler pockets are cylindrical and plugged from the outside with plugs. The plugs are secured in place with the steel balls in the complementary pockets. The pockets on the rotor adapter shaft are elongated in the axial direction of the rotor adapter shaft to allow tilting of the rotor adapter shaft.