Abstract: A positive displacement pump and thrust bearing assembly having a housing defining an internal fluid chamber having front and rear inner walls. A rotor having a front face and a rear face is positioned in the fluid chamber. Each of the faces of the rotor is spaced apart from a corresponding inner wall of the fluid chamber to define predetermined clearances therebetween. A shaft is connected at a first end to the rotor to effect rotation thereof. The shaft extends through a bore in the housing. The thrust bearing assembly includes an adjusting collar and a thrust bushing. The adjusting collar is attachable to the shaft and includes a central bore through which the shaft extends. The thrust bushing is secured within the bore of the housing by a locking ring that prevents longitudinal movement of the thrust bushing toward the fluid chamber. The adjusting collar is rotatable with the shaft and is in relative rotatable engagement with the thrust bushing.