A positive displacement pump (12) includes a rotor (50) having a front face (62) and a rear face (54) positioned in the fluid chamber (22). Each of the faces of the rotor is spaced apart from a corresponding inner wall (28,26) of the fluid chamber (22) to define predetermined clearances therebetween. A shaft (44) is connected at a first end to the rotor to effect rotation thereof and extends through a bore (40) in the housing. A thrust bearing assembly (100) includes an adjusting collar (102) and a thrust bushing (110). The adjusting collar (102) is attachable to the shaft and includes a central bore through which the shaft extends. The thrust bushing (110) 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 (102) is rotatable with the shaft and is in relative rotatable engagement with the thrust bushing. The adjusting collar and the thrust bushing prevent movement of the shaft and the rotor in a direction away from the adjusting sleeve and toward the rear inner wall of the fluid chamber (22) such that the predetermined clearances between the faces (54, 62) of the rotor and the internal walls (26,28) of the housing are substantially uniformly maintained as the rotor rotates.