Abstract: A rotary lobed rotor pump has externally adjustable wearplates in the rotor casing juxtaposed to the parallel ends of rotating lobed rotors. Resilient members such as Belleville springs are adjusted by compression by means external of the rotor casing so that the springs move the wearplate(s) into a design clearance with the parallel rotor ends. No pump disassembly is required to accurately set the clearance. An internal pressure surge by-pass is provided which permits outward movement of either or both wearplates resulting in an increased gap with the rotor ends causing a decrease in the rate of rise in fluid pressure in the internal rotor casing cavities in effect causing rotor slippage.