Abstract: A fluid pump assembly comprising a circular drive shaft including a flat side, and an impeller hub including an inner surface formed in part by a circular bore receiving the drive shaft and by a generally crescent-shaped keyway adjoining the bore and having opposite ends, one of the ends being blunt, and an arcuate surface extending between the ends. The assembly also comprises a retainer key positioned in the keyway and having a flat surface complementary with the flat side of the drive shaft, an arcuate surface complementary with the arcuate surface of the keyway, and a blunt end complementary with the blunt end of the keyway.