Bell & Gossett bronze impeller for B&G Series 1531 pumps with 1-1/4" shafts, size 4AC. Also fits Series 1510 4AC pumps. Impeller diameter must be specified when ordered as this impeller is trimmable. Impeller diameter will be found on the pump's I.D. tag. Impellers are installed on the end of the pump shaft, which spins the impeller in turn pumping water, or other liquid, throughout your system.