Abstract: An earth boring bit is shown of the type used to cut a shaft in the earth. The earth boring bit has a central bit body which is adapted to be driven to cut a shaft and which has an outer conical surface thereon. An outer ring segment carried on the central bit body has a conical surface for matingly engaging the central bit body conical surface whereby torque transmitted to the outer ring is transmitted through the mating conical surfaces to drive the central bit body to cut a shaft. The mating conical surfaces are preloaded to assure drivable contact between the surfaces. The effective cutting diameter of the earth boring bit can be varied by interchanging the outer ring segment.