A zero clearance bearing assembly comprising a housing defining a generally cylindrical bearing chamber with a pair of ball bearing units disposed therein. One unit is adjacent an axial thrust reaction surface of frusto-conical configuration and a shoulder on an associated shaft urges the unit into engagement with the surface. The other unit is press fit on the shaft and also has a shaft shoulder urging it toward the thrust surface. An air impeller on the shaft urges the same toward the other bearing unit and the thrust unit reacts to both the impeller load and an axial preload spring engaging the other bearing unit. One or both of the bearing units is provided with an improved radial preload device comprising a spring and a pair of circumaxially spaced reaction surfaces opposing the same.