Rolling element bearings capable of both axial and radial restraint are readily removably mounted on the shaft of each screw rotor so as to allow a clearance type radial fit between the bearing and shaft. A washer engages the inner race of the bearing which also engages a shim. A bolt squeezes the inner race between the washer and shim to secure the bearing in place while the shim thickness/height determines the position of the rotor within the casing.