Through Bore Bearings are compatible with the REV DUO System and are used with 5mm Hex Shafts recommend for the DUO Extrusion System to provide a low friction rotating shaft support. These low profile bearings fit in any of the 3mm thick brackets with a the 9mm ID bearing seat including: REV-41-1313, REV-41-1303 and REV-41-1304. Use shaft collars to secure the shaft and the bearings.