This is the 41.5mm Flange Ring which is required for attaching any of the Belt Cutters or Multi Cutters, Master and Basic, to the Fein Angle Grinder. This ring also fits other 4½” grinders with a 41.50mm flange. Other manufacturers grinders have flanges that are 42.50mm and 44mm, both of which are included in every set. Please call or check beforehand. If you own a Fein, you’ll definitely need this flange.