This filter clamp is designed to support our P-1 and P-3 towers which are 3.750" outside diameter.  This size is most often used only on the filter towers that are black in color, or that measure 3 3/4" OD.  This clamp will NOT work on BAUER and MAKO filter towers that are 4" OD. (see our part number CLAMP-4000 for 4" clamps).  One set includes 2 clamp halves and a screw and nut.  Two sets are generally required for proper support.