The interchangeable insert for standard bits features a 1/2" opening. The 4-3/8" OD allows this insert to fit into a standard plate with a 3-11/16" opening. Fits The Following Rockler Router Table Plates: Rockler Router Plate C: Porter Cable 7518; Porter Cable 7519; Porter Cable 7538; Porter Cable 7539; Milwaukee 5625-20; Triton TRA001; Triton MOF001. Rockler Router Plate D: Bosch 1619-1619EV; Freud FT2000; DeWalt 625.