Sturdy plastic inserts can be interchanged to ensure the bit opening is as close to the router bit diameter as possible. Compatible with Rockler Phenolic & Aluminium Router Plates. Inserts have multiple bit openings to match the router bit in use. Set includes 1 x insert with 32mm (1-1/4") opening, 1 x insert with 38mm (1-1/2") opening, 1 x insert with 64mm (2-1/2") bit openings, and 1 x solid insert.