This is the most convenient and versatile solution to changing the bit-hole diameter in all JessEm router plates and lift systems. Molded from glass filled poly-carbonate for long life and durability and featuring JessEm's innovative Tab-Loc™ design. You can install and remove our insert rings with the flick of our two-prong insert ring wrench. This 4 piece set includes 1/4", 1/2", 1-3/8" (with counter bored hole for use with standard template guide bushings), and 2-1/2" diameters center hole sizes.