Made from 356 heat-treated cast aluminum, tougher than cast iron. Oil impregnated bronze bushings with extra long guide ways for accuracy and long life. With two 2-1/8″ spur bit guide holes and extra long backset blades. Adjusts for 2-3/8″, 2-3/4″, and 5″ backsets. The maximum thickness that can be drilled using this jig is 2-1/2″. The minimum is 1-1/4″.