Grover Titan 4-String Bass Tuning Machines - 20:1 Ratio Self aligning thrust bearings. Special butterfly button. Durable cast housing. End thrust bearings. Worm meshes perfectly with gear. Safety-type string with cutaway shoulders to provide slip-over. Two screws in housing apron provide greater holding power. May be used with all modern bass guitar headpieces. Require 11/16" headstock holes. Individually hand tested. Guaranteed for life. Screws and bushings included.