Grover 502BC Roto-Grip Locking Rotomatic Tuners, 3-Per-Side, Black Chrome The newest addition to the Grover family, these locking machine heads utilize the famous 18:1 Rotomatic design with a thumb screw mechanism which locks the string into place. Insert the string through the post hole, wrap around once, then lock by tightening with the thumb screw. Perfect replacement for the most standard machine heads. Includes all mounting hardware. 3 Treble and 3 Bass, Set of 6.