The Grover® 138 Series deluxe machine heads are mounted three to a plate for the vintage look that has been popular for decades. The stamped back plate provides a sturdy base for mounting vertically or horizontally on the head stock. The housings are securely fastened to the base insuring proper gear and worm alignment for accurate tuning. Bushings and screws included. Hole Size Required: 8.8mm ~ 3/8"