The Tune-O-Matic bridge is a fixed bridge used on electric guitars, typically those made by Gibson. Fixed bridges permit the use of the tremolo arm, but the upside of using a fixed bridge is that there are fewer moving parts and adjustments are typically a lot more straightforward. The bridge controls the length and height of the string. The former, if incorrectly set, will cause poor intonation. The latter can impede your playing if too high and create an irritating buzzing sound if too low. Therefore it’s essential to adjust the Tune-O-Matic so the string length and height are correct.
Plug the guitar into an electric tuner and tune it up in the standard way. If the tuner display points left, the string is flat. If it points right, the string is sharp. If it points to the center, the string is in tune. Correct sharp notes by loosening the machine head and correct flat notes by tightening it. It’s essential the strings are in tune when you adjust the string length.
Play fret 12 of each string and monitor the tuner reading. Fret 12 is an octave higher than the open string. On a correctly intonated guitar, the reading at fret 12 is identical to the reading for the open string. The length of the portion of string between saddle and nut determines intonation accuracy. This is called the “vibration length.” Note which strings have incorrect intonation and whether it's sharp or flat.
Tighten or loosen the saddle screw for each string, depending on the intonation. If the intonation of a string is sharp, turn the intonation adjustment screw clockwise. This increases the vibration length and lowers the pitch of the note at fret 12. Make a 90-degree adjustment to begin with, check the intonation again and readjust if necessary. If the intonation is flat, make the same sized adjustment counterclockwise. These adjustments move the saddle, which alters the vibration length of the string.
Slide a feeler gauge between the B string and top E string. Measure the distance between fret 12 and the underside of the top E string. Note the clearance. Measure the clearance between fret 12 and the underside of the bottom E string. Note down the clearance. Although string height, or “action,” is matter of preference, the clearance shouldn’t be less than 1/16-inch, otherwise the string will vibrate against the fret. The maximum clearance shouldn’t exceed 1/4-inch, otherwise it will be difficult to press the strings down.
Loosen the machine heads of all strings so the strings sit slack against the neck. The tension of the strings applies down-force on the bridge, meaning you can’t adjust string height with the strings fully tensioned.
Rotate the thumb-screws on either side of the bridge. To lower the bridge rotate them counterclockwise and to raise it rotate them clockwise.
Check the clearance between strings and fret with the feeler gauge. Make further adjustments if required.
Make further, small adjustments until both strings have the same clearance.
Keep your guitar away from doors, heaters and anything that can cause temperature changes. This warps the wood, which necessitates regular bridge adjustments.