When a guitar is new, the neck should be set to be relatively flat from the factory, but string tension, humidity, and other factors can cause the neck to bow out of tune over time. The truss rod, which runs down the interior of the neck beneath the fret board, is turned to adjust the guitar neck. Excessive forward bow, also known as relief, can be reduced this way.
Before adjusting the neck of your guitar, keep in mind that many beginners make the mistake of thinking that if the string action is too high, a truss adjustment will solve the problem. Because of the way strings vibrate when strummed or plucked, a guitar neck usually needs some forward relief to keep strings from buzzing against frets. Adjusting the neck into a backward bow to lower action will make the guitar sound worse, not better, if a truss adjustment is unnecessary.
Though a standard formula would be nice, the ideal amount of relief in a guitar neck varies from instrument to instrument and is determined a variety of factors such as string gauge and playing style. Before making any changes, make sure the relief is satisfactory. If it appears to be in good condition but the action is high or the guitar buzzes, a nut, saddle, or fret adjustment may be required instead. Continue adjusting the truss rod if the neck has a lot of bow.
Depending on the guitar model, the truss rod adjustment head can be found in various locations. It’s either under the pickguard or at the base of the headstock on electric guitars, and it may necessitate removing the neck. It is located at the base of the headstock or accessible through the sound hole on steel-string acoustic guitars. A small plastic or wooden plate held in place screws on the headstock sometimes covers the truss rod. Double truss rods are found on many 12-string guitars and bass guitars, and they should be adjusted a professional.
Truss rod heads are designed differently, so you’ll need to get the right tool in some cases before you can adjust the guitar neck. A hex nut is welded to the top of some rods. An Allen wrench is required for many guitars, including Martins and Fenders®, while a nut socket is required for Gibsons and Taylors®. To avoid damaging the truss rod head, use the correct size wrench or socket.
Mark the position of the truss head before beginning the adjustment. Not only will you be able to see where you started, but you will also be able to return to the original setting if desired. With all strings in place and tuned to pitch, make sure to adjust the guitar neck.
The back bow of the neck is increased tightening a truss rod (turning it clockwise). Its forward bow is increased loosening it (turning it counterclockwise). A typical truss head adjustment requires significantly less than a quarter turn.
Regardless of the type of relief you require, begin loosening the truss a small amount and slowly turning it. This shouldn’t require a lot of torque. You might be better off taking the guitar to a shop than risking breaking the truss rod if the truss does not budge with moderate effort.
Slowly adjust the truss as needed, assuming the truss turns easily enough. Allow the guitar neck to settle in between adjustments turning the truss in small increments and checking the relief.
It’s best to take the truss to a shop for advice if it turns easily but doesn’t seem to be affecting neck relief. Because truss adjustments are so inexpensive, the only reason to do them yourself is for the pleasure of personally caring for your instrument(s).