As the weather changes from season to season, the wood in the neck of your bass guitar can bend or straighten slightly. If you know how to adjust the truss rod on a bass guitar, you can counteract those changes and keep your bass sounding great.
Changes in temperature, pressure, and humidity can cause the wood to bend. The change in position can cause the strings to either pull away from the frets or rest against them; this makes playing almost impossible because the strings need space to vibrate freely over the entire length of the neck.
There needs to be a slight gap between the strings and the frets. To determine how much action is enough
Press the E string down at the first fret with your left hand and hold it.
Press the E string down at the last fret with your right hand.
This should open up a gap between the strings and the 7th and 12th frets (about the thickness of a credit card).
If the curvature of your bass neck is not correct, you need to adjust the truss rod. To adjust the action, turn the screw in the truss rod to change the curvature of the neck.
If the gap is too large, insert the Allen wrench or Phillips screwdriver into the screw (located either on the headstock or at the other end of the neck) and then tighten the truss rod by turning the screw clockwise.
If your strings buzz when you play on the first four frets (near the headstock), you need to loosen the truss rod by turning the wrench or screwdriver counterclockwise.
Generally, you'll want to use the small Allen wrench that comes with your bass. If you lose this wrench, you can get another one from your local music store or the bass manufacturer. On other basses, the screw of the truss rod requires a Phillips screwdriver.
Don’t try to force the truss rod with anything that doesn’t quite fit. If you strip the truss rod, it’ll cost you. Turn the truss rod only between one-quarter and one-half of a turn per day. You need to allow the wood to settle before you do any more adjustments.
On some basses, you have to remove the neck from the body in order to reach the truss rod screw. Don’t attempt to loosen the screws at the back of the bass that hold the neck in place without first loosening the tension of the strings. Otherwise, the neck will snap off, stripping away the wood that holds the screws.