Truss rod/neck bow adjustment
Contrary to what many people believe, a dead straight neck is not the most desirable aspect of an instrument set up for playing. Due to the distance a vibrating string moves (deflection) the neck requires a small amount of upward bow to prevent the strings from buzzing on frets. Adjusting the balance between string tension (which bows the neck upwards) and the truss rod resisting (or assisting) this pull, the player can have control over the playability of the instrument.
This guide was written from the perspective of setting up a fast-playing instrument with a low setup such as an Ibanez RG/JEM or other similar instrument. Different players and their respective differing styles may require marginally different measurements dialling in to those quoted.
Firstly, the neck needs checking as to whether or not it has a suitable amount of forward bow. This is done by placing a steel straightedge (or similar item with a dead straight edge) lengthwise down the center of the fingerboard with the guitar tuned to pitch and in the player's position.
"Player's position" is how the guitar would be oriented if it were sitting in your lap for playing. If you try this procedure with the guitar flat on its back or other orientation, the neck may not be in it's natural position. Gravity acting on the mass of the neck and the headstock can cause it to bow marginally into a different position which throw off the measurements you are trying to gauge.
Ensure that one end of the straightedge is touching the center of the first fret and the other is touching the center of the last fret.
Now take a feeler gauge (you can purchase one of these at most automotive stores) and check the clearance at the 7th fret. If there is less than .005"/0,13mm clearance, the truss rod will need loosening in order to reduce its resistance to string tension, thereby increasing the amount of neck relief present. If there is a larger clearance then the opposite is true; the truss rod will require tightening to increase resistance and decrease neck relief.
Tightening or loosening of truss rods should only be carried out in small steps. Sensitive truss rods can sometimes require less than a fraction of a turn, plus it can take time for the wood in necks to move into a new shape. A rushed setup may yield the correct clearances initially, however necks may continue to move over a longer period; hours normally. Patience more than pays off when dialling in the perfect setup!
Should the clearance be too small, the neck is too straight. If you are unable to locate the straightedge over the first and last frets, the neck is in fact bowed backwards. Both of these situations require that the truss rod nut be loosened by turning it counterclockwise. Do this gradually in increments of 1/10 of a turn and recheck the clearance each time.
If the clearance at the 7th fret was more than .015"/0,13mm you will need to tighten the truss rod by turning the nut in a clockwise direction. Remember to move it only in 1/10 increments as it can move significantly with each adjustment and recheck the clearance each time after the neck settles.
If your truss rod is a single-acting type such as those found on Gibson guitars (where the adjustment nut is threaded onto the rod as opposed to welded) or vintage Rickenbacker-style rods it is possible that these steps may not correct all necks. If you are attempting to adjust out a back bow (the neck is bent backwards into a convex shape where you cannot seat the straightedge onto the first/last frets) and adjustment leaves the truss rod nut loose (string tension alone does not induce forward bow) the neck will likely require professional adjustment or more risky methods of forcing the neck into shape. By all means head over to the forums for advice if this is the case.
Below you will see pictures of the common types and where the truss rod nut is located.
On many guitars you will find the truss rod nut located underneath a cover on the headstock.
View of a standard Allen wrench style adjustment truss rod.
Heel end truss rod adjustment
For some guitars you will find the truss rod adjustment on the other end of the neck which may mean you will have to remove the strings and take the neck off to make an adjustment. This does mean that you will not be able to make the adjustments with the neck under string tension or in the player's position. Plan adjustments ahead before removing the neck!
If you do not have a straightedge to help you check the neck relief you can either use a capo at the 1st fret and manually fret the strings at the last fret or have a friend hold down the strings at these frets. If you do not have a set of feeler gauges you can use a thin piece of cardboard such as a playing card to measure clearance under the outer strings at the 7th fret. The card should barely slide under the string without lifting it, so adjust your neck to the proper setting from the results you observe.
Pictured above are the two most common style of truss rod adjustment tools an Allen Wrench on the left and a barrel style socket wrench on the right, If your adjusting the neck on say an older style Strat neck you may need to use a flat bladed screwdriver instead.
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