In any setup on a guitar with an adjustable neck, the first step will always be to adjust the truss rod. A guitar neck that is properly adjusted will have a slight amount of relief (not completely flat), which is what allows each fretted note to have a clear shot to the bridge at all points of the neck.
When the neck needs adjusting
With a truss rod that is functioning properly, there are only two things that might require the neck to be adjusted:
- A change in string gauge (higher or lower tension) or
- A change in weather humidity (which can cause the neck to expand or contract).
For instance, a customer who decides to set up a guitar for 12-gauge heavy strings, but was playing 9s on it before, will probably need to have the truss rod tightened to counter the higher string tension. Also, a guitar that has developed high string action in the winter might also need to have the neck tightened to counter the lower humidity levels (and conversely, let out in the summer with higher humidity).
Why adjust the truss rod?
Adjusting the truss rod as it relates to setup and playability is a big deal because it dictates how much space will be under the string in the middle of the neck, as opposed to the string action height at the nut or bridge. Slide players will want a lot of relief since they don’t want the slide to hit the frets when playing, and conversely, shredders will want the neck to be almost flat, with next to no relief, since they want the guitar to play very fast and are not hitting the strings very hard with their fretting hand. Everyone is different and will want the guitar to play for their technique; it is the job of the guitar tech to make the guitar play for that specific purpose and make it work.
In this regard, the truss rod is at the heart of the setup. I have found that .02” of neck relief at the middle of the neck is a great starting point. This is roughly the thickness of a business card. You can use a feeler gauge to find this exact thickness. As long as the frets are truly level, you can make almost any guitar with standard-gauge strings play great with this amount of neck relief.
How to adjust the truss rod
To do this, you will tune the guitar up to pitch and then capo the guitar at the first fret. This capo will take the nut out of the equation and help make for a much more accurate measurement. Then, suppress the 12th fret on the low E string. This creates a situation where the low E string itself can be used as a straight edge. The amount of neck relief in the middle of the 1st to 12th fret span should be .02” and can be measured with a feeler gauge (approximately at the sixth fret). You are measuring the amount of space in-between the top of the sixth fret and bottom of the string when suppressing the low E string at the 12th fret.
If there is very little space or if the neck is back-bowed and there is no space at all, you will loosen the truss rod until there is .02” of space in between the top of the sixth fret and the bottom of the string. Loosening the truss rod is a counter-clockwise motion of the truss rod wrench, and you should re-tune the guitar after every half turn of the truss wrench.
If there is an excessive amount of space in between the top of the sixth fret and the bottom of the string when suppressing the low E string at the 12th fret, you will need to tighten the neck until there is .02” of space. To do this, you will turn the truss rod wrench in a clockwise motion, and you should re-tune the guitar after every half turn of the truss wrench.
Standard vs. custom relief
When you start setting up guitars, it is very helpful to use the .02” feeler gauge as your guide, but the more times you make this adjustment, you will quickly memorize what this amount of space looks like. You can then use this as a starting point when setting up guitars for different players. Some players will want a little more space under the string, and some will want almost none at all, but .02” is a great starting point.
Also, it is important to note that bass guitars will need a little extra relief in the neck, as the strings are much thicker, and therefore have more side-to-side movement when played. For basses you should use .03” as a starting point.