Let’s begin by reviewing what a truss rod is, and what it does. Simply put, a truss rod is a metal rod inside a guitar neck. It lends support to the neck, and makes it possible to adjust its “relief”, or the bowed shape it takes while under tension of the strings. On one end of the rod there is a threaded nut, and adjustments are made by tightening or loosening it.
There are several different truss-rod designs. Some are single-rod designs, and some are double. Some are adjustable at the headstock, others at the heel. Though their mechanisms of operation differ, the effect they have on the neck is always the same. When you tighten the adjustment nut, the neck becomes straighter (less relief). When you loosen the nut it becomes more bowed (more relief).
When you set up a guitar for the first time, it is nearly always necessary to adjust the truss rod. As time goes on, you may need to adjust it again periodically, to compensate for the effects of humidity, temperature, and string tension. The truss rod is just one part of a system of components that all contribute to your guitar’s action and playability. Other parts of the system include the bridge, saddles, and string nut. Signs that point specifically to a misadjusted truss rod include buzzing just in the first few frets, and unusually high action across the middle frets only.
If you think your truss rod might need adjustment, start by checking your neck’s relief. Some luthiers use a straight-edge for this, but I find it easier to use the “capo method”. With my axe strung up to pitch, I place a capo at the first fret. Then I hold the guitar in playing position and use a finger of my picking hand to fret a string up near where the neck joins the body. This turns the string itself into a perfect straight-edge, and I am able to easily gauge the curvature of my neck by measuring the gap between the bottom of the strings and top of the frets.
There is no exact gap dimension that is perfect for all guitars and players. The correct truss rod setting is mostly a matter of playing style and preference. Players with a light touch can get away with lower action and less neck relief. Heavy strummers often prefer just the opposite. Finding a setting that works for you and your guitar will take some trial and error. One thing no player wants is “back-bow”, wherein the truss rod has been adjust so tightly that the neck is actually arching backwards.
I don’t use rulers or feeler-gauges to check exact gap dimensions. I go by sight and feel. Using my free hand, I lightly depress each string, and visually gauge how far it travels before making contact with the frets. I concentrate on the middle area of the neck, around frets 5-10. I like to see the smallest of gaps, no bigger than the thickness of a business card. Any larger than that, and I’ll try tightening the adjustment nut just a bit. If the gap is smaller than that, or if there is not gap at all (indicating back-bow), I will loosen it.
If, after checking your bridge, saddles, string nut, and neck relief, you have determined that you need to adjust the truss rod on your guitar, the first thing you’ll need to do is locate its adjustment nut. Sometimes this is easy, and it will be openly accessible via a small hole near the nut, or a spoke-wheel or side adjust mechanism near the heel. Other times it’s a bit more tricky. Depending on how your guitar is constructed, you may need to remove a truss rod cover, or even remove the neck from the body.
Once you have located the nut, and before you do any fiddling, you can use a fine-tip marker to draw small reference lines on the nut and the channel opening. Knowing that you can always get it back to exactly where you started should things go wrong will help you feel more confident about making adjustments.
Now remove the capo and slacken your strings. As you prepare to make an adjustment, try loosening the nut first by turning it to the left (remember the old adage: righty-tighty, lefty-loosey) a quarter turn or so. It’s extremely unlikely that you’ll damage anything by loosening the truss rod nut just a bit, and moving it in this direction first will give you a chance to get a feel for the amount of force required to turn it. Return the nut to the “zero position” using the reference lines you made.
Based on what you found while examining your neck, you can now tighten or loosen the adjustment nut in small increments (perhaps an eighth or quarter turn at a time) to begin moving your neck to where you want it to be. After each adjustment, use the capo method to check your progress. The effects of your changes will be most apparent in the middle of the neck. If you loosened the nut, you should see that the neck became slightly more bowed, and the gap between strings and frets became bigger. If you tightened the nut, the headstock will have pulled backwards, causing the neck to become straighter, and the gap to become smaller. If you tightened the nut too much, the headstock will have pulled too far back, and the center of the neck will be touching the strings with no gap.
Once you feel the neck is where you want it, remove the capo, retune the guitar, and play it. Check to see if the symptoms that prompted you to adjust the truss rod (buzzing, poor action, etc.) have been solved. Continue this cycle of adjusting, checking, and playing until the neck feels right to you. Also remember that it may take a day or two for your changes to fully settle in. Be sure to check and play your guitar in the days following your truss rod adjustment, and adjust further if necessary.
Adjusting your truss rod isn’t as hard as it seems. Play close attention to what you’re doing, make small incremental changes, and check your progress as you go. Your axe will be playing its best in no time!
Disclaimer: Improper adjustment of a truss rod can damage your neck. If after reading this blog post you still don't feel comfortable adjusting your truss rod, or aren't sure of what you are doing, stop, and take your neck to a professional. If turning your rod's adjustment nut requires an extreme amount of force, stop, and take your neck to a professional.