Adjusting a guitar neck is one of the most common tweaks performed on a guitar. Wood being sensitive to atmospheric fluctuations (temperature, humidity, etc), periodic adjustment is generally required to maintain good action. It is a fairly simple procedure, usually well within the scope of the careful guitarist.
Virtually all electrics and the vast majority of acoustics provide for adjustment. Structurally, what is required is a steel truss rod in the neck—essentially a long, slender bolt that runs down the center of the neck beneath the fingerboard. The rod’s purpose is to counter the tension of the strings at pitch. The tension on an acoustic is around 150 pounds (significantly less on electrics). Acoustic 12-strings must contend with upwards of 250 pounds of stress (which explains why many players tune their 12-stringers down a whole tone).
Classic guitars (which have relatively wide and robust necks compared to other guitars, and use nylon strings, which exert much less tension) traditionally have no truss rods. Modern classics have gotten away from this omission, but there are still many brands of classic guitars which eschew the use of a truss rod.
The guitar’s tension rod is secured at one end of the neck, and the other end, bearing the adjustable nut, is at the other. By tightening or loosening the nut, the tension is increased or decreased, allowing the neck to move just enough to be tweaked into optimal relationship to the strings.
The adjustment nut can be at either end of the neck. On Gibsons, Guilds, and many other more vintage designs, the nut is located in the headstock of the guitar, near the tuning machines. Look for a cover plate right behind the string nut of the guitar, or (as in many Fender electrics) you will see the adjustment nut protruding through the neck beneath the string nut.
Other designs place the nut at the high end of the neck, near the neck/body joint. Older Fenders provide the adjustment nut at the high end of the neck, often requiring that the neck be removed from the body to facilitate adjustment. Many acoustics these days have the adjustment nut inside the body cavity near the heel block, visible through the sound hole.
Most new instruments come with the appropriate tool for adjustment. It can be a simple hex wrench (Allen wrench), or a socket wrench (generally somewhere around a 1/4″ socket). Some use a screwdriver. You’ll want to procure the proper tool for adjustment, as otherwise you can damage the adjustment nut, making future adjustments difficult or impossible.
Check the action by sighting down the neck, like you were aiming a rifle. The strings create an absolute straight line to which you can compare the contour of the fingerboard. I like to sight the neck from each end of the guitar to gain a more complete perspective.
The tension of the strings generally cause the neck to want to bow upwards, increasing the distance from string to fingerboard, thereby making the action too “high” (visualize a bow and arrow to grasp the idea here). You’ll want to tighten the rod to counter this tension, straightening the neck back into its optimal relationship to the strings. Move the nut no more than a quarter turn at a time, sighting down the neck after each tweak to determine if enough tension has been applied.
Retune and play the instrument after each tweak, listening for buzzes. Some fret rattle is not unusual in well-adjusted guitars, but too much is annoying and indicates that you have over-compensated, creating a bit of a hump in the neck which causes the strings to rattle against the frets, especially at the low end.
Warning: Do NOT apply too much stress to the adjustment nut. The nut should move fairly easily. Attempting to tighten the rod too much can cause the rod to break, or the threads to strip. You can assist the process by holding the body of the guitar securely on the workbench while you apply some additional pressure on the neck as you tighten the nut. Try some 3-in-1 oil or WD-40 to help lube the threads, but if the nut doesn’t want to move, don’t force it. A broken rod will rattle inside the neck, and is expensive to fix.
Sometimes you will find that a neck cannot be properly adjusted at all. This can be caused by two things:
1) The neck has such an extreme bow that the adjustment rod is insufficient to pull the action back into alignment.
2) The neck is naturally back-bowed – this means that even the tension of the strings is insufficient to straighten the neck, and you’ll end up with a loose nut on the adjustment rod in your attempts to tweak it. If slight, this condition can sometimes be helped by raising the bridge height a bit. But if the back-bow is significant, the remedy is to remove the frets, replane the fingerboard, and install new frets. You could also replace the neck altogether. These procedures are quite possible in theory, but only a very valuable instrument warrants this kind of extreme fix, and should definitely be performed by an experienced guitar technician.
Other weird conditions can include twists in the neck, where one side can be straightened but the other side cannot, or humps in the middle of the neck so that the low end action cannot be made right unless the high end action is sacrificed. Also check for high frets which can cause rattle—these can be tapped down into an otherwise straight neck to eliminate the rattle.
CAVEAT: Adjusting a neck is quite often a simple affair, and one which the inexperienced guitar player can definitely perform. However, be careful! If you are the least bit insecure regarding your ability to perform the tweak properly and safely, don’t push it—take your beloved axe to a professional guitar tech. It’s generally just a few bucks to have it done right. Well worth the money.
Copyright 1998 Jeff Foster. All Rights Reserved.