No single part of a guitar is more terrifying or mysterious to many guitarists than the humble truss rod. For years we have been told that we can ruin our precious and fragile guitars by just looking at them funny. But truss rods are very simple devices and are nothing to be afraid of. So let’s dispel the fears and the myths.
What is a Truss Rod?
The truss rod is a thin steel rod fitted into a curved channel within the neck of electric and acoustic steel-stringed guitars and basses (see the diagram below). Its purpose is to help counteract the forces on the neck from the tension of steel strings. Without a trussrod, the headstock would be pulled forward, bowing the neck and affecting string height and playing comfort. The truss rod can be adjusted with an adjusting screw or nut located either at the headstock or the body end of the neck. When we tighten the adjusting nut, the truss rod is pulled by the nut, putting it under tension. This causes the truss rod to try to straighten within the curved channel, pushing the middle of the neck upwards and bending the neck backwards, countering the forward pull of the strings.
At the low string heights favoured by modern guitar and bass players, a small amount of forward bow is useful. This forward bow is known as “neck relief“. Note: this is the geographical definition of “relief” as “difference in height from surroundings”, not relief as in “relaxed” or “relieved of stress”). Without neck relief the strings may not vibrate freely without making contact with other frets. That unwanted contact with frets causes a stinging or buzzing noise (sometimes called “fret buzz” or “string buzz“). By tightening or loosening the adjusting screw of the truss rod, we can adjust the amount of neck relief, which helps us to avoid fret buzz without the strings being unnecessarily far from the neck.
If neck relief is too low or flat, we risk buzzing, particularly on the lower frets. With too much neck relief can raise the string height over the middle of the guitar neck, which if compensated by lowering the saddles, could result in buzzing on the highest frets.
Measuring Neck Relief
To accurately measure neck relief, you’ll need a capo and feeler gauges. Start with the guitar tuned to pitch and hold it in its normal playing position. Do not lie the guitar on its back (I’ll explain why in another post all about gravity). Use the capo to hold the strings down at the 1st fret. Then press the low E-string down at the fret closest to the neck/body joint (sometimes called the “body fret”, usually around the 15th or 17th fret, depending on the guitar). If there is any forward bow in the neck, there will be a small gap between the string and the frets. Keeping the string pressed to the body fret, use the feeler gauges to to measure the size of that gap where it is largest, which is usually at the 7th or 8th fret.
For reference, an acceptable range of neck relief runs from almost dead flat (i.e. no relief) up to about 0.5 mm gap (.020″). Factory specification for most of the major guitar manufacturers will fall within this range, for most players 0.2 mm to 0.35 mm gap (0.008″ – 0.014″) will work just fine.
If you don’t have feeler gauges, you can use short cuttings from a 9, 10, 11, and 12 gauge strings (Tip: bend the string piece into a U-shape and glue it to a lollipop stick or piece of wood to make a handy re-useable gauge). Failing that, a standard business card is about 0.3 mm (0.012″) and can be a useful guide – if the card fits easily between the string and the fret with contact perhaps reduce the relief slightly, as most players won’t need any more neck relief than that.
Despite what many people will tell you, there is no right or wrong where neck relief is concerned, provided there is no excessive string buzz. Some people prefer a perfectly straight neck while some people like a lot of relief (e.g. some country or slide players who also prefer high action). The condition of the frets also has an influence on neck relief – worn frets or poor fret work will result in inconsistent fret height up and down the neck, which may need more relief and/or string height to avoid string buzz in some playing positions.
It is important to realise that our own unique playing style and picking has a big influence on how much neck relief is required. Factory settings, or your favourite guitar player’s setup specs, are only a guide and may not suit you. See my post “Your Own Personal Fretbuzz” for a more in-depth discussion.
Adjusting your Truss Rod
While the location of the truss rod nut may vary from one guitar to another, the method of adjusting it is always the same and very simple. Hold the guitar with the truss rod’s adjusting nut facing you and the neck pointing away from you. To add relief and curve the neck more, loosen the truss rod by turning to the left (counter-clockwise). To straighten the neck and have less relief, we tighten the truss rod by turning to the right (clockwise). The rule of thumb is very simple and applies to almost any standard screw, nut, tap, faucet, or valve: left is loose, right is tight!
Single- and Dual-Action Truss Rods
Truss rods come in two ‘flavours’. The first, which we described above and is the most common, is sometimes known as single-action or one-way truss rod and, as mentioned above, bends the neck against the forces from string tension. The second type of truss rod is known as a dual-action, two-way, or bi-flex truss rod, which works just like the one-way rod but can also bend the neck in the same direction as the string tension, which allows the possibility of correcting a back-bowed neck.
The good news is that you really don’t have to worry about which is which. Whichever one your guitar might have changes little where guitar set up and adjustment are concerned. The neck relief you’ll need won’t change, the direction we turn the adjusting screw won’t change, and your adjustment procedure won’t change – tune up, test and measure, adjust, test, repeat as necessary. It’s that simple. And no matter which type of truss rod you have, if you are unable to adjust the neck relief as needed, it’s time to bring your guitar to an expert.
Coming up: Part Two – Myth Busting!
Can you damage the neck by mis-adjusting your truss rod? Do truss rods affect the entire neck? Can taking all the strings off at once damage a guitar neck? Should a neck be left to ‘settle’ after truss rod adjustment? Can you adjust string action with the truss rod? Click here for part two of this article where I answer these questions and bust the most common truss rod-related myths.
Edited 5 Jan 2018: Thanks to Ian for pointing out an error on the page where I confused the terms compression and tension. Much appreciated, Ian!