Part Two: Truss-Rod Adjustment
It’s important to understand the role the truss-rod plays in affecting the action. The the primary function is counteracting string pull to relieve or add bow to the neck under string tension. The ‘action’ (all the parts that anchor the strings, divide their length, and apply tension to them) work together in a balancing act that, when carefully adjusted, keep the instrument playing optimally.
Truss-Rod Lore: The function of the truss-rod has been the subject of some debate. Basic physics dictate that “a string of a given diameter and material at pitch is always at the same tension”. Some would suggest that the when the truss rod is adjusted to counteract the strings pull, which finely controls the amount of relief at the middle of the strings length, this adjustment has no other effect on playability or tone.
Another school of thought, which we at ModGuitar subscribe to, suggests that wood and strings and the other parts of the instrument being imperfect leave room for fine tuning, and while under tension are reactive. Such that subtle adjustments can make a neck feel springy or stiff, thereby affecting the ‘feel’ of the instrument.
It will be your quest to experiment (carefully!) and come to your own conclusions. Please be very careful when making truss-rod adjustments, particularly on older more fragile instruments. It’s very easy to accidentally strip the adjustment parts or your tools, and at the worst, snap off the end of a truss rod which can leave the instrument unplayable.
Before making truss-rod adjustments first measure the string relief. Relief is the amount of space between the bottom of the string and the top of the fret.
Relief is the bow in the middle of the neck under tension set to compensate for the nature of a strings vibration which expands in the middle of it’s length in an elliptical pattern when plucked. This is easily observed by simply plucking a string and looking closely at it with a strong light behind it.
Tools required for Measuring neck relief, and Truss-Rod adjustment
A capo to hold down the string at the first fret.
Feeler Gauge Set – the gauge of .010″ is a good starting point for a medium action.
Truss Rod Adjustment Tool (usually a hex or Allen key for Fender’s, and a small adjustment wrench for Gibson’s)
Truss Rod adjustment, beginning with measuring neck relief
The Procedure For Measuring Relief.
With the guitar fully stung up and tuned to pitch.
- Put the capo on at the first fret. Use the feeler gauge leaf that says .012″/.305mm (this is a medium amount of relief for an electric guitar strung with 10s). With one hand, press the string down at the highest fret – being careful not to push down too hard which can make the string deflect upward.
- Slide a feeler gauge over the 7th fret and under the string. This is a little tricky, and requires a steady hand. The idea is to just barely brush the bottom of the string while sliding the gauge as evenly as possible over the top of the fret. Practice will give you a ‘feeling’ for it.
- If there is no contact between the feeler gauge and the string, or the relief is lower move up or down in the feeler gauge range until you’ve pinpointed how much relief is present. There are fancier ways of measuring and adjusting relief, but this one is simple and as accurate as you need.
- Once you’ve established how much relief your instrument has, you can then adjust the truss-rod to dial in the amount that correct amount. Make all truss rod adjustments with well fitting tools and in small increments (1/4 turns). If you find that you have forward bow (no relief) or back bow (to much of relief) your neck may be warped or the truss rod already broken.
- A feeler gauge of .012″/.305mm represents a medium amount of relief. Speed players, preferring a light and low action might prefer .008″/.203mm, or even a flat neck. Players using heavier strings, acoustic guitars, to facilitate a strumming style of playing might want .014″/.356mm to.016″/.406mm.
- You may need to make small adjustments, let the instrument rest, then make further fine adjustments.
Most set-neck guitars use a threaded nut for truss-rod adjustment which is located under a plastic cover at the head stock end of the guitar. Bolt on necks usually have a hex nut that is adjusted thru a small opening at the head-stock or neck heel (heel end adjustments may require removal of the neck for adjustment). Make sure you have the correct size tool for the job.
Some Luthiers prefer to slacken the strings to make truss-rod adjustments, particularly on vintage instruments. Unless you need to make major adjustments, you can carefully adjust the truss rod under full tension for most newer guitars.
Making Truss Rod Adjustments
- The truss-rod for most instruments tightens (flattens the neck against string tension) by turning clock-wise. Bow is created by loosening the truss-rod counter-clockwise.
- Hold the neck firmly or on a workbench. You can also set the guitar standing up on the floor, held steady with your legs. Carefully turn the rod a 1/4 turn at a time. The motion should be firm and fluid – You may feel some resistance if the parts have settled over time. If you find it hard to turn, or it seems to offer a lot of resistance, or does not seem to effect the amount of relief -STOP-, there may be something wrong with the truss rod. Never force the truss rod, it should turn easily. if the truss rod is not easily turned or adjustments seem to have no effect the instrument will need to be taken to a luthier for repair.
- Make small adjustments, pausing to let the insturment rest, check the tuning, and take measurement of the relief.
You’ll find that as you adjust the truss-rod the relief changes in a measurable manner. You will be able to set it to your exact amount using the feeler gauges. Sometimes a guitar won’t seem to have enough range of adjustment or one of the other problems outlined above. That’s when it’s time to take it in and let a pro see what they can do for you.
Truss Rod Tips And Conclusions:
It’s a good idea to recheck your truss-rod adjustments after a day or two. Don’t be tempted to spray WD40, or other lubricants or oils into or around the truss rod. These chemicals can weaken the wood and/or degrade the finish and glue seams holding the neck together. If the truss rod nut is removable, a tiny amount of lithium grease or Vaseline can be applied to the thread of the nut.
Seasonal weather changes may require minor adjustment Spring and Fall for optimum playability.
Truss rod adjustment is easy to do yourself, but it does take some awareness and a careful hand. Like many guitar repair and modification tasks, experience is the best teacher. If you’re leery about making these adjustments, take it to a pro and ask them to show you how they make the adjustment.