Neck Evaluation and Truss Rod Adjustment
The truss rod makes the neck stronger against the tension of the strings in electric and acoustic guitar (with Steel strings). The truss rod adjustment is as easy as loosen or adjusting a nut.
The most common causes of why we should do this neck evaluation are: changes in the strings caliber, free buzzing sound, the “action” is too high (the string are far away from the fretboard), some notes are not clear, or the instrument has tuning issues despite a perfect bridge calibration. With truss rod adjustment and evaluating the action, relief, straightness of the neck, you will be on the way of getting your instrument to be executed on the greatest way.
Seeing and adjusting in the playing position
The first must-do thing is carefully looking at the neck. Stick your nose to the end of the headstock and use all the straight borders that you can find along the neck. You will be able to see straightness, curves, top arch or down arch. If the curve is upward, push the neck against string (frontward). If the curve is downward pull the neck backward. Check both directions (up and down) until you are able to see the difference.
All necks are adjustable adjusting o loosening a bolt. This bolt may be located at the top of the headstock. Les Paul guitars hide the bolt behind a cover meanwhile the last Stratocasters have them semi-covered or in the other side hidden in a hole inside the body. In old Stratocaster guitars, you can find it at the end of the fretboard. In some acoustic guitars, you can see it through the soundhole. Find the bolt in your guitar and choose the right tool to calibrate it. Usually you should use a Hex key.
Adjusting the bolt (clockwise) you will straighten up an elevated curve decreasing the reliefe. Loosening it, you will get a low curve that separates itself pulling the strings outside.
Cover a Surface with paper or cloth so you don’t damage your guitar finish and let it lay on its side in the position you would play it. Place it against the light for the ruler test. Place the ruler on the neck between the third and fourth string. Place it from the 1st to the 17th fret covering most of the fretboard. Hold the ruler with one hand while you are adjusting the bolt with the other hand. Look the light under the ruler.
If the ruler oscillates on the neck caused by a low curve, loose the bolt (counter clockwise), looking that the neck is approaching the ruler. If the strings are pulling the neck upward in a wide curve, adjust bolt (clockwise) till you see the ruler resting on all the frets. If you loosen up the bolt at that moment, the neck will come back to a position with a little curve. That slight curve is controlled and it’s called “relief”. Remember how it looks. When you adjust the bolt, it’s better first to totally loosen it up and then adjust it slowly until you feel that it’s beginning to change the curve without forcing it. From this point, a quarter of half turn usually straighten up a neck against the tension of the strings. It would be rare that you must give a full turn to the bolt. A cracking noise will make you notice that you’ve reached the limit and you must stop.
Adjusting too much may ruin the bolt or the truss rod, or both! It’s a good idea to lubricate them before adjusting, especially if it’s been a long time since the last time you don’t calibrate the neck.
Understanding the relief
The relief is the key for strings close to the neck, few buzzes and a comfortable action. If you push a string, the biggest movement occurs in the center of the string length. Loosening the truss rod, the neck gets more relief, allowing a bigger space for the vibrating portion of the string between the pushed fret and the bridge. The relief can also eliminate the buzzing of strings on the first frets where the strings are lower. The different amounts of relief are necessary depending on the guitarist stile and the strings caliber, scale length, and action setting. Try to have the experience adjusting the relief inside and outside the neck playing the guitar on each adjustment just to feel the effect.
You may need to increase o decrease the height of the bridge after calibrating the neck. In fact, a professional could calibrate the neck and the strings height simultaneously.
Not all guitars need relief. In fact, I always try to get a guitar play it with the neck perfectly straight and then I usually add a little relief to get rid of the fret buzzing. The straight necks have low and fast action. Necks with too much relief are harder to play a little heavy at the middle of the neck. With too much relief they can buzz on high notes. It’s important to understand how the relief is related first with the complete neck and then, in second place with the top of it.
The relief is measured inserting a gauge on the top of the fret and the lower part of the ruler in the lowest point of the relief. Usually it’s founded at the middle of the neck between the nut and the assemble point between the neck and the body (around 7th and 8th fret). The specifications for relieves are listed in another section but it is from .004” to 0.12”.
The best instrument for checking the relief is an 18 inches’ ruler that reaches at least the 15th oy 17th fret. If you don’t have a ruler, you can use any string as one keeping it pressed, in this case, on 1st and 17th fret.
This technique is also used for fixing the strings height. With the capo you can eliminate the nut height and with one hand you can press the strings and with other one you measure the relief and adjust the truss rod.
Study the following picture to understand how a perfect neck looks. The relief must disappear gradually while the ruler goes near the body. There should never be a curve on the ends of the neck. It’s ok if the last part drops slightly. If the superior part of the neck called “tongue” arises instead of being straight or drop, you have a symptom called “rising tongue”.
The relief should be measured with a long ruler to consider the whole neck and then be checked again carefully from the 7th, 8th or 9th fret. If the area shows a hole where it should be straight, then, you won’t be able to measure the real relief because of this “second relief”. Many necks have this problem and the only solution is to do some repairing work and not adjustment.
This problem is easy to solve with a notched ruler to fit in the frets, allowing to read the fretboard instead of the frets (they could be wasted in some areas and not in others giving a bad reading of the neck)
After adjusting the relief of your neck, place a ruler on the neck and slide it towards the body. You should feel that it’s falling fret to fret on its movement because each fret should be a little higher than the next one. The at the 7th and 10th should feel the level being straight the rest of the way. As we said before, it’s ok if you feel a little drop around the 15th fret. This fall make clean high notes possible.
Note: if the ruler is too long it can fall in the pickup or bridge areas while sliding it. That’s why you need different lengths of rulers.
At the beginning it may sound pretty easy, but while you’re doing it the results may be very complexes. The different kinds of truss rods with the different qualities and kinds of wood used produce different results. Even adjusting the necks of two identical instruments may have different results.
If you notice that something isn’t going right, or you have any doubts, look for a trained person and pay for the adjustment, ask if you can watch. A trained hand knows immediately if the truss rod is working fine or not.
You should know that some guitar has two bolts, each one on each side of the neck, and others has to truss rods, which makes everything more complex.
I recommend that before adjusting your guitars truss rod (truss rod adjustment), look carefully and make a diagnose of it. Do the same thing with your friends guitars. That way you will be training your eyes and hand to choose which is going to be next step.