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Before adjusting anything make sure your guitar is strung up correctly and that your neck has the correct amount of relief and is not excessively bowed or warped. If your neck is bowed you first need to adjust the truss rod and check that the nut is good. If your neck is warped it will require a more extensive repair. Also check that the angle of the tremolo unit is correctly set and not floating at an angle. This would require setting up prior to any work on the rest of the instrument. In general it is recommended that all other avenues of instrument setup are checked before resorting to the use of shims otherwise one can easily end up going backwards and forwards finding that adjustment of one things changes those of another! Shimming a neck should be the last resort if all other setup adjustments run out of usable range. Try to imagine the strings of your guitar as a flat plane and the fretboard as a parallel plane running underneath them. The angle of the top plane which contains the strings is controlled by the position of the tremolo unit and the nut. The angle of the lower plane which is the the fret board is controlled by the neck pocket of the body. If your setup is perfect these two planes will have a more or less equal distance between them at any point. If your guitar doesn't look this way try adjusting the height of the tremolo bridge unit first. This will usually take care of the problem unless you find your action becoming too high or too low equally across the length of the fret board. If adjusting the height of the bridge corrects the problem but leaves you with too high or low of an action (distance between the strings and fretboard) or the bridge unit is left excessively high or low then you will need to to use shims to adjust specific areas of the instrument's geometry. Shims are commonly used in two different areas of the neck. One is under the nut and the other is directly under the heel in the neck pocket of the body. Nut shims are usually made out of one or more thin sheets of metal such as brass or steel. Shims located in the neck pocket are usually made out of wood rather than metal as the pressure between the two mating faces can deform the wood of the neck or body. In either case you can produce your own shim by using a sheet of paper, a business card or preferably a slice of hardwood veneer such as Maple. For shims in the neck pocket you might need to fold or layer paper stock 3-4 times to get the required thickness needed then trim to fit properly. Softer cardboard stock may compress in use creating a thinner shim than expected. A nut shim acts as a spacer between the nut and neck raising and lowering the distance of all of the strings at the headstock end. A neck pocket shim acts as a spacer between the neck and body, changing the angle from which the neck protrudes out away from the body. First determine if the distance between the strings and fretboard is too close either at the headstock end of the neck and remedy this if so. This can be determined by fretting the strings at the 3rd fret (or fitting a capo) and measuring the clearance between the first fret's crown and the strings. In the case of the string clearance being too low under the first fret, progressively add shims under the nut until a clearance of at least 0.005"/0,13mm is achieved with the strings fretted as described. You can now fret strings at the first fret (or move the capo here) and adjust the bridge height until the strings are a more equal distance from the fret board down the entire length of the neck. If adjusting the distance between the strings and the fretboard at the body end requires an excessive correction in bridge height you can place shims in the neck pocket to create a more appropriate neck angle and correct this problem. If the strings are higher on one side or the bridge sits at an uneven angle side-to-side, placing a shim in the neck pocket parallel with the length of the neck on the respective side raises the entire neck down that side when the neck is reattached. It is important to check that the neck does not possess any kind of twist or warp as this cannot generally be corrected through simple adjustment/shimming and will require professional repair. If the bridge is set too low in the body a shim can be fitted at the back end of the neck pocket (the end nearest to the bridge) to increase the neck angle. The opposite approach can be taken if the bridge is set too high on the body. A slice of veneer cut to cover the entire surface of the neck pocket can be progressively sanded thinner at one end to achieve a more permanent angled shim however creating layered paper shims is often more than adequate. In some instances you might find that you need the shim to raise only one corner of the two planes as described above. In these cases make a smaller shim and place it in the appropriate area of the neck pocket. Of course upon removing the nut from the neck or the neck from the body, if you find a shim already there determine what action it was doing in the first place then make the necessary corrections using as few shims as possible.
Contrary to what many people believe, a dead straight neck is not the most desirable aspect of an instrument set up for playing. Due to the distance a vibrating string moves (deflection) the neck requires a small amount of upward bow to prevent the strings from buzzing on frets. Adjusting the balance between string tension (which bows the neck upwards into "upbow") and the truss rod resisting (or assisting) this pull, the player can have control over the playability of the instrument. This guide was written from the perspective of setting up a fast-playing instrument with a precise low setup such as an Ibanez RG/JEM or other similar instrument. Different players and their respective differing styles may require marginally different measurements dialling in to those quoted. A little on truss rods.... In their most basic form, a truss rod is designed to add stiffness (or "resistance to bending") to a neck under string tension. Originally, they were simple non-adjustable reinforcement bars set into the neck. Gibson introduced the first adjustable rod which was set into a curved channel. When the adjustment nut was tightened, the rod tried to straighten itself out, taking the neck with it. These single-acting rods are surprisingly effective and reliable when installed correctly and well-maintained. Double-acting rods are a much more flexible device which allow the corrective force of the truss rod to act both against and with the tension of the strings, carrying the neck both ways if required. Other rod and neck adjustment methods exist, however the fundamental purpose is to give the owner (or tech) control over how the neck bends in use. "I heard that a straight neck is ideal...." For the most part, it is! A bendy pretzel neck is no use to anybody. However, it has to be borne in mind that strings need room to vibrate. A surgically-straight neck can produce ultra-low action, which is great until you come to playing it. String buzz is a BIG problem! The ideal neck shape is one that has a very mild curve upwards. That simple small amount of "relief" gives strings room to breathe whilst still allowing low action in the upper positions. Note! This tutorial makes a few assumptions which you need to be confident about checking before using these techniques. Most importantly, it assumes a neck that is well-made and has not warped; that the fretwork is straight and even with no humps and dips beyond the usual curvature of a neck. These are not problems that a truss rod adjustment alone can remediate and should be fixed first. With some single-acting and vintage Rickenbacker-style rods it is possible that these steps may not correct all necks. If you are attempting to adjust out a back bow where the neck is bent backwards into a convex shape (you are unable to seat the straightedge onto the first/last frets) and adjustment leaves the truss rod nut loose (string tension alone does not induce forward bow) the neck will likely require professional adjustment or more risky methods of forcing the neck into shape. Simply, the bow in the neck has also bent the convex truss rod channel straight or beyond into a concave type of curve. Tightening the truss rod in this condition will make the back bow more severe rather than doing what is otherwise expected. By all means head over to the forums or consult a professional tech for advice if this is the case. Inspection Firstly, the neck needs checking as to whether or not it has a suitable amount of forward/up bow. This is done by placing a steel straightedge (or similar item with a dead straight edge) lengthwise down the center of the fingerboard between the 3rd and 4th strings, with the guitar tuned to pitch and in the player's position. "Player's position" is how the guitar would be oriented if it were sitting in your lap for playing. If you try this procedure with the guitar flat on its back or other orientation, the neck may not be in it's natural position. Gravity acting on the mass of the neck and the headstock can cause it to bow marginally into a different position which throw off the measurements you are trying to gauge. If you prefer working on your instrument laid on the bench, that is fine also however it's good to know why an instrument might start acting slightly different once picked up and played normally! Ensure that one end of the straightedge is touching the center of the first fret and the other is touching the center of the last fret. Using a feeler gauge (you can purchase one of these at most automotive stores) check the clearance at the 7th fret. If there is less than .005"/0,13mm clearance, the truss rod will need loosening in order to reduce its resistance to string tension, thereby increasing the amount of neck relief present. If there is a larger clearance then the opposite is true; the truss rod will require tightening to increase resistance and decrease neck relief. If you do not have a straightedge to help you check the neck relief you can either use a capo at the 1st fret and manually fret the strings at the last fret (or have a friend hold down the strings at these frets) and use the strings themselves as straightedges. If you do not have a set of feeler gauges you can use a thin piece of cardboard such as a playing card to measure clearance under the strings. The card should barely slide under the string without lifting it. Tightening or loosening of truss rods should only be carried out in small steps. Sensitive truss rods can sometimes require a small fraction of a turn to significantly alter neck shape. Additionally, it can take some time for the wood in necks to "move into the new shape" and reach equilibrium....don't go cranking on the rod if it doesn't co-operate immediately! A rushed setup may yield the correct clearances initially, however necks may continue to move over a longer period. Patience more than pays off when dialling in the perfect setup! An hour between truss rod adjustments is satisfactory with a whole re-check of the neck the following day is good practice. Should the clearance be too small, the neck is too straight. If the straightedge is unable to sit over the first and last frets, the neck is in fact bowed backwards ("backbow"). Both of these situations require that the truss rod nut be loosened by turning it counterclockwise. Do this gradually as described previously and recheck the clearance each time, allowing the neck to resettle as appropriate. If the clearance at the 7th fret was more than .015"/0,13mm you will need to tighten the truss rod by turning the nut in a clockwise direction. Remember to move it in fractional increments (less than 1/8th) as it can move significantly with each adjustment; recheck the clearance each time after the neck settles. Below you will see pictures of the common types and where the truss rod nut is located. On many guitars you will find the truss rod nut located underneath a cover on the headstock. View of a standard Allen wrench style adjustment truss rod For some guitars you will find the truss rod adjustment on the other end of the neck which may mean you will have to remove the strings and take the neck off to make an adjustment. This does mean that you will not be able to make the adjustments with the neck under string tension or in the player's position. Plan adjustments ahead before removing the neck! Heel end truss rod adjustment In the case of spoke wheel adjustment nuts at the heel end, it is possible to adjust the rod using a screwdriver or Allen key by parting the 3rd and 4th strings to gain access to the wheel. The two most common style of truss rod adjustment tools are Allen wrenches and barrel style socket wrenches. If you're adjusting the neck on (for example) an older style Strat neck at the heel you may need to use a flat bladed screwdriver instead. Always check that the tool is the correct size before applying force; a stripped or broken adjustment nut is a far greater headache than a badly set up neck! Step 1: Introduction and headstock area Step 2: Trussrod and neck bow adjustment Step 3: Nut height check and adjustment Step 4: String height and bridge adjustment Step 5: Adjusting the intonation of a guitar Step 6: Adjusting pickup height