When you buy a new guitar, you might notice slight changes. This is because in the first few weeks to months of its assembly, the neck and action will move a little bit. Should your instrument have been at a store for some time, it's likely it's been adjusted. However, if you order a guitar or purchase a newly arrived instrument from a store, you'll probably see or feel changes in the neck, resulting in the action rising. These are not uncommon, and, if you are confident you can make the necessary adjustments, you may do so by following these steps:
1) Get a small metal ruler that begins its graduation marks at the very end of the instrument, preferably, one that with increments down to 1/32". These are easier to use and see the graduation marks than a 1/64" ruler. You can find these at most hardware and home improvement stores and these are inexpensive, generally only a few dollars.
2) In addition to the ruler, you'll need a business card, or, a .012″ feeler gauge, which can be found in auto parts stores.
3) Get a ratchet socket wrench, along with a 3" drive extension, and a 3/8" deep socket or a 5/16" deep socket, depending on your particular instrument. To know which, just look at the truss rod adjustment card that came with your guitar.
4) Using a capo, which you can buy from a local music shop, place it lightly over the first fret. Depress the low E string, or the top or thickest gauge string, until the string touches the 12th fret. Place the business card or .012″ feeler gauge under the low E string at the 6th fret. You ought to be able to slide the feeler gauge or card without having to manually raise the string.
If there is a lot of room between the card or feeler gauge and string, the truss rod should be gently turned clockwise to straighten out the neck. Should the gauge or card slide under the string with moving the string itself, the neck is under too much pressure. Though some players like the action to be higher at the 12th fret, the best range is between .005″- .012″ at the 6th fret. You'll find that the relief is more or less under the high E string than the bass string, but understand this is normal. If the low E bass string is touching at the 6th fret, your instrument will probably buzz during playing.
You can adjust the truss rod by slightly separating the D and G strings and placing the ratchet inside the access point. You'll have to make slight adjustments to get the socket into the right place, but you'll eventually feel it engage with the truss rod nut. To straighten the neck, turn the truss rod clockwise, to add more relief, turn it counterclockwise. You're aiming to find an area where there's little resistance to turning the rod. This is the neutral position.
In general, the adjustment should end with a slight turn toward the clockwise side of the neutral area. The reason for doing this is to ensure the adjustment holds. This is the same thing as tuning down and then tightening the strings to pitch when restringing your instrument. By doing this, the proper amount of pressure is applied against the threads.
If the next on your guitar is too stiff, you might have to turn the truss rod to the counterclockwise side of the neutral area to provide the right amount of relief to the next of your instrument. Eventually, this will be reversed after playing your instrument for some time and turning the truss rod back to the clockwise side of the neutral area.
Seasonal and Humidity Change Adjustment
As the seasons change and/or the humidity changes, the truss rod might have to be adjusted. While the truss rod does straighten the neck or provide relief, it is not the primary action adjustment. The purpose of the truss rod is to balance the relief in the next and to comply with your style of play. String pull and humidity changes can be dealt with at the saddle of your instrument.
Once your neck is properly adjusted, you can make additional adjustments at the nut or at the saddle. The nut will wear over time, so, the lower it is slotted, the more likely it will be that you'll have to get a new nut for your instrument. Generally, if you bar fret all six string between the 2nd and 3rd fret, there ought to be .005″ clearance between the barred strings and the 1st fret. Should there be more than .005″ of clearance, the nut slot is higher than it ought to be on the guitar.
After making an adjustment, use the metal ruler to measure the action at the 12th fret. Low action on a guitar is 3/32″ under the 12th fret on the bass E string and 2/32" at the treble E string. Medium action is set at 7/64″ on the 12th fret on the bass E string and 5/64″ on the treble E string.
If the action is higher, do not immediately lower the saddle, instead, check your instrument for signs of being over-humidified. Conversely, if the action is low, the top might have dropped and the instrument needs to be humidified before raising the saddle.If you are traveling and are going through areas where the humidity levels change, or, you live in an area that humidity levels change a lot from summer to winter, you might consider having a few saddles cut so you can make easy adjustments to your guitar. You can also protect your instrument be keeping it in a controlled environment, like its case, for best protection.