Before we adjust the truss rod, we should take a look at what a truss rod is. A truss rod, essentially, is a metal rod that is inserted into a channel or cavity in the neck to help with support and straightness. Truss rods stiffen the neck and help eliminate relief and back-bow in the neck. There have been many different styles of truss rods since they were first invented. The early truss rods were not adjustable.
Early Martin guitars had a variety of non-adjustable truss rods. These truss rods were often a T-bar or piece of square tubing simply glued into the neck. The primary purpose for these truss rods was to stabilize the neck and keep it from bowing. If the neck did bow because of extreme weather conditions or faulty craftsmanship, there was nothing you could do to fix the neck short of tearing it apart, straightening the neck, and inserting a new truss rod. Martin used these non-adjustable truss rods up until 1984. For more information about the history of Martin guitar truss rods, please visit the Martin truss rod page. "Modern" truss rods are adjustable. This means they not only help reinforce the neck but also help bend the neck back into a straight position.
Today, almost all guitars are fitted with adjustable truss rods. Adjustable truss rods can be adjusted using an allen wrench, screw driver, or nut driver in either the headstock or the heel of the neck. Acoustic guitar truss rods come in two main styles: single action or one-way truss rods and dual action or two-way truss rods.
In short, single action truss rods bend the neck one way by tightening the truss rod. Loosening a single action truss will not "bend" the neck in the other direction; it simply relieves the pressure from the truss rod and allows the string tension to bend the neck in the opposite direction.
A dual action truss rod, on the other hand, does bend the neck in both directions. Tightening or loosening a dual action truss rod will bend cause the truss rod to bend in either direction. For more detailed information about how truss rods work, please visit the truss rod page.
Steps for checking the straightness of your guitar neck are the same for both acoustic and electric guitars.
Checking the straightness of your neck is relatively simple if you have a straight edge or a notched straight edge. Simply play the straight edge against your frets or fretboard and shine light behind it. If the neck is not straight, light will shine through the gaps.
If you do not have a straight edge, you can simply use your strings as a straight edge. Press down the string at the first fret and press down the string on the fret where the body and neck meet. The string will become a straight line between the two frets. You may want to use capos to hold the string down. Once the string is fretted on two frets, you can measure the distance between the frets and the string with feeler gauges. If there is relief in the neck, the 6th or 7th fret will have the most distance between the string and the fret. The distance between the string and frets will decrease the closer you get to the capoed frets. The opposite will be true if the neck has back-bow. If the distance between all the frets and the string are equal, the neck is straight and does not need to be adjusted.
Remember as I wrote on the action adjustment page, there is no right or wrong amount of relief in your neck to an extent. Usually a slight amount of relief is appropriate. Just like anything else in adjusting guitars, neck relief is a player's preference. It depends on the style of instrument and player. You should try to adjust your truss rod until your neck is flat and play it. Then you can continue to add a slight amount of relief until the neck feels comfortable. Since the neck changes with the seasons, this adjustment will be fairly often. You will get used to how much relief you prefer in your neck. The average relief at the 7th fret is about .007 inches.
Before we adjust the truss rod I want to warn you. BE VERY CAREFUL. Do not over tighten the truss rod. Remember the truss rod is a piece of metal that we are applying pressure to in order to bend it and the neck. If you over tighten the truss, you can cause any number of problems. You could snap the truss rod, warp the fretboard, or twist the neck. All of these problems are rather involved and expensive repairs. It is best just to be careful and not over tighten the truss rod in the first place.
A good thing to do before following the steps below is to loosen your truss rod a full turn and measure the neck to see what happens. If the truss rod is already all the way tightened and the neck still has relief in it, you may break something by continuing to tighten the truss rod beyond its limits.
There are two basic styles of acoustic guitar truss rod access points: in the headstock or peghead and in the heel. Here are the steps to adjusting both of these truss rod setups.
Many acoustic guitars' truss rods are only accessible in the heel of the neck inside the guitar. There are a few different reasons for building a guitar neck like this. With the truss rod access inside the guitar, there is no need for a truss rod cover cluttering up the headstock. It is also easier to build a guitar with a truss rod with access inside the body.
I know what you are thinking. How am I supposed get inside the body to adjust that? It is pretty simple if you have the right tools. Most truss rods like this either have a nut or hex key slot in them. You will need either an allen wrench or a nut driver to adjust this truss rod. As you can imagine, getting leverage on your tool while your hand is in the sound hole can be difficult. That is why I suggest getting an extra long wrench or driver and bending it at a 45-degree angle about a third of the way down from the hand. This will allow you to turn the wrench without sticking your hand down in the sound hole.
Here are some steps to help adjust this type of truss rod.
It's as simple as that. Remember to only adjust the truss rod about a 1/8 turn at a time.
Many acoustic guitars have truss rods that are accessible through the headstock/peghead. This truss rod is much easier to adjust than the truss rods with access in the body. It is very easy to tell if your acoustic has this style truss rod because it will have a decorative truss rod cover to hide the access point on the headstock. Most of these truss rods can be adjusted with an allen wrench or nut driver. Here are the steps to adjusting an acoustic guitar truss rod in the headstock.
With all truss rod adjustments, do not drastically adjust the truss rod. Sometimes it takes time for the neck to settle into its new shape. If it feels like you have to keep tightening or loosening the truss rod for a single adjustment, retune the guitar to pitch and let it sit for a few minutes. This will give your guitar time to settle into its new shape.
Remember, the truss rod adjustment is only one step in lowering or adjusting the action on your guitar. Do not keep adjusting the truss rod if the action is not ideal. The truss rod is only mean to straighten the neck. The rest of the steps to completely setting the action on your acoustic guitar can be found on the adjusting your acoustic guitar action page. I cannot stress enough to be careful while adjusting the truss rod. There are many highly involved and expensive problems that you can be caused by over-tightening the truss rod.