Guitar Truss Rod Research Paper

Different Styles of Acoustic Guitar Truss Rods.

How to adjust an Acoustic Guitar
Truss Rod through the Sound Hole

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.
After you have checked your neck for relief or back bow, loosen the strings. You don't have to loosen the strings to adjust the truss rod, but it can be difficult to get in the sound hole with the strings in place.
Use your allen wrench or nut driver to adjust the truss rod. BE VERY CAREFULLY not to tighten the truss rod too much at once. Only turn the screw 1/8 of a turn at a time. Tightening the truss rod (turning it clockwise) will eliminate relief in the neck or add back-bow. Loosening the truss rod (turning it counter-clockwise) will add relief in the neck.
After you have turned the screw 1/8 of a turn, retune the guitar. It is important to have tension on the neck. Without the string tension on the neck, you cannot see the extent to which you are moving the neck with each truss rod adjustment. Then you can measure the relief in the neck again.
Repeat these steps as much as necessary until your neck is straightened.
It's as simple as that. Remember to only adjust the truss rod about a 1/8 turn at a
time.

I put a truss rod in all of my Masters series guitars. Although this is not traditional in classical guitars, I find it a most useful remedy when a guitar neck changes due to climate changes or age. I like for a guitar to play very well, and even subtle adjustments in the neck can make a big difference in how easily a guitar plays.

This truss rod works both forward and backward, which means it can adjust for both a back bow or a forward bow.
It is accessed through the sound hole.

Here's how to do it:

It's easiest to do with the strings loosened, although I personally do it with the strings up to pitch so I can look at the neck under tension. If you do it up to pitch, just lift up the 2nd and 3rd string at the nut and move them over to the 1st string nut slot. Do the same thing with the 4th and 5th string, moving them into the 6th string nut slot. Then you can reach into the sound hole between the 3rd and 4th strings. The strings hurt the forearm a bit, but it's manageable.

Set the guitar on a workbench with the neck pointing to the left, tilt the body toward you so the sound hole is facing your belly. Then reach in with the 9/64 inch Allen wrench and put it into the Allen nut, which appears through the very upper cross brace of the soundboard, right by the end of the neck. Although it will offer a certain resistance to turning, it doesn't take much movement to cause a change.

Turn counter clockwise (imagine looking at the Allen nut) to add relief or correct a back bow. Turn clockwise to reduce the relief or correct too much forward
bow.

It's easiest to remove the wrench by lifting the guitar up on it's bottom end and just let the wrench drop into your hand.

Sounds easy, huh? It usually works very well. The hardest part is getting the wrench into the allen nut, since you can't see it. But with a little practice it's easy.

Don't forget that a neck is not supposed to be straight - it's supposed to have a little forward bend, called relief. I measure it with a straight edge and a feeler gauge at the 6th or 7th fret, or I just sight down the edge if the finger board. I like between .003' and .008" relief on the bass side, and a bit less on the treble side. I don't consider it a repair problem until the forward bow gets above .011". If a neck is perfectly straight I'll usually not worry about it, but any back bow at all makes it impossible to get a low action without buzzing. Just leaving the guitar strung to pitch will often pull a back bow out, but if not, something needs to be done. (And that's another story.)

Yes, the Palo Escrito has a two way acting truss rod. Until recently it has been 5/32" size allen wrench, and in the last two months has changed to conform to our Master series wrench, which is 9/64". These are both standard allen sizes.

There is no tradition for using truss rods in classicals, but the usefulness is the same as in any other type of guitar. It not only helps to correct real problems that can develop from changes in climate, age, etc, but it helps to set up a brand new instrument into perfect playing condition with the neck relief exactly right. Installing truss rods was one of the best decisions I've ever made.