Article by David Lambert
There are things that you need to know before you start. Your guitar is expected to have an adjustable rod that runs down the center of the neck. Most likely your guitar will feature a truss rod channel that is cut down the center of the neck which receives the rod. The rod is set loosely in this channel and the only attachment to the neck is commonly at the very end of the rod.
These truss rods come in 3 basic varieties:
Fixed Reinforcement: This is a typical placement you will see in Martin Guitars and some of the handmade guitars. The reinforcement consists of a rigid bar that is secured to the neck and has no adjustability. The steel configuration can be tube-shaped, a steel tee or steel rectangular bar stock. If you find an aluminum rod, it will normally be a T8 spec grade aluminum, which conforms for use in aircraft.
One Way Truss Rod: A one way truss rod permits tightening of the rod to bow the neck upwards. You cannot bend the neck downwardly with a one-way rod.
Bi-Directional Truss Rod: The most versatile form of a truss rod is the bi-directional rod, which allows adjustment in both directions. By tightening the nut (clockwise) you bow the neck upward, and loosening the nut (counter-clockwise) has the reverse effect. Unless you are dealing with a distorted or turned neck, the two-way rod will correct nearly any neck problem.
Neck Relief: This term is the amount of bow that is present in a guitar neck. It can be a forward or backward bow. There is often a misunderstanding of the terms used to describe neck relief. The two terms used are backward bow and forward bow.
Forward Bow: This is a appropriate condition for your guitar neck. At least a modest amount. If a neck is 'cupped' or bends downwards, it has a forward bow. If you would hold a tightened string between the 12th fret and the 1st fret and the distance that exists between the top of the 6th fret to the bottom of the string would be your neck relief.
Backward Bow: This is not a proper condition in your guitar neck. The biggest trouble that a backward bow condition has for the guitar neck is the likelihood of buzzing strings and high string action. This is where an adjustable truss rod is worth its' weight in gold. Implementing the same experiment as above, with the string running from the 12th to the 1st strings will show you that the string will not clear the middle of the neck at all, but curve over it.
Measuring Neck Relief: There is, of course a magic measurement that we want to shoot for, with neck relief for our acoustic instruments. For the first step you will want to position a capo just behind the first fret and apply a number of tension on it - just enough to bring the string in firm contact with the first fret. Don't over-tighten the capo, as this may be the cause for the string rise slightly. Next you should press the string down to contact the 12th fret with your finger.
Now, with a feeler gage, measure the space between the crown of the 6th fret and the bottom of the string. If your measurement is someplace between 0.010" and 0.015" you are in outstanding condition. If you find that you have slightly more than this, do not worry too much about it, unless it is unreasonable. If you discover that your measurement is less than the low end of 0.010", you should make a truss rod adjustment, to bring the neck back into satisfactory tolerance.
Please Note:
Suitable neck relief occurs between the nut and the Middle of the neck. If you find that the forward bow occurs in an another location other than noted above, you are likely dealing with a bulge or other abnormality in the neck, which can be a more severe neck problem.
Tools Required for Measuring Neck Relief:Feeler Gage; Steel Ruler in Decimal Inches or Millimeters; (2) Guitar Capos
Neck Relief Adjustment:
This subject will be dealt with in our article on How to Adjust a Guitar Truss Rod.
For more information about this article, visit: Guitar Neck Relief
Truss Rod Adjustment