An upbowed neck will bend up slightly when you hold the guitar flat in front of you, while a backbowed neck is going to bend downward slightly. To check the straightness of the neck, hold it at eye level and look straight down the neck, or lay it flat on a table or bench and look at the neck at eye level. There’s another way to check the straightness of your guitar neck, but you’ll need a helper. Press a string down at the first and 14th frets. Have your helper line up a ruler next to the string you’re pressing down at the sixth fret. There should be approximately 0. 01 of an inch (about 0. 25 millimeters) between the string and the fret.
An adjustable truss rod is either one-way or two-way – also known as single-action or double-action. A one-way rod will only straighten the neck of your guitar against string tension and upbow, while a two-way rod can also correct a backbowed neck. With a one-way truss rod, there’s no way to adjust a backbowed neck. However, if you have a newer guitar you typically have a two-way truss rod, since these became standard in the 1980s.
Check the truss rod to see what kind of tool you’re going to need for the job. Typically it will either have a nut or a hex key slot. If your truss rod is only accessible through the sound hole, you’ll probably want a longer allen wrench or nut driver to turn it so you’re not trying to stick your whole hand in the sound hole. If your truss rod is accessible from the headstock, you don’t need to worry about the sound hole. You just need to unscrew the screws holding the truss rod cover in place. When you’re adjusting the truss rod from the headstock, don’t loosen your strings – you need to have them tuned to pitch so you have appropriate tension on the neck and can see the extent to which you’re adjusting it.
Remember “righty-tighty, lefty-loosey. " Turn the truss rod screw to the right to straighten an upbow, and to the left to straighten a backbow. Place a mark on the nut so you can tell where it was when you started. Don’t turn the screw more than 1/8 of a turn at a time. This will keep you from adjusting too much.
This isn’t something you can just eyeball with loose strings. The neck must have correct tension on it for you to be able to tell whether you’ve straightened it enough or not.
If you don’t have a set of nut files, you can typically find them at a luthier supply shop, as well as at many music stores. You’ll also need a feeler gauge so you can measure the action at each fret and file accordingly.
Use a ruler to measure first. It should be 0. 3 inches or about 7. 5 millimeters from the string to the first fret. If it’s greater than that, keep checking the distance using larger feeler gauges until the string moves because the gauge is too big to fit. The distance between the string and the fret is the thickness of the largest feeler gauge that doesn’t cause the string to move. Repeat this with each of the six strings.
Place your nut file in the notch and file carefully, going in the direction of the headstock at the same angle. Only file a small amount at a time, as you cannot replace material once you’ve filed it down and you don’t want to file it down too much. When you think you’re done, replace the string, tune it, and measure it again to see if you need to refile or if you’ve corrected your problem.
The saddle serves the same purpose as the nut, controlling the height of the guitar strings. If you lowered the action at the nut, you must also lower the action at the bridge or your tone will be off. The strings are strung through the bridge, and their tension holds the saddle in place. It isn’t glued in place. Saddles may be either straight or compensated. A compensated saddle is curved to compensate for the tone of the strings and help keep the guitar in tune. This is why if you want to lower the action at the bridge, you always sand down the bottom of the saddle, never the top.
Most acoustic guitars take 2/32 of an inch (about 1. 5 millimeters) of action for the first string and 3/32 of an inch (about 2. 3 millimeters) of action for the sixth string. If your action is more than that, you’ll want to lower it.
Use your string winder to detune your guitar until the strings are loose and floppy. Don’t take your strings off the tuner.
The lower three strings should give you enough room to slide the saddle out, provided the other three strings are really loose and floppy. You still don’t need to remove your strings from the tuners unless the strings go through the bridge. If you do have a string-thru bridge, this process will take a little longer because you’ll have to remove the strings from the tuners as well to get the saddle out.
One way to do this is to place a piece of double-stick sand paper on a level table or work bench. Get the ruler you used before and determine how much you want to sand down your saddle. Mark your saddle across with a pencil. Then all you have to do is sand until you get to the pencil line. Keep in mind that if you sand your saddle down too low, your strings will be too long. You also don’t want to take more than you need. Be cautious and only sand down a little at a time. You can always repeat the process if you didn’t sand enough, but if you sand too much you won’t be able to put it back.
Measure the action again and play your guitar a little bit to see if you like it. You may want to repeat the process and sand down a little more. Keep in mind that industry standards are just that, but each guitarist has their own personal preference for how much action they like.
