With your guitar in tune, you’re going to check to make sure that there are no warps or bends in the bridge that could cause the action to be uneven and make your guitar harder to play.
Turn the guitar over and look down the other side as well. The neck should be straight on both sides. You can also look down through the center of the headstock with your guitar resting on its back edge on the table. From this angle, you can more easily see if there’s a bend in the neck. If this is your first time sighting the neck of your guitar, you may not know exactly what to look for.
Repeat the tap test with all 6 strings of your guitar. Since it takes practice to sight a guitar neck, this test can help you confirm your findings.
Take care not to lose the truss rod cover or the screws. You can stick them temporarily to a piece of tape or the sticky part of a sticky note to keep them together. Some acoustic guitars are only accessible through the soundhole of the guitar. You’ll be better able to get at the truss rod if you loosen your guitar strings so they aren’t in your way.
If you’re accessing your truss rod through the soundhole of your guitar, you’ll be using a longer truss rod wrench to reach it. Keep this in mind when adjusting it so you don’t tighten or loosen it more than necessary.
If this is your first time making this adjustment, you may have to go through this process 4 or 5 times before you get a straight neck. Just have patience. More experienced luthiers can tell exactly how much they need to turn the truss rod just by sighting the neck.
You can use any reference point between the 8th and the 12th fret. Just make sure you use the same one every time. While you can use any sort of ruler for this measurement, string action gauge height rulers have much smaller increments, which will enable you to be more accurate.
If you fret lightly, you may feel more comfortable with a lower action and you likely won’t have to worry about too much buzz. However, more aggressive players typically want higher action to avoid too much fret buzz. Acoustic guitars typically need slightly higher action than electric guitars. Keep that in mind if you have both or are switching from one to the other.
For electric guitars, the default action is typically 6⁄64 in (0. 24 cm) on the bass side and 4⁄64 in (0. 16 cm) on the treble side. For acoustic guitars, the default action is typically 7⁄64 in (0. 28 cm) on the bass side and 5⁄64 in (0. 20 cm) on the treble side.
You may not want to adjust the action at the nut at all if you frequently use alternate tunings or switch to different gauge strings. It will only make it difficult for your strings to sit right and you may have trouble keeping your guitar in tune.
The largest feeler gauge that will fit between the string and the first fret is the distance between the string and the top of the fret. This will give you an idea of how much the nut needs to be filed down (if at all). You can also measure with a gauge height ruler. The standard measurement at the 1st fret is 0. 30 in (0. 76 cm).
You can also rest the loosened string lightly on the top of the nut. However, don’t try this until you have some practice as it can damage the nut.
You may want to place a piece of plastic on the headstock between the tuners to protect it. Otherwise, you may accidentally file into the headstock.
For middle strings, make sure the strings are loose enough that they aren’t resting directly on the nut.
Electric: 6⁄64 in (0. 24 cm) on the low side, 4⁄64 in (0. 16 cm) on the high side Acoustic: 7⁄64 in (0. 28 cm) on the low side, 5⁄64 in (0. 20 cm) on the high side Bass: 7⁄64 in (0. 28 cm) on the low side, 5⁄64 in (0. 20 cm) on the high side
For a neck radius of 7. 25 in (18. 4 cm), string height should be 5⁄64 in (0. 20 cm) on the low side and 4⁄64 in (0. 16 cm) on the high side. For a neck radius of 9. 5 to 12 in (24 to 30 cm), string height should be 4⁄64 in (0. 16 cm) on the low side and 4⁄64 in (0. 16 cm) on the high side. For a neck radius of 15 to 17 in (38 to 43 cm), string height should be 4⁄64 in (0. 16 cm) on the low side and 3⁄64 in (0. 12 cm) on the high side.
Use your string winder to loosen your strings until they’re floppy. You may not need to remove them entirely, which can save you some time. However, you do need to loosen them enough that you can remove them from the bridge. If your bridge is wedged pretty tight, you may need to use pliers to get it out. Take care not to scratch your guitar or damage the bridge or saddle.
When sanding, be careful to keep it completely even. An uneven saddle will ruin the tone of your instrument. Do a little at a time, then replace your strings, bring your guitar back up to pitch, and check the action again. Be prepared to repeat this action several times, especially if you don’t have much practice.
More than a specific measurement, pay attention to how the guitar plays and sounds. If you’re getting a lot of fret buzz when you play your guitar, you’ve got your action too low. If it’s not comfortable to play or you’re having trouble fretting strings, you may have it set too high. Play around until you find what works best for you.
