Correct Nut Slot Height
As to action height, I've seen Martin repair estimates where they refer to 3 and 2 and 3 1/2 and 2 1/2. These are 32nds of an inch measured from the top of the 12th fret to the bottom of the E strings when the nut slots and truss rod are where you want them as both of these adjustments impact the height measurement. What you're thinking of- and I know because I had the same confusion for awhile- would result from slots that are cut BELOW fret height. That's not what David is saying. The correct nut height is the height of the frets and the saddles take care of action off the 1st fret and all the others. Try this- put a capo on the 2nd fret.
After adjusting the truss rod and setting the string saddle heights at the bridge, you can now make the third adjustment, which is to the nut slots. There are three aspects of the nut slots that you will be addressing in the setup process in order to make sure that they are cut correctly:
- The nut slot depths,
- The slot widths, and
- The slot angles.
When to adjust the nut slots
You typically make adjustments to the nut after setting the bridge, as the heights of the nut slots are a function of the overall string angle. The string angle is determined by the string action height at the bridge. The individual slot depths are cut so each string can clear the first fret.
You might notice that some guitars fresh off of the showroom floor are a little difficult to play in the first position (at the first five frets). This is because the manufacturer will cut the nut slots on the high side on purpose. Then a local guitar tech at the store can adjust them to customer taste. It is much easier to cut a nut slot down than to raise it by either filling the slots or shimming the entire nut up.
The effect of nut slots that are too high
Nut slots that are too high create a situation where the guitar is difficult to play in first position. They can also throw off the tuning and intonation. Nut slots that are too low will create what many players refer to as a “sitar” effect. This is where the string isn’t able to clear the first fret and rattles against it, creating a string buzz similar to a sitar.
The ideal nut slot height, in terms of playability, is subjective — much like string action height at the bridge. Some players like the slots to be cut up a little higher (like slide players), and some players want them cut down as low as possible so the guitar plays very easy with no fight.
You can measure the nut slot heights by placing a feeler gauge in between the bottom of the string and the top of the first fret when the guitar is up to pitch and after the neck and bridge are adjusted. This is why you use a capo at the first fret in all previous steps — to take the nut slot heights out of the equation, saving them for last.
Measuring the nut slot heights
Correct Nut Slot Height Calculator
You can start this process by making sure the guitar is up to pitch. Then read the nut slot depths. As a starting point, shoot for .02” in between the top of the first fret and the bottom of the string at the first fret on the wound strings (low E, A and D), and .018” for the plain strings (high G, B, and E). On acoustic guitars with a wound G, you will also shoot for .018” of space since this a thinner-wound string. The feeler gauge should be placed on top of the first fret, sliding underneath each string.
If there is extra space, then the slot is cut too high and will need to come down until the string lightly touches the top of the gauge. If the gauge pushes the string up, then the slot will need to be filled and re-cut to raise the slot.
Raising the nut slots
You can fill and re-cut a nut slot by wicking either bone dust or the rolled-up fibers of a Q-tip in the slot with thin Cyanoacrylate glue. Using a super glue accelerator to instantly toughen the material, trim the excess, and then re-cut the slot.
Widening the nut slots
Machine T Slot Nuts
The individual slot widths will be a function of the individual string gauges and will determine what gauged file you use on each nut slot. As a rule, you typically want to cut a nut slot slightly wider than the actual string so the slot does not bind or choke the strings at the nut. This can cause tuning issues.
Sometimes I will get a guitar in the shop to repair or replace tuners because the string is making a “clicking” sound when tuning. The customer assumes that the tuners are not working properly. However, most of the time the slots were binding the strings and hanging them up at the nut. I can fix the issue by simply widening the nut slots .002” over the size of the actual string width and cutting the slots correctly.
If, for instance, you are changing your string gauges from 10’s to 11’s, you will also want to widen the slots to accommodate the thicker string gauges. Again, these nut slot height measurements are workable specs to shoot for. But the customer will ultimately decide what feels right, as they can dramatically affect the playability of the setup.
Cutting at the proper angle
The last aspect of nut slot adjustment is the angle at which it’s cut. This is determined by the string pull of the headstock (either straight string pull or angled). Whenever you are cutting a nut slot, you will always direct the file in the slot to the corresponding tuning key.
String Height At Nut
On a Gibson, (which has symmetrical offset tuners) you will direct the file with a slight angle pointing to each tuner post. Gibson and Martin guitars have a tilt-back headstock design, so you will want to angle each nut slot cut with a slight downward angle that matches the headstock pitch, leaving the highest point of the slot at the front of the nut.
On Fender guitars with straight string pull, you’ll cut straight and in-line with the tuners. On Fender guitars with a scooped headstock design, you will want almost no angle in your cuts because there is almost no break angle in the string after it leaves the nut slot on the way to its corresponding post. If you cut a nut slot on a Fender guitar with an angle, you might create a string buzz on the open notes. You can almost always fix this issue by filling the nut slots and then re-cutting them correctly.