String action is determined by several factors -

• the angle at which the neck joins the body
• the height of the nut - or, more specifically - the height of the cuts in the nut above the fretboard
• the height of the saddle in the bridge
• the hieght of the frets above the fretboard

How we perceive action is also affected by the tension of the strings - the higher the tension, the harder it is to depress the string, and the "harder" we perceive the action to be.

Before you consider adjusting your string action, you need to consider a number of design factors or design compromises that were involved in setting the action originally -

• low action means easier playing; but it also means that the loud notes are likely to cause the string to slap against the upper frets and create an unwanted percussive sound
• the action has been set so as to achieve optimum intonation across the 6 strings

So, you have decided you want you action lower - or higher - then how do you go about it. I shall take the basic approach here.

The key is the height of the saddle in the bridge. Have a look at the saddle and if it projects at least 2mm above the bridge then there is scope to lower the action.

To lower the action -

• Make an estimate of how much you want the action lowered - a typical classic al guitar has an action of between 3mm and 4mm between the strings and the 12th fret.
• If you want 0.5mm lower action at the 12th fret then you will need to lower the saddle in the bridge by 2x this amount
• Loosen all 6 strings
• Slide the saddle out of its slot in the bridge - taking care to note its orientation
• Put the saddle into a steel vice, upside down, protruding above the vice jaws by exactly the amount you want to reduce its height. The vice is important here. It should have completely flat jaws so that you can reduce the height of the saddle in such a way that its undersurfact will be completely "true." This is crucial. The underside needs to be true so that when it is replaced in the slot in the bridge, it makes the maximum possible contact at its base with the bottom of the slot in the bridge. It is all too easy to file down the saddle so that it has a "hump back bridge" profile or a depression. It needs to be completely true to have maximum contact with the bridge. If it does not, you may find that the tonal quality of the guitar is significantly reduced.
• using a metal file, reduce the height of the saddle so that it is level with the top of the jaws of the vice.
• Replace the saddle in its slot maintaining the orientation it had before it was removed.

If you want to raise the action, you will need to -

• Remove the saddle as when reducing its height
• Put some packing into the slot in the bridge
• Replace the original saddle.

The packing referred to above ideally is a strip of wood - we always use the very thin strips used by luthiers for purfling. If you don't have access to this then a sliver of card will do. The objection to card is that it may muffle the sound transmitted from the strings, via the bridge, to the soundboard.

Before reducing the height of the saddle, a wise precaution is to obtain a second saddle of the same dimensions as the first. Armed with this, you can take a file to the first saddle knowing that if you go wrong you have one in reserve and all is not lost.

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