I’ve spent the last 15 years of my life doing repairs for local Philly musicians. Many of those repairs were fixing old guitars that were worth less than the repairs needed to make them work. Once in a while a customer would pay far too much to save an old guitar for sentimental reasons but usually they’d balk at the estimated price I’d give them (and I tend to very reasonable with my pricing). So I am constantly trying to devise new ways of fixing a guitar without the long hours usually needed to make it work again. The most common problem on an old acoustic is high action. The usual solutions are to lower the saddle and adjust the neck but when you’ve taken those to the limit your only option at that point is change the neck angle.
The usual way of changing the neck angle is to completely remove the neck and re-set it. The problem with that approach is, the neck rarely comes off completely without a fight. It is very common to steam the neck off but this is also time consuming and just not an affordable approach to fixing and old $100-$200 guitar.
Old acoustic guitar necks are glued in with a dove-tail joint but they are never fitted very tightly. There is always room below the joint. I’ve been fairly successful at “shimming” the neck on an acoustic guitar much the way you shim the neck of a bolt-on style guitar. For this example, I’ll use an old Harmony Monterrey.
The first step is to remove the fret where the neck meets the body. If there is no fret right above the meeting place, choose the one closer to the nut. The next step is to saw though the fretboard with a fret saw at that spot, down to the neck wood, in this case mahogany (see pic 1). Now you need to remove the fret board that is glued to the body. You can try heating it up with an iron. Once the wood has become hot, you need to pry off the fret board with a metal spatula. I use one sold through Stu Mac. Once the board is off you have a clear view of the dovetail. My next step is to cut out wedge shaped pieces of wood and lightly hammer them into the space below the bottom of the dove-tail joint (see pic 2). This will tilt the neck away from the body and increase the neck angle, which is what you need for lower action.
At this point take a straight edge and lay it on the board so that it also rests on the bridge saddle. This will give you an idea of how low the action will be. once you are happy with the new angle trim the shims to the level of the dove-tail (pic 3). Strip all the old glue off and re-glue the fret board on. I like to lay the original fret in place to allow for it to be hammered back in later. I clamp the board using a radius block (also a Stu Mac product) for even pressure (pic 4). Once the glue is dry, hammer or glue the original fret back in and string it up. If you did it right the action should be much better. You can now fine tune the action at the saddle.
The drawback to using this method is that you make a gap at the heel of the neck which can be seen when you look down at the guitar when in playing position (pic 5). I fill this gap on both sides of the heel with either wood filler or latex wall grout and touch up the color with wood staining markers. You may also notice that the action, while low up to the body gets higher over the body. This can be fixed by making a wedge shaped shim that fits under the fret board before you glue it on. But remember, this repair is design to be quick and cheap and since most people don’t play above the 12-14Th fret, I skip this part.
The result is far from picture perfect, but that’s not the point. This repair allows you to enjoy an old guitar that would otherwise be a wall hanger for a lot less time and money spent. One could always spend more time on cosmetics but once again the price will start to sky-rocket.