The string tension on your acoustic guitar depends on a few factors. String gauge, scale-length and tuning all play a part but if you assume somewhere around 200 pounds of pressure, you'll be in the ball-park.
If you sat there with 200lbs on your shoulders for years, odds are you might begin to buckle a bit. Your acoustic guitar holds up better than you would but that tension can take its toll.
If you have any steel-string acoustic guitar for long enough, chances are good it'll need a neck reset at some point in its life. That string tension alters the geometry of the instrument and the most obvious way is that the action creeps up to a point were it's uncomfortable or awkward to play. When a reset will be needed is anyone's guess. Different guitars are, well, different. Could be five years, could be fifty.
Most guitars have some additional height in the saddle to allow it be lowered, taking the action down with it. This buys some time but, eventually, the same thing can happen. It's not unusual, on older guitars, to see a saddle that's been lowered repeatedly and is little more than a sliver, barely above the bridge.
Might be a good time for a reset, then.
A reasonable rule of thumb is that, the plane of the frets should be at the same height as the top of the bridge (that's the wooden bit and not the white saddle). Putting a longish straight-edge on the frets can show you what the story is, as in the photo above. As you can see, it contacts a few millimetres below the bridge-top. Sighting down the frets from the headstock can give you a good idea visually if you don't have a long enough ruler.
What happens in a neck reset?
Basically, we're trying to re-adjust the geometry of the guitar and neck so that straight-edge in the photo gets raised enough to touch or clear the bridge. That means changing the angle at which the neck joins the body.
To do this, the neck has to be removed and some wood taken off part of the heel.
A neck reset on a bolt-on acoustic
Ahh, a bolt-on neck…
Dubious arguments about tone aside, if your acoustic guitar has a bolt-on neck, it does make a neck reset a little easier. The first step, you see, is getting that neck off and the easier that is for me, the cheaper it is for you. Bolt-on necks mean less hassle trying to get glue-joints to release.
The first image clearly shows the bolts in the neck block (we're looking inside the acoustic guitar here). Straightforward. Excellent.
The second image is the inside of the 'top'. The image is taken with a mirror lying inside the guitar. It's always a good idea to get an idea what's going on in here before starting major surgery, especially as bolt-on necked guitars from different manufacturers vary in how the handle things in this area. That block of wood glued to the extension and shoulder-brace, for instance, is worth some consideration.
A little work to get that block to disengage and some work on the fingerboard extension is all that's needed here. You can see the way neck and body fit together relatively clearly above.
Something I wasn't expecting was to encounter an epoxy-like material in the body mortice around the neck tenon. It was in the area around that white tape (marked with an X in the photo on the left). Because it wouldn't adhere well to this tape, I'm guessing its job was simply to act as a sort of gap-filler to ensure help ensure a solid connection here. Whatever, I noted it for reassembly and cleaned up the residue.
Incidentally, I noticed a hairline crack in the heel between the two sockets for the bolts. It was pretty small and probably unlikely to cause problems but I made it good before proceeding to work the wood in this area.
The reset itself is done by removing a 'wedge' shape of wood from the heel—more at the bottom, graduating to none at the top where it meets the fingerboard. Calculating the amount to remove can be done by a relatively simple formula but I tend to do that only to get in the ball-park and then finish by eye.
The tape in the left photo gives me a line indicating the wood to be removed. There are a few ways to go about this but I like to bevel down to this line and then bring the sides to meet it.
Like this. Wood is carefully removed from the sides of the heel now. Very carefully. It would be very easy to mess this up. I don't want to remove any wood from that far end where the heel intersects the fingerboard. Doing that would actually move the neck closer to the bridge and muck up the intonation. What we want to do is to take out that wedge shape I mentioned earlier. When that's gone, the neck joins the body at a slightly increased angle and this means better action.
Most of the work is done as shown in the left photo. When the bulk is gone, I'll test fit to the body and I'll remove the remainder of wood using sandpaper between body and neck as in the right image. This helps perfectly shape the heel-fit to the body.
It's important to check alignment often. As well as the set angle, I'm checking for side-to-side alignment to make sure I don't take too much off one side of the heel. That would point the neck too much to one side or the other—not good.
Once I'm happy with the fit and alignment, it's time to reassemble. In this case, thanks to the construction of this guitar, it's an easy job. Bolted back on and a little glue in the appropriate places—especially on that little block of wood we found earlier on the end of the neck—and we're good to go.
And this is what we're looking for. The straight-edge along the frets just skims the top of the bridge. I'll need to make a new saddle to replace this one that's now far too low but that's no problem. We don't need to do any fretwork on this guitar as a consequence of the neck reset (frequently that's not the case) so, all in all, it's been a good day.
I'll try to pull together some photos of a more involved neck reset soon. Anything with a glue-in, dovetail neck involves messy work, steaming out the neck.
For now, though, I've strung up this baby and it's sounding (and more importantly), playing great.
I suspect this deserves a celebratory tea.