05/08/2026
Hello everyone,
Ian here. I wanted to take a moment to make a nerdy post about restoring the geometry and playability of older acoustics.
The project we are looking at today is a 1974 Martin 00-18. The only work ever performed on this guitar, before it came to us was a pickguard replacement. It had lived in Arizona for at least 20 years. The climate, coupled with the years of string tension had taken a toll on it. The bridge area was extremely bellied, the neck relief was 0.020”, the frets were worn out and the action was a whopping 8/64th of an inch while the saddle was flush with the top of the bridge.
So, we had quite a few components to attend to in order to restore the playability of this lovely guitar.
The first step is disassembly. Frets come out, neck comes off, bridge comes off. The second step is the belly reduction and re-humidification. We have these purpose built aluminum cauls. One fits the bridge plate, the other the bridge footprint. Those two areas are moistened with warm water and the cauls are then clamped into place. This process basically does the exact opposite of what would happen if a guitar was left in a hot car (and we suspect that may have happened to this one at one point in time). The moisture and clamped cauls allow the joint between the bridge plate and top to slowly slip back into place. This takes weeks. Three weeks in this case under clamps in climate control.
Once that bit of geometry is corrected, we can start putting Humpty Dumpty back together. The bridge is re-installed and then we move forward with the neck reset. We use two tricks that I find pretty cool on neck sets with guitars that have no truss rods. First, two reference holes are drilled under the last fret position before removal. They are very shallow, and don’t go through the top. This allows the installation of two reference pins to keep the alignment constant during the resetting process. Without this step, one is fighting two dimensions while working: The pitch AND the alignment. So, unless that string alignment needed correcting, which this didn’t, this allows us to leave it alone, and focus on the pitch.
The second cool trick is the partial board plane before puling the neck. Since it has no truss rod, and a hilarious amount of relief, we need a structure that’s closer to the final target to use a reference for the neck angle. The board is levelled on the Plek machine, but only until the 14th fret. The fingerboard extension is kicking up, but the neck reset will change that, so we don’t touch that portion yet. With two dummy frets in place at the 1st and 12th fret, we can now establish the final geometry with greater precision.
After the neck is re-installed, the final board plane is executed. It only takes an extra 0.005” off the board, but we are now ready for frets.
There are targets we need to hit in a neck reset. And with no truss rod, we also have to nail the relief. We are shooting for 0.005” relief under the low E, 0.004” under the high e with absolute minimal fret removal. Some careful compression fretting was necessary, but this was achieved on this guitar with removing no more than 0.0015” on some frets.
The second target is saddle height. Too low, and the voice is weak, and the longevity of the neck reset shortened. Too to high: the top is choked, and the tension to the top can bring the belly back in a hurry, and even split the bridge. The prefect score is 1/8” saddle protrusion, which is 0.125”. On this guitar we nailed the action at 5/64” on the low E, 3/64th on the high e with a saddle protrusion of 0.120”.
So there we go. A nerdy game of precision to make something work optimally for another generation.
