Fig. 4. A shim made from a Popsicle stick or scraps of wood leaves a gap between the neck pocket and neck. Eventually, pressure from the neck screws will warp the neck as it tries to fill the void.
Shimming dos and don’ts. I’ve seen just about every type of material used to shim a neck, including a matchbook cover, metal washers, a broken Popsicle stick, wood scraps (Fig. 4), and a guitar pick (Fig. 5).
Fig. 5. Another no-no: Using a guitar pick as a shim.
For starters, don’t use paper or plastic, and above all, only use a “full-pocket” shim. Here’s why a full-pocket shim is essential: When someone uses, say, a matchbook cover or guitar pick, it leaves a gap between the body and neck. Over time, the screws holding the neck force it to fill that void. As a result, the neck warps and the end of the fretboard looks like a ski jump. Next thing you know, the fretboard has to be sanded to remove the warp and then refretted. If you’re lucky, the neck will have a thick enough fretboard to allow resurfacing. If not, the neck is a total loss. Either way, it’s going to cost you hundreds of dollars.
To avoid these problems, use a full-pocket shim that fits inside the entire neck pocket and is shaped like a thin wedge, with the slightly taller end facing the bridge This raises the end of the fretboard just a bit, which is what you want when correcting a neck angle that’s too low.
What you’ll need. The best material for a shim is a thin piece of maple veneer. Most bolt-on necks are made of maple, so it makes sense to make a shim from the same material. The wood can be easily shaped to fit the neck pocket and angled to the proper degree. I typically start with a piece of maple about .060" thick.
The project requires a small hobby saw, 80-grit self-adhesive sandpaper attached to a flat plastic or wood sanding block, a belt sander, a hole punch, a pencil, and some superglue.