Building a well-crafted electric guitar or bass requires patience. As tempting as it can be to rush or cut corners, the most successful builds ultimately go to those with the patience to take all the necessary steps, no matter how tedious. One such step, often skipped by the impatient or the inexperienced, is properly drilling the guide-holes for all the wood-screws that must be used. So let’s talk about guide-holes, see why they are so critical, and figure out how to drill them properly.
Two Paths You Can Go By
To begin with, there are two kinds of guide-holes. The first type is called a clearance-hole, and it is used when the purpose of the wood-screw is to draw two pieces of wood together. A clearance-hole is drilled just large enough for the screw to pass through without biting into the wood. Trying to join two pieces of wood together without a clearance-hole in one of them usually results in a gap between them. This happens because the screw tightens into the first piece of wood before it has tightened into the second. Drilling a clearance hole into the first piece of wood allows the screw to draw the two materials together before it cinches tight.
The second type of guide-hole is called a pilot-hole. A pilot-hole is used when a screw will be driven into a single piece of wood. It relieves the pressure the screw will exert as it wedges itself into place. Without a pilot-hole, this pressure puts the wood at risk of splitting or cracking, usually along the grain lines.
The neck joint of a “bolt-on” style electric guitar is a perfect example of both clearance-holes and pilot-holes. Because the purpose of the neck screws is to draw the neck and the body together, clearance-holes are drilled in the body. They allow the screws to pass through the body and pull the neck up against it as they are tightened. Pilot-holes are drilled into the neck itself, to relieve pressure on the wood and keep it from splitting as the neck-screws bite into it.
The Pilot Program
Most of the guide-holes you will be required to drill when assembling a guitar are pilot-holes, for all of the various screw-mounted hardware; things like tuners, string trees, strap buttons, control plates, output jacks, and pickup rings. Let’s talk about how to do this properly.
In order to be effective, pilot-holes must be drilled to the correct size and depth. There is a common misconception that pilot-holes should be as small as possible. Untrue. Pilot-holes that are too small do not fully mitigate the possibility of splitting the wood. Pilot-holes should be as large as they can possibly be, leaving just enough wood for a screw to bite into and hold securely.
A properly drilled pilot-hole should be the same diameter as a screw’s "root" or "minor" diameter. This is the center core that runs the length of the screw within the threads. This will relieve the necessary pressure, while leaving ample wood for the threads. It is also important to know the screw’s "thread" or "major" diameter size, or the outermost diameter of the threads. To find these dimensions, use a digital caliper. Measuring the outer threads quickly gives you the thread diameter. For proper bite, the pilot-hole you drill must be smaller than this.
If the screw’s threads are large enough you can measure its root diameter size by using the sharpened nose of the caliper between the threads. If the screw is too small for this, look for a drill bit that is about 30% smaller than the major diameter size. I never rely on my eyeballs for this, or on the manufacturer’s dimensions stamped on the bit. I always use my caliper to measure both the screw and the bit.
To drill your pilot-hole to the correct depth, first pass your wood-screw through the hardware it will be mounting, and then hold your drill bit alongside it. You can now easily see how deep the hole must be. Mark this depth by wrapping a small piece of masking tape around the bit. As you drill the hole, keep an eye on the masking tape. When it reaches the surface of the wood, your hole is the correct depth.
As you drive the screw into the wood you should feel some resistance, but not too much. If you really have to work at it, the pilot-hole is too small. Stop. Don’t force anything. Besides splitting the wood, you also run the risk of twisting the head off the screw, leaving the shank embedded in the wood – a vexing problem with no easy fix. Be patient and remain calm. Back the screw out, re-measure, and re-drill the hole. For guide-holes that will be repeated multiple times, like tuner mounting screws, always drill one and test to make sure it’s right before drilling the rest.
Fixing a Hole
Some other things to consider: If the wood you drilled your pilot-hole into has a hard finish on it, you'll want to chamfer the edges. "Chamfering" is the process of creating a slight bevel around the lip of the hole. This bevel "pushes back" the brittle layer of paint around the immediate edge of the hole, so that the screw's threads don't have to cut through it, risking paint chips, cracks, and flakes.
Also, don't forget to take into account the species of wood you are drilling into, and its properties. Some woods are soft and forgiving, while others are hard and brittle. Larger pilot-holes are required in the latter. Roasted Maple, for example, requires larger pilot-holes than standard Maple. As always, pay attention to how much resistance you feel as you drive the screw into the wood. If you feel too much, stop and reassess the pilot-hole size.
One last thing: never trust a manufacturer's pre-drilled guide-holes. While such holes are convenient for marking location, you can't assume their dimensions are correct. This is primarily because the manufacturer has no idea which wood-screws you intend to use. Secondarily, the wood your body or neck is made of might have expanded or contracted as it adjusted to your local temperature and humidity, altering the original size of the pilot-holes. For proper fit it is always necessary to measure. On a related note, you should never assume pre-drilled holes for mounting hardware are a perfect fit either. Manufacturing tolerances on hardware vary, and pieces like bridge studs and tuner bushings should always be measured and compared to the actual size of the guide-hole.
Taking the time to measure everything and drill proper guide-holes may seem tedious and time-consuming, but the problems that could arise from not drilling them properly are exponentially worse. As you learn and gain experience, it is always useful to practice on scrap wood. Even after you have become proficient with drilling guide-holes, it never hurts to take a practice swing or two before stepping up to the plate.
Do you have any more tips or tricks about guide-holes? How about disaster stories? Share them with us!