If you do much of this guitar modifying or upgrading, sooner or later you’re going to need to make a hole bigger.
For example, if you replace some of the vintage-style tuners with more modern ones, there’s a good chance you’ll need to enlarge the tuner post hole in the headstock. This is actually one of the most common reasons for bigger holes. Moving from metric 'import' pots to sturdier CTS or Bourns is another common requirement for larger holes.
Now, you’ll be tempted just to take a drill bit, the size of your new tuner, and jam it in the smaller hole.
It’s usually a recipe for gouged wood, terrible chips, and mangled guitars.
You can’t centre the bit properly, you see. And as soon as the flutes bite into the wood, it’ll generally snag or yank itself in crazy ways. Even if you manage to get all the way through, instead of a nice circular hole, it’ll be shaped like an abstract painting of an egg.
What’s to be done?
Option 1: The Step Drill Bit
For most jobs, I’d recommend a step drill bit. This is a conical bit that increases size in increments.
IMG Step drill
Because it starts with smaller sizes, you can usually find a size to fit your current (smaller) hole well. This helps to keep the bit centred as you drill.
Then, just step up the size of your hole until you’re where you need to be.
Get yourself some metric and imperial step drills and you’ll be sorted for most guitar hole-related jobs.
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Step drills are mostly intended for sheet metal work so they’re made for thin materials. This means the height of each step increment is sometimes too short to get all the way through thicker pieces of guitar.
Some bits have larger steps and that might do the trick. Or, sometimes you can drill from each side of the hole and meet in the middle. That’s often enough to do the trick. If not, you’ll be left with a little in the middle that you can file out with a rat-tail file.
Or you can use a reamer to get rid of more of the remaining wood in the centre of the hole and follow that with a file to save a little work.
Reamers? Oh, yeah… They’re option 2.
Option 2: The Reamer
A reamer is sort of like a continuous step drill. It has cutting flutes like a drill bit. The best ones do not have flutes all the way around — instead, one half of their circumference is smooth and the other half has the cutting edges. This helps keep them centred in the hole (the ones you generally find in the hardware store have cutting edges all around and aren’t as ‘clean’ to use).
Because they’re tapered, on anything other than thin materials (say an archtop top), you’ll need to work from both ends of the hole and you’ll end up with a similar ‘hump in the middle’. Again, a thin round file will take care of that.
Stew Mac carries good reamers. Their Endpin Jack Reamer for instance is invaluable for enlarging acoustic endpin holes to accept a pickup output jack. They also have some reamers for bridge pins (useful because different pins have different tapers — one instance where you don’t want a straight hole) and heaps of other stuff.
The bottom line
I’d get some step drills, if I was you. You’ll find them massively useful for this sort of stuff and they’ll cover most bases. Wait to get a reamer until you know you need a reamer (you’ll know).