Important! Before you drill, be sure to check the underside of the sink so you won't be drilling into a brace or other obstruction that would make installation of the faucet difficult or impossible.
Faucets require a hole in the sink or countertop for installation.
A standard (non-air-gap) faucet, with or without tubing attached, needs a hole that's 7/16" in diameter or larger. You can install standard faucets comfortably in holes up to 1.25". It's possible to install in a 1.5" hole, but it's hard with the standard faucet escutcheon. You can find oversized escutcheons, but they aren't always pretty.
An air gap faucet needs a hole that's at least 3/4" in diameter, and it should be no larger than 1 1/4".
If you have an existing hole that's slightly larger, you'll need to find an oversized escutcheon to take up the slack. Try a local hardware or plumbing supply store.
If you have an existing hole that's too small, there is no remedy other than to drill it larger. (We loaned our hole stretcher to a customer who did not return it.)
Drilling smaller holes in a stainless steel sink is usually quite easy. A 1/2" hole, for example, can be drilled with a standard metal bit from the hardware store. It's easiest to drill a small pilot hole, then follow with a 7/16" or 1/2" bit. The cone-shaped “Unibits” that drill an increasingly larger hole work great on stainless sinks. Larger holes are difficult in stainless. In hardware-store quality tools, a good sharp “hole saw” is your best choice.
Many of the new sinks that appear to be porcelain are actually quite thin and quite easy to drill with a standard masonry drill bit. If in doubt, consult the sink manufacturer for advice.
Drilling older porcelain or cast iron sinks should probably not be attempted unless you have special equipment (a Relton Sink Cutter, for example).
Tip #1: If there is simply no place to drill a hole in a sink, you can sometimes drill into the countertop next to the sink.
Tip #2: If you're buying a new sink, you can usually get the seller to furnish a sink with the hole you need.
(whole house & well units)