The acronym NPT describes pipe and fittings with tapered threads that follow National Pipe Thread specifications. The "1/4" portion of "1/4-18 NPT" refers to pipe diameter and the "18" refers to the number of threads per inch. Taps require a starter hole slightly smaller than the breadth of the tap's cutting edges. Because manufacturers produce 1/4-18 NPT taps to a standard size, the starter hole drill bit size is uniform across all tap makes and models.
Drill Bits and reamers
The two most common methods of creating starter holes for taps are drilling with just a drill bit or drilling with both a bit and reamer. Reamers are cutting tools that remove a small amount of material from a predrilled hole's interior circumference. To use a reamer, machinists drill a slightly undersized, primary hole to the desired depth a with standard drill bit and follow it with the reamer to achieve a precise interior diameter. While both the drill-and-reamer and drill-only methods are suitable for cutting pipe threads, following your bit with a reamer ensures precise dimensions. Note that if you use a reamer to complete a tap's starter hole, you must use a slightly undersized drill bit to perform the primary cut.
1/4-18 NPT Starter Hole Drill Bit
If you intend to use the drill-only method to prepare the starter hole for a 1/4-18 NPT tap, use a 7/16 inch drill bit. If you choose to use the drill-and-reamer methods, use a 27/64 inch bit, followed by a 1/4-18 tapered pipe reamer. In either case, you should resist the temptation to use old drill bits. Although sharp and designed to perform repeated cuts, drill bits eventually wear and become slightly smaller in diameter. Tapping threads requires precise holes; purchase new bits if you doubt the condition of your bits.
Drilling and Tapping Tools
Power drills and drill presses are the most common tools for drilling taps' starter holes. Reamers and taps mount into specially designed manual tools, called tap wrenches, or into power drills or drill presses. If a reamer or tap doesn't fit into your power drill or drill press chuck, you might need an adapter fitting, called a tap driver.
Drilling and Tapping Tips
Clean, straight holes are important to successfully tapping pipe threads. To ensure that drills, reamers and taps create uniformly sized cuts, you should clean metal flakes and dust from the holes after performing every cut. Although tap wrenches and power drills are suitable for tapping threads, many builders prefer to use drill presses, which guarantee a straight cut with drill bits, reamers or taps.