Drill Bit Size for a 1/4-20 Tap (and How to Tap It)

1/4-20 and 1/4-28 tap drill sizes

The tap drill makes the hole you cut threads into. The clearance drill makes a hole the bolt slides through. Use a close fit when parts need to line up precisely, and a free fit when you want some slop for alignment.

Which drill bit for a 1/4-20 tap: your options

Percent of thread shows how deep the threads end up. More is not always better: deeper threads make the tap much harder to turn, which is why charts aim for about 65–75% (Tameson marks 65–75% as the recommended range).

Thread percentages are calculated with the standard formula: (major diameter − drill) ÷ (1.299 ÷ TPI). For 20 TPI that full thread depth is 0.06495 in, as in Tameson's percent-of-thread table.

Thread-forming (roll) taps are different. They push metal instead of cutting it and need a larger hole. Use the drill size printed on the tap or the tap maker's chart.

Other common tap drill sizes

Every size from #0 to 1 in, plus metric, is on the tap drill size chart.

How to tap a 1/4-20 hole

1. Mark and center-punch the spot so the bit doesn't wander.
2. Drill #7 straight. A drill press or drill guide helps. For a blind hole, drill deeper than the thread you need, because the tap's tip doesn't cut full threads.
3. Chamfer the edge lightly with a countersink so the tap starts square and the top thread doesn't lift.
4. Use cutting fluid suited to the metal.
5. Start the tap square. Turn it about half a turn to a full turn, then back it off a quarter turn to break the chip. Repeat.
6. Finish blind holes with a bottoming tap if you need threads close to the bottom.
7. Clear the chips and test with a 1/4-20 bolt. It should turn in by hand.

How to check you really have 1/4-20

• Diameter: calipers read about 0.25 in (6.35 mm) across the threads, often a hair under.
• Pitch: a thread gauge reads 20 TPI. Without a gauge, count the threads along 1/2 in of the bolt: you should count 10.
• Look-alike: M6 x 1.0 is close in size (6.0 mm, 0.236 in) but has a 1.0 mm pitch, about 25.4 threads per inch. A 1/4-20 tap ruins an M6 hole, and the other way round.

Common mistakes

• Drilling with a 1/4 in bit. That is the bolt's own size. Nothing is left to thread.
• Using the 1/4-20 drill for 1/4-28. The fine thread needs a #3 (0.213 in). A #7 hole is smaller than the fine thread's root, so the tap would have to cut more than a full thread.
• Mixing up 1/4-20 and M6. They look alike and don't interchange.
• Forcing the tap. If it suddenly gets hard to turn, back it out, clear the chips and add fluid. A snapped tap in a hole is hard to remove.
• Tapping a crooked hole. A tap follows the hole. If the hole leans, the bolt will too.

Not sure it's 1/4-20? Snap it

Snap the old bolt next to a ruler and Mechy tells you the size, thread and length. If the thread has to be exact, confirm it with a thread gauge or a nut you know is 1/4-20 before you drill. Identify a screw or bolt from a photo.