Tap drills
1/4-20 tap drill size
The tap drill for 1/4-20 is #7, which is 0.2010 inches or 5.11 mm. That leaves about 75 percent thread engagement.
1/4-20 tap drill, clearance and engagement
| Series | UNC (coarse) |
| Major diameter | 0.2500 in (6.35 mm) |
| Threads per inch | 20 |
| Tap drill | #7 (0.2010 in / 5.11 mm) |
| Thread engagement | 75% |
| Drill for 75% thread | 0.2013 in |
| Clearance, close fit | 17/64 (0.2656 in) |
| Nearest metric drill | 5 mm (0.1969 in) |
What 1/4-20 is used for
The default general-purpose coarse thread in the inch system. Machine screws, jigs and fixtures, T-slot tables, camera and tripod hardware in the 1/4-20 form, and most shop-built weldments.
If you do not have that drill
13/64 at 0.2031 in is 0.0021 larger than a number 7 and drops engagement to about 72 percent, which is fine for steel and better for stainless. Do not substitute 7/32 at 0.2188; that leaves roughly 56 percent and a weak thread.
How to tap a 1/4-20 thread
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Drill the hole
Use a #7 drill, 0.2010 inches or 5.11 mm. Drill square to the surface and clear the swarf on deep holes.
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Start the tap square
Start with a taper tap and check it is square in both axes within the first two turns. A tap started crooked will cut a crooked thread all the way down and cannot be corrected.
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Cut and break the chip
Turn forward about half a turn, then back a quarter turn to break the chip. Use cutting fluid on steel and stainless; aluminium and brass usually need none, though a wax stick helps in aluminium.
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Check the fit
Run the mating fastener in by hand. It should turn freely without side play. If it binds, the hole was undersized or the tap wandered.
Common questions
What size drill do I need for a 1/4-20 tap?
- The tap drill for 1/4-20 is #7, which is 0.2010 inches or 5.11 mm. That leaves about 75 percent thread engagement. 13/64 at 0.2031 in is 0.0021 larger than a number 7 and drops engagement to about 72 percent, which is fine for steel and better for stainless. Do not substitute 7/32 at 0.2188; that leaves roughly 56 percent and a weak thread.
What is the clearance hole for a 1/4-20 bolt?
- 17/64, which is 0.2656 inches. That is a close fit. A normal fit is one drill size larger and a loose fit two, and the looser classes are the sensible choice when the parts have to line up without reaming.
How is the 1/4-20 tap drill worked out?
- Take the major diameter and subtract the engagement fraction times 1.299 divided by threads per inch: 0.2500 minus 0.75 times 1.299 divided by 20 gives 0.2013 inches. The nearest standard drill is #7 at 0.2010 inches.
Can I use a metric drill for a 1/4-20 tap?
- The closest standard metric drill is 5 mm, which is 0.1969 inches against 0.2010 for #7. That changes engagement to about 82 percent, so check that figure is acceptable for your material before substituting.