Tap drills
10-24 tap drill size
The tap drill for 10-24 is #25, which is 0.1495 inches or 3.80 mm. That leaves about 75 percent thread engagement.
10-24 tap drill, clearance and engagement
| Series | UNC (coarse) |
| Major diameter | 0.1900 in (4.83 mm) |
| Threads per inch | 24 |
| Tap drill | #25 (0.1495 in / 3.80 mm) |
| Thread engagement | 75% |
| Drill for 75% thread | 0.1494 in |
| Clearance, close fit | #11 (0.1910 in) |
| Nearest metric drill | 4 mm (0.1575 in) |
What 10-24 is used for
The coarse-thread number 10, common in sheet metal assemblies, electrical panels and general fabrication where the mating material is softer.
If you do not have that drill
5/32 at 0.1562 is very close to a number 25 at 0.1495 but tightens engagement to about 84 percent. A number 24 at 0.152 is the better substitute.
How to tap a 10-24 thread
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Drill the hole
Use a #25 drill, 0.1495 inches or 3.80 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 10-24 tap?
- The tap drill for 10-24 is #25, which is 0.1495 inches or 3.80 mm. That leaves about 75 percent thread engagement. 5/32 at 0.1562 is very close to a number 25 at 0.1495 but tightens engagement to about 84 percent. A number 24 at 0.152 is the better substitute.
What is the clearance hole for a 10-24 bolt?
- #11, which is 0.1910 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 10-24 tap drill worked out?
- Take the major diameter and subtract the engagement fraction times 1.299 divided by threads per inch: 0.1900 minus 0.75 times 1.299 divided by 24 gives 0.1494 inches. The nearest standard drill is #25 at 0.1495 inches.
Can I use a metric drill for a 10-24 tap?
- The closest standard metric drill is 4 mm, which is 0.1575 inches against 0.1495 for #25. That changes engagement to about 60 percent, so check that figure is acceptable for your material before substituting.