Tap drills
1/2-13 tap drill size
The tap drill for 1/2-13 is 27/64, which is 0.4219 inches or 10.72 mm. That leaves about 78 percent thread engagement.
1/2-13 tap drill, clearance and engagement
| Series | UNC (coarse) |
| Major diameter | 0.5000 in (12.70 mm) |
| Threads per inch | 13 |
| Tap drill | 27/64 (0.4219 in / 10.72 mm) |
| Thread engagement | 78% |
| Drill for 75% thread | 0.4251 in |
| Clearance, close fit | 33/64 (0.5156 in) |
| Nearest metric drill | 10.5 mm (0.4134 in) |
What 1/2-13 is used for
The workhorse coarse thread for structural bolting, machine feet, threaded rod and anchor hardware in the inch system.
If you do not have that drill
11/32 is far too small. Stay with 27/64 at 0.4219; the next size up, 7/16 at 0.4375, is the tap size itself and will not cut a thread at all.
How to tap a 1/2-13 thread
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Drill the hole
Use a 27/64 drill, 0.4219 inches or 10.72 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 1/2-13 tap?
- The tap drill for 1/2-13 is 27/64, which is 0.4219 inches or 10.72 mm. That leaves about 78 percent thread engagement. 11/32 is far too small. Stay with 27/64 at 0.4219; the next size up, 7/16 at 0.4375, is the tap size itself and will not cut a thread at all.
What is the clearance hole for a 1/2-13 bolt?
- 33/64, which is 0.5156 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 1/2-13 tap drill worked out?
- Take the major diameter and subtract the engagement fraction times 1.299 divided by threads per inch: 0.5000 minus 0.75 times 1.299 divided by 13 gives 0.4251 inches. The nearest standard drill is 27/64 at 0.4219 inches.
Can I use a metric drill for a 1/2-13 tap?
- The closest standard metric drill is 10.5 mm, which is 0.4134 inches against 0.4219 for 27/64. That changes engagement to about 87 percent, so check that figure is acceptable for your material before substituting.