1-8 tap drill size
The tap drill for a 1-8 UNC thread is a 7/8 drill (0.8750 in, 22.22 mm). It leaves about 77 % thread. The closest metric drill is 22.2 mm, which leaves 78 % thread.
Choose a drill
The standard drill plus the closest metric sizes. Each option redraws the section above.
1-8 thread data
How much thread you need
Thread percentage to aim for by material.
Bars run from 50 % to 90 % thread. Typical shop practice for cut taps. Harder, tougher materials take less thread to save the tap; soft materials need more engagement to hold.
How the percentage is worked out
- TPI
- 8 threads per inch
- D
- 1.0000", major diameter
- drill
- 0.8750" (7/8)
Why not 100 %?
Going from 75 % to 100 % thread adds only about 5 % strength but roughly triples the torque needed to turn the tap, so the standard drill leaves 65–80 %.
No number or letter drill in the shop? The metric drills above are the closest substitutes, with the thread percentage each one leaves.
Other UNC threads
Common questions
What size drill for a 1-8 tap?
Use a 7/8 drill (0.8750 in, 22.22 mm). It leaves about 77 % thread.
Can I use a metric drill for 1-8?
Yes. A 22.2 mm drill gives about 78 % thread.
How tight should a 1 grade 5 bolt be?
About 644 ft-lb dry for 1-8 UNC.