Home › Tap drill chart › 1-8
1-8 tap drill size
Use a 7/8 drill (.8750 in / 22.23 mm) to tap 1-8 UNC. That gives about 77% thread engagement. Closest metric drill: 20 mm.
1-8 thread specifications
| Thread | 1-8 UNC (coarse) |
|---|---|
| Major (nominal) diameter | 1.0000 in / 25.40 mm |
| Threads per inch | 8 (pitch 3.175 mm) |
| Theoretical drill for 75% thread | .8782 in (22.31 mm) |
| Recommended tap drill | 7/8 = .8750 in |
Alternative drills and % of thread
Any of these drills will tap 1-8. Smaller drills give more thread but make tapping harder and raise the risk of breaking the tap, especially in stainless steel and in small sizes. 60–65% thread is common in tough materials. 75% is the general-purpose standard.
Tips for tapping 1-8
- Use cutting fluid, and with a hand tap back off about half a turn every turn or two to break the chip. Aluminium and soft materials are the exception: they tap cleanly with spiral-point taps.
- For blind holes, drill deeper than the thread you need. Allow at least 3–4 pitches beyond the last full thread for a plug tap (0.500" for 8 TPI).
- Thread engagement of 1–1.5 × the diameter (25.4–38.1 mm) is enough for most steel parts. Softer materials need more.