1-8 Tap Drill Size & Thread Specs
Everything for a 1-8 UNC thread on one page: tap drill (7/8), clearance holes, wrench and hex key sizes, and the nearest metric thread.
SP-TD-01 · 1-8
1-8 specs
SizePlate
1-8 UNC · tap with
7/8 drill
.8750″
22.22 mm · 77% thread
- Metric drill
- 22 mm · .8661
- Clearance
- 1-1/64 / 1-1/32
- Wrench
- 1-1/2″
- Cap screw key
- 3/4″
Thread
- Nominal size
- 1″ · 25.4 mm
- Major diameter
- 1.0000″
- Threads per inch
- 8
- Pitch
- .1250″ · 3.175 mm
- Series
- UNC (coarse)
- UNF version
- 1-12
Drilling
- Tap drill (75%)
- 7/8 · .8750
- Metric substitute
- 22 mm · .8661
- Tap drill (≈50%)
- 59/64 · .9219
- Close fit
- 1-1/64 · 1.0156
- Free fit
- 1-1/32 · 1.0313
Tools
- Hex bolt wrench
- 1-1/2″
- Hex nut wrench
- 1-1/2″
- Socket head cap key
- 3/4″
- Button head key
- —
- Flat head key
- —
Nearest metric
- Thread
- M24 × 3.0
- Major diameter
- 24 mm
- Pitch
- 3 mm
- Tap drill
- 21 mm
- Interchangeable
- No
Not interchangeable means the threads look close but won't run together: forcing one into the other strips both.
Drill size by thread percentage
Larger drill, less thread, easier tapping. 75% is the usual target.
| Drill | Decimal | mm | Thread |
|---|---|---|---|
| 22 mm | .8661 | 22 | 82% |
| 7/8 | .8750 | 22.22 | 77% |
| 22.5 mm | .8858 | 22.5 | 70% |
| 57/64 | .8906 | 22.62 | 67% |
| 23 mm | .9055 | 23 | 58% |
| 29/32 | .9063 | 23.02 | 58% |
| 59/64 | .9219 | 23.42 | 48% |
| 23.5 mm | .9252 | 23.5 | 46% |
Neighboring sizes
Questions
What does 1-8 mean?
A 1″ nominal diameter with 8 threads per inch. It’s the coarse (UNC) thread for that size; the fine version is 1-12.
Can I use a 22 mm drill instead of 7/8?
Usually. 22 mm is .8661″ against .8750″ for 7/8, which changes the thread engagement by a few percent at most. The thread-percentage table above shows exactly how much thread each nearby drill leaves.