7/8-14 Tap Drill Size & Thread Specs
Everything for a 7/8-14 UNF thread on one page: tap drill (13/16), clearance holes, wrench and hex key sizes, and the nearest metric thread.
SP-TD-01 · 7/8-14
7/8-14 specs
SizePlate
7/8-14 UNF · tap with
13/16 drill
.8125″
20.64 mm · 67% thread
- Metric drill
- 20.5 mm · .8071
- Clearance
- 57/64 / 29/32
- Wrench
- 1-5/16″
- Cap screw key
- 3/4″
Thread
- Nominal size
- 7/8″ · 22.22 mm
- Major diameter
- .8750″
- Threads per inch
- 14
- Pitch
- .0714″ · 1.814 mm
- Series
- UNF (fine)
- UNC version
- 7/8-9
Drilling
- Tap drill (75%)
- 13/16 · .8125
- Metric substitute
- 20.5 mm · .8071
- Tap drill (≈50%)
- 53/64 · .8281
- Close fit
- 57/64 · .8906
- Free fit
- 29/32 · .9063
Tools
- Hex bolt wrench
- 1-5/16″
- Hex nut wrench
- 1-5/16″
- 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 |
|---|---|---|---|
| 51/64 | .7969 | 20.24 | 84% |
| 20.5 mm | .8071 | 20.5 | 73% |
| 13/16 | .8125 | 20.64 | 67% |
| 21 mm | .8268 | 21 | 52% |
| 53/64 | .8281 | 21.03 | 51% |
Neighboring sizes
Questions
What does 7/8-14 mean?
A 7/8″ nominal diameter with 14 threads per inch. It’s the fine (UNF) thread for that size; the coarse version is 7/8-9.
Can I use a 20.5 mm drill instead of 13/16?
Usually. 20.5 mm is .8071″ against .8125″ for 13/16, 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.