7/8-9 Tap Drill Size & Thread Specs
Everything for a 7/8-9 UNC thread on one page: tap drill (49/64), clearance holes, wrench and hex key sizes, and the nearest metric thread.
SP-TD-01 · 7/8-9
7/8-9 specs
SizePlate
7/8-9 UNC · tap with
49/64 drill
.7656″
19.45 mm · 76% thread
- Metric drill
- 19.5 mm · .7677
- 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
- 9
- Pitch
- .1111″ · 2.822 mm
- Series
- UNC (coarse)
- UNF version
- 7/8-14
Drilling
- Tap drill (75%)
- 49/64 · .7656
- Metric substitute
- 19.5 mm · .7677
- Tap drill (≈50%)
- 51/64 · .7969
- 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 |
|---|---|---|---|
| 49/64 | .7656 | 19.45 | 76% |
| 19.5 mm | .7677 | 19.5 | 74% |
| 25/32 | .7813 | 19.85 | 65% |
| 20 mm | .7874 | 20 | 61% |
| 51/64 | .7969 | 20.24 | 54% |
| 20.5 mm | .8071 | 20.5 | 47% |
Neighboring sizes
Questions
What does 7/8-9 mean?
A 7/8″ nominal diameter with 9 threads per inch. It’s the coarse (UNC) thread for that size; the fine version is 7/8-14.
Can I use a 19.5 mm drill instead of 49/64?
Usually. 19.5 mm is .7677″ against .7656″ for 49/64, 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.