9/16-18 Tap Drill Size & Thread Specs
Everything for a 9/16-18 UNF thread on one page: tap drill (33/64), clearance holes, wrench and hex key sizes, and the nearest metric thread.
SP-TD-01 · 9/16-18
9/16-18 specs
SizePlate
9/16-18 UNF · tap with
33/64 drill
.5156″
13.1 mm · 65% thread
- Metric drill
- 13 mm · .5118
- Clearance
- 37/64 / 19/32
- Wrench
- 13/16″
Thread
- Nominal size
- 9/16″ · 14.29 mm
- Major diameter
- .5625″
- Threads per inch
- 18
- Pitch
- .0556″ · 1.411 mm
- Series
- UNF (fine)
- UNC version
- 9/16-12
Drilling
- Tap drill (75%)
- 33/64 · .5156
- Metric substitute
- 13 mm · .5118
- Tap drill (≈50%)
- 17/32 · .5313
- Close fit
- 37/64 · .5781
- Free fit
- 19/32 · .5938
Tools
- Hex bolt wrench
- 13/16″
- Hex nut wrench
- 7/8″
- Socket head cap key
- —
- Button head key
- —
- Flat head key
- —
Nearest metric
- Thread
- M14 × 2.0
- Major diameter
- 14 mm
- Pitch
- 2 mm
- Tap drill
- 12 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 |
|---|---|---|---|
| 12.8 mm | .5039 | 12.8 | 81% |
| 12.9 mm | .5079 | 12.9 | 76% |
| 13 mm | .5118 | 13 | 70% |
| 33/64 | .5156 | 13.1 | 65% |
Neighboring sizes
Questions
What does 9/16-18 mean?
A 9/16″ nominal diameter with 18 threads per inch. It’s the fine (UNF) thread for that size; the coarse version is 9/16-12.
Can I use a 13 mm drill instead of 33/64?
Usually. 13 mm is .5118″ against .5156″ for 33/64, 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.