#8-32 Tap Drill Size & Thread Specs
Everything for a #8-32 UNC thread on one page: tap drill (#29), clearance holes, wrench and hex key sizes, and the nearest metric thread.
SP-TD-01 · #8-32
#8-32 specs
SizePlate
#8-32 UNC · tap with
#29 drill
.1360″
3.45 mm · 69% thread
- Metric drill
- 3.5 mm · .1378
- Clearance
- #18 / #16
- Wrench
- 11/32″
- Cap screw key
- 9/64″
Thread
- Nominal size
- #8 · 4.17 mm
- Major diameter
- .1640″
- Threads per inch
- 32
- Pitch
- .0312″ · 0.794 mm
- Series
- UNC (coarse)
- UNF version
- #8-36
Drilling
- Tap drill (75%)
- #29 · .1360
- Metric substitute
- 3.5 mm · .1378
- Tap drill (≈50%)
- #27 · .1440
- Close fit
- #18 · .1695
- Free fit
- #16 · .1770
Tools
- Hex bolt wrench
- —
- Hex nut wrench
- 11/32″
- Socket head cap key
- 9/64″
- Button head key
- 3/32″
- Flat head key
- 3/32″
Nearest metric
- Thread
- M4 × 0.7
- Major diameter
- 4 mm
- Pitch
- 0.7 mm
- Tap drill
- 3.3 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 |
|---|---|---|---|
| 3.3 mm | .1299 | 3.3 | 84% |
| 3.4 mm | .1339 | 3.4 | 74% |
| #29 | .1360 | 3.45 | 69% |
| 3.5 mm | .1378 | 3.5 | 65% |
| #28 | .1405 | 3.57 | 58% |
| 9/64 | .1406 | 3.57 | 58% |
| 3.6 mm | .1417 | 3.6 | 55% |
| #27 | .1440 | 3.66 | 49% |
| 3.7 mm | .1457 | 3.7 | 45% |
Neighboring sizes
Questions
What does #8-32 mean?
A #8 nominal diameter with 32 threads per inch. It’s the coarse (UNC) thread for that size; the fine version is #8-36.
Can I use a 3.5 mm drill instead of #29?
Usually. 3.5 mm is .1378″ against .1360″ for #29, 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.