What size do I need?
Drill Bit Size for a 10-32 Tap (Plus 10-24 and 8-32)
Short answer
Use a #21 drill bit (0.159 in) for a 10-32 tap. That drill bit size for a 10-32 tap leaves about 75% of a full thread. For the coarse 10-24 thread, use a #25 (0.1495 in). For 8-32, use a #29 (0.136 in). If the screw only needs to pass through, drill a #9 (0.196 in) or a #7 (0.201 in) instead.
Tap drill sizes for 10-32, 10-24 and 8-32
| Thread | Tap drill | Tap drill (in) | Approx. thread | Close-fit clearance | Free-fit clearance |
|---|---|---|---|---|---|
| #10-32 UNF | #21 | 0.1590 | 76% | #9 (0.1960 in) | #7 (0.2010 in) |
| #10-24 UNC | #25 | 0.1495 | 75% | #9 (0.1960 in) | #7 (0.2010 in) |
| #8-32 UNC | #29 | 0.1360 | 69% | #18 (0.1695 in) | #16 (0.1770 in) |
| #8-36 UNF | #29 | 0.1360 | 78% | #18 (0.1695 in) | #16 (0.1770 in) |
| #6-32 UNC | #36 | 0.1065 | 78% | #27 (0.1440 in) | #25 (0.1495 in) |
The tap drills match across the University of Pennsylvania, Little Machine Shop and Tameson charts, with one exception: Tameson lists a #26 (0.147 in) for 10-24. The percent-of-thread column is calculated with the standard formula, (major diameter − drill) ÷ (1.299 ÷ TPI).
10-32 tap drill substitutes
No #21 in your drill index? These are the nearest bits and what they do to the thread.
| Drill | Size (in) | Size (mm) | Approx. thread | Notes |
|---|---|---|---|---|
| 5/32 in | 0.1563 | 3.97 | 83% | Tight. Harder to tap, more risk of a broken tap in steel |
| 4.0 mm | 0.1575 | 4.00 | 80% | Slightly tight, fine in aluminum and plastic |
| #21 | 0.1590 | 4.04 | 76% | The standard |
| #20 | 0.1610 | 4.09 | 71% | Slightly looser, easier tapping |
| 4.1 mm | 0.1614 | 4.10 | 70% | Slightly looser |
| #18 | 0.1695 | 4.31 | 50% | Little Machine Shop's 50% drill, for tough steel |
How to tell 10-32 from 10-24 and M5
All three are close to 3/16 in (4.8–5 mm) across the threads, so diameter alone won't tell you.
| Thread | Diameter | Threads per inch | Pitch (mm) | Threads in 1/2 in |
|---|---|---|---|---|
| #10-32 | 0.190 in (4.83 mm) | 32 | 0.79 | 16 |
| #10-24 | 0.190 in (4.83 mm) | 24 | 1.06 | 12 |
| M5 x 0.8 | 0.197 in (5.0 mm) | 31.75 | 0.80 | about 16 |
Use a thread gauge if you can. A 10-32 and an M5 x 0.8 have almost the same pitch, so a 10-32 screw can start in an M5 hole and then bind or strip. Treat them as different threads.
How to tap a 10-32 hole
- Center-punch, then drill #21 straight. For a blind hole, drill deeper than the threads you need.
- Lightly chamfer the hole so the tap starts square.
- Add cutting fluid suited to the metal.
- Turn the tap about half a turn, back it off a quarter turn to break the chip, and repeat. Small taps like #10 snap easily, so go gently.
- Clear the chips and test with a known 10-32 screw.
Common mistakes
- Using the 10-24 drill for a 10-32 tap (or the other way). The fine thread needs the bigger #21; the coarse thread needs the smaller #25.
- Grabbing a 3/16 in bit. At 0.1875 in it is close to the screw's own diameter, so almost nothing is left to thread. It's too small for clearance too.
- Treating M5 as 10-32. Close, but not interchangeable.
- Forcing a small tap. #8 and #10 taps break with little warning. If the tap gets stiff, back it out and clear the chips.
Not sure what thread you have? Snap it
Snap the old screw next to a ruler and Mechy tells you the size, thread and length. For a 10-32 vs M5 call, confirm it with a thread gauge or a nut you know fits. Identify a screw from a photo.
Questions
What size drill bit for a 10-32 tap?
A #21 drill bit (0.159 in). It leaves about 75% thread. The nearest metric bits are 4.0 mm (tighter) and 4.1 mm (looser).
Can I use a 5/32 drill bit for a 10-32 tap?
It works, but 5/32 in (0.156 in) leaves about 83% thread, so the tap turns harder and is more likely to break in steel. A #21 is the standard.
What size drill bit for an 8-32 tap?
A #29 drill bit (0.136 in). All three charts we checked list #29 for both 8-32 and 8-36.
What size drill bit for a 10-24 tap?
A #25 drill bit (0.1495 in). Some charts list #26 (0.147 in), which gives slightly more thread.
What size hole does a #10 screw pass through?
Drill a #9 (0.196 in) for a close fit or a #7 (0.201 in) for a free fit. The same clearance holes work for 10-24 and 10-32.
Sources
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