8-32 Tap Drill Size
Unified coarse (UNC) · 32 threads per inch · ASME B1.1, class 2B
What size drill bit for a 8-32 tap?
To scale section of the tapped hole
Drill options % of thread
| Drill | Inch | mm | Thread | 2B |
|---|---|---|---|---|
| 3.3 mm | 0.1299 | 3.30 | ✗ | |
| 3.4 mm | 0.1339 | 3.40 | ✓ | |
| #29Use | 0.1360 | 3.45 | ✓ | |
| 3.5 mm | 0.1378 | 3.50 | ✓ | |
| #28 | 0.1405 | 3.57 | ✗ | |
| 9/64" | 0.1406 | 3.57 | ✗ |
By material Aluminum, brass, mild steel: #29 (69%) · Stainless, tool steel: 3.5 mm (65%)
75% (orange tick) is the usual target. Hard or gummy metals like stainless are often tapped at 60 to 65% to spare the tap; aluminum, brass and plastic take 75% or a little more. The last column checks the hole against the 2B minor diameter limits (0.130 in to 0.139 in).
8-32 UNC ASME B1.1, class 2B
| Major diameter | 0.1640 in / 4.17 mm |
|---|---|
| Threads per inch | 32 TPI (pitch 0.0313 in) |
| Pitch diameter (basic) | 0.1437 in |
| Minor diameter 2B | 0.1302–0.1389 in |
| Thread-forming tap drill | #25 0.1495 in |
Clearance hole ASME B18.2.8
For a screw that passes through the part. Normal fit is the default; close fit locates the part more accurately but leaves no room for hole position error.
Hole depth rule of thumb
- 0.164 inthread length for full strength in steel (1 × D)
- 0.246 inin cast iron or brass (1.5 × D), up to 0.328 in in aluminum
- +0.156 inextra drill depth below the thread for a plug tap (5 pitches), +0.063 in with a bottoming tap
Shop notes
Of the small UNC sizes, #8-32 has the narrowest drill window. Only three drills in the table land inside the minor diameter limits: 3.4 mm, the standard #29 and 3.5 mm. Everything else misses one way or the other.
The #29 (0.1360 in) gives about 69% thread, low but sensible on a screw this size. The 3.4 mm metric drill fills the thread better at 74% and is worth using in aluminum where you want more engagement. The trap is on the large side. The 9/64 looks like a natural fractional neighbour, as does the #28, and both cut only about 58% thread, which leaves the hole above the 0.1389 in maximum minor diameter. A screw will still go in, but the engagement you gave away does not come back.
Watch for M4
M4 x 0.7 is 0.157 in across against 0.164 in for an 8-32, and the pitches (0.7 mm and 0.794 mm) are close enough that an M4 screw can start in an 8-32 hole before it binds. Going the other way, the M4 tap drill (3.3 mm) gives about 84% thread on an 8-32 and is under its minimum minor diameter, so a hole drilled from a metric chart is a poor start for an 8-32 tap.
Questions
What size is a #29 drill bit?
A #29 drill is 0.1360 in (3.45 mm), between 1/8 (0.1250 in) and 9/64 (0.1406 in). It is the standard tap drill for #8-32.