#6-40 UNF Thread Dimensions and Tap Drill
#6-40 UNF is the Unified fine thread for the number 6 size (major diameter 0.138 in, 3.51 mm): 40 threads per inch, a pitch of 0.02500 in (0.635 mm), on a 60° profile. ASME B1.1 lists it in the UNF series. Against the coarse #6-32 UNC it has 40 threads per inch instead of 32, a stress area 12% larger and a lead angle of 3.74° instead of 4.83°.
- Tap drill#3377% thread, 2.87 mm
- Pitch40 TPI0.635 mm
- Major diameter0.1380 in3.505 mm
- Pitch diameter0.1218 in3.093 mm
- Minor diameter (nut)0.1109 in2.818 mm
- Tensile stress area0.0101 in²6.55 mm²
- Clearance hole#18normal fit, ASME B18.2.8
Thread profile
#6-40 UNF basic dimensions
| Dimension | in | mm |
|---|---|---|
| Major diameter d = D | 0.1380 | 3.505 |
| Pitch P | 0.02500 | 0.635 |
| Pitch diameter d2 = D2 | 0.1218 | 3.093 |
| Minor diameter, internal thread D1 | 0.1109 | 2.818 |
| Minor diameter, external UNR thread d3 | 0.1082 | 2.749 |
| Height of fundamental triangle H | 0.0217 | 0.550 |
| Thread height, internal (5H/8) | 0.0135 | 0.344 |
| Thread height, external (11H/16) | 0.0149 | 0.378 |
| Crest flat of external thread (P/8) | 0.0031 | 0.079 |
| Crest flat of internal thread (P/4) | 0.0063 | 0.159 |
| Property | Value |
|---|---|
| Threads per inch | 40 |
| Flank angle | 60° (30° each side) |
| Lead angle at the pitch diameter | 3.74° |
| Tensile stress area As | 0.0101 in² (6.55 mm²) |
Basic (nominal) sizes per ASME B1.1, before tolerances. A UN external thread with a flat root has the same basic minor diameter as the nut (D1); d3 applies to the rounded UNR root.
#6-40 UNF limits of size (class 2A bolt, 2B nut)
Classes 2A and 2B are the general-purpose fits for commercial bolts, screws and nuts. The 2A external thread has an allowance of 0.0008 in, so its maximum size is that much below basic; the 2B nut starts at the basic size.
| External thread 2A | Max (in) | Min (in) | Tolerance (in) |
|---|---|---|---|
| Major diameter | 0.1372 | 0.1321 | 0.0051 |
| Pitch diameter | 0.1210 | 0.1184 | 0.0026 |
| Internal thread 2B | Max (in) | Min (in) | Tolerance (in) |
|---|---|---|---|
| Minor diameter | 0.119 | 0.111 | 0.0080 |
| Pitch diameter | 0.1252 | 0.1218 | 0.0034 |
Limits of size from ASME B1.1 Table 2.
#6-40 UNF tap drill size
Tap drill for #6-40 UNF: #33 (0.1130 in, 2.87 mm)
This leaves about 77% thread in the tapped hole.
#33 is the drill listed for #6-40 UNF in standard tap drill charts. It is 0.0250 in under the 0.1380 in major diameter.
The 2B minor diameter of the finished nut thread must lie between 0.111 and 0.119 in. The recommended drill falls inside that band.
Other drills that work for #6-40 UNF
A larger drill cuts less thread and taps more easily; a smaller one gives a fuller thread but loads the tap harder.
| Drill | mm | in | % thread | Within 2B limits | Thread engagement |
|---|---|---|---|---|---|
| 7/64" | 2.78 | 0.1094 | 88% | no | |
| #35 | 2.79 | 0.1100 | 86% | no | |
| #34 | 2.82 | 0.1110 | 83% | yes | |
| 2.85 mm | 2.85 | 0.1122 | 79% | yes | |
| #33recommended | 2.87 | 0.1130 | 77% | yes | |
| #32 | 2.95 | 0.1160 | 68% | yes | |
| #31 | 3.05 | 0.1200 | 55% | no |
% thread = (major diameter − drill diameter) ÷ (1.299 × pitch). See the tap drill chart for how to choose a percentage.
Clearance hole for #6
The clearance hole depends only on the 0.1380 in diameter, not on the thread series. The normal fit, a #18 drill, is the usual choice for general assembly.
| Fit | Drill | Hole Ø (in) | Hole Ø (mm) | Clearance on Ø (in) |
|---|---|---|---|---|
| Close | #23 | 0.1540 | 3.91 | 0.0160 |
| Normal | #18 | 0.1695 | 4.31 | 0.0315 |
| Loose | #13 | 0.1850 | 4.70 | 0.0470 |
Nominal drill sizes from ASME B18.2.8.
#6-40 UNF strength
SAE J429 starts at 1/4 in, so the SAE grades give no proof load for #6-40 UNF. For an estimate, multiply the tensile stress area of 0.0101 in² by the strength of the material: for example, 60,000 psi gives 609 lbf.
#6 pitches compared
All standard pitches on the #6 diameter. A finer pitch gives a shallower thread, a larger core and a smaller lead angle; a coarser pitch is faster to assemble and more tolerant of damage and plating.
| Thread | TPI | Pitch Ø (in) | Minor Ø D1 (in) | Stress area (in²) | Tap drill |
|---|---|---|---|---|---|
| #6-32 UNC | 32 | 0.1177 | 0.1042 | 0.00909 | #36 |
| #6-40 UNF | 40 | 0.1218 | 0.1109 | 0.0101 | #33 |
Closest metric thread
No metric thread is interchangeable with #6-40 UNF. The nearest is M3.5×0.6, which is 0.01 mm smaller in diameter with a pitch of 0.600 mm against 0.635 mm. A nut may start on the wrong thread and then bind or strip, so identify by both diameter and pitch.
| Thread | Major Ø (mm) | Pitch (mm) | TPI | Ø difference (mm) |
|---|---|---|---|---|
| #6-40 UNF (this thread) | 3.505 | 0.635 | 40.00 | — |
| M3.5×0.6 | 3.500 | 0.600 | 42.33 | -0.005 |
How the #6-40 UNF dimensions are calculated
Every basic dimension follows from the major diameter and the pitch:
- P = 0.02500 in = 1 ÷ 40 threads per inch
- H = 0.866025 × P = 0.0217 in
- d2 = d − 0.649519 × P = 0.1380 − 0.0162 = 0.1218 in
- D1 = d − 1.082532 × P = 0.1380 − 0.0271 = 0.1109 in
- d3 = d − 1.190785 × P = 0.1380 − 0.0298 = 0.1082 in (external root, 11H/16 deep)
- As = 0.7854 × (d − 0.9743 × P)² = 0.7854 × 0.1136² = 0.01014 in²
- Lead angle = arctan(P ÷ (π × d2)) = 3.74°
Related sizes and tools
Sources: ASME B1.1 (profile, series, limits of size and stress area), ASME B18.2.8 (clearance holes), SAE J429 (mechanical properties) and the tap drill tables of Machinery’s Handbook. How this site checks its data. Last reviewed 29 September 2026.