#3-56 UNF Thread Dimensions and Tap Drill
#3-56 UNF is the Unified fine thread for the number 3 size (major diameter 0.099 in, 2.51 mm): 56 threads per inch, a pitch of 0.01786 in (0.454 mm), on a 60° profile. ASME B1.1 lists it in the UNF series as a secondary size. Against the coarse #3-48 UNC it has 56 threads per inch instead of 48, a stress area 7% larger and a lead angle of 3.72° instead of 4.44°.
- Tap drill#4573% thread, 2.08 mm
- Pitch56 TPI0.454 mm
- Major diameter0.0990 in2.515 mm
- Pitch diameter0.0874 in2.220 mm
- Minor diameter (nut)0.0797 in2.024 mm
- Tensile stress area0.00523 in²3.37 mm²
- Clearance hole#32normal fit, ASME B18.2.8
Thread profile
#3-56 UNF basic dimensions
| Dimension | in | mm |
|---|---|---|
| Major diameter d = D | 0.0990 | 2.515 |
| Pitch P | 0.01786 | 0.454 |
| Pitch diameter d2 = D2 | 0.0874 | 2.220 |
| Minor diameter, internal thread D1 | 0.0797 | 2.024 |
| Minor diameter, external UNR thread d3 | 0.0777 | 1.974 |
| Height of fundamental triangle H | 0.0155 | 0.393 |
| Thread height, internal (5H/8) | 0.0097 | 0.246 |
| Thread height, external (11H/16) | 0.0106 | 0.270 |
| Crest flat of external thread (P/8) | 0.0022 | 0.057 |
| Crest flat of internal thread (P/4) | 0.0045 | 0.113 |
| Property | Value |
|---|---|
| Threads per inch | 56 |
| Flank angle | 60° (30° each side) |
| Lead angle at the pitch diameter | 3.72° |
| Tensile stress area As | 0.00523 in² (3.37 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.
#3-56 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 0.0007 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 | 0.0983 | 0.0942 | 0.0041 |
| Pitch diameter | 0.0867 | 0.0845 | 0.0022 |
| Internal thread 2B | Max (in) | Min (in) | Tolerance (in) |
|---|---|---|---|
| Minor diameter | 0.0865 | 0.0797 | 0.0068 |
| Pitch diameter | 0.0902 | 0.0874 | 0.0028 |
Limits of size from ASME B1.1 Table 2.
#3-56 UNF tap drill size
Tap drill for #3-56 UNF: #45 (0.0820 in, 2.08 mm)
This leaves about 73% thread in the tapped hole.
#45 is the drill listed for #3-56 UNF in standard tap drill charts. It is 0.0170 in under the 0.0990 in major diameter.
The 2B minor diameter of the finished nut thread must lie between 0.080 and 0.086 in. The recommended drill falls inside that band.
Other drills that work for #3-56 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 |
|---|---|---|---|---|---|
| 5/64" | 1.98 | 0.0781 | 90% | no | |
| #47 | 1.99 | 0.0785 | 88% | no | |
| #46 | 2.06 | 0.0810 | 78% | yes | |
| #45recommended | 2.08 | 0.0820 | 73% | yes | |
| 2.1 mm | 2.10 | 0.0827 | 70% | yes | |
| #44 | 2.18 | 0.0860 | 56% | yes |
% thread = (major diameter − drill diameter) ÷ (1.299 × pitch). See the tap drill chart for how to choose a percentage.
Clearance hole for #3
The clearance hole depends only on the 0.0990 in diameter, not on the thread series. The normal fit, a #32 drill, is the usual choice for general assembly.
| Fit | Drill | Hole Ø (in) | Hole Ø (mm) | Clearance on Ø (in) |
|---|---|---|---|---|
| Close | #36 | 0.1065 | 2.71 | 0.0075 |
| Normal | #32 | 0.1160 | 2.95 | 0.0170 |
| Loose | #30 | 0.1285 | 3.26 | 0.0295 |
Nominal drill sizes from ASME B18.2.8.
#3-56 UNF strength
SAE J429 starts at 1/4 in, so the SAE grades give no proof load for #3-56 UNF. For an estimate, multiply the tensile stress area of 0.00523 in² by the strength of the material: for example, 60,000 psi gives 314 lbf.
#3 pitches compared
All standard pitches on the #3 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 |
|---|---|---|---|---|---|
| #3-48 UNC | 48 | 0.0855 | 0.0764 | 0.00487 | #47 |
| #3-56 UNF | 56 | 0.0874 | 0.0797 | 0.00523 | #45 |
Closest metric thread
No metric thread is interchangeable with #3-56 UNF. The nearest is M2.5×0.45, which is 0.01 mm smaller in diameter with a pitch of 0.450 mm against 0.454 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) |
|---|---|---|---|---|
| #3-56 UNF (this thread) | 2.515 | 0.454 | 56.00 | — |
| M2.5×0.45 | 2.500 | 0.450 | 56.44 | -0.015 |
How the #3-56 UNF dimensions are calculated
Every basic dimension follows from the major diameter and the pitch:
- P = 0.01786 in = 1 ÷ 56 threads per inch
- H = 0.866025 × P = 0.0155 in
- d2 = d − 0.649519 × P = 0.0990 − 0.0116 = 0.0874 in
- D1 = d − 1.082532 × P = 0.0990 − 0.0193 = 0.0797 in
- d3 = d − 1.190785 × P = 0.0990 − 0.0213 = 0.0777 in (external root, 11H/16 deep)
- As = 0.7854 × (d − 0.9743 × P)² = 0.7854 × 0.0816² = 0.00523 in²
- Lead angle = arctan(P ÷ (π × d2)) = 3.72°
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.