#2-56 UNC Thread Dimensions and Tap Drill
#2-56 UNC is the Unified coarse thread for the number 2 size (major diameter 0.086 in, 2.18 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 UNC series. Finer threads on the same diameter are #2-64 UNF.
- Tap drill#5069% thread, 1.78 mm
- Pitch56 TPI0.454 mm
- Major diameter0.0860 in2.184 mm
- Pitch diameter0.0744 in1.890 mm
- Minor diameter (nut)0.0667 in1.693 mm
- Tensile stress area0.00370 in²2.39 mm²
- Clearance hole#38normal fit, ASME B18.2.8
Thread profile
#2-56 UNC basic dimensions
| Dimension | in | mm |
|---|---|---|
| Major diameter d = D | 0.0860 | 2.184 |
| Pitch P | 0.01786 | 0.454 |
| Pitch diameter d2 = D2 | 0.0744 | 1.890 |
| Minor diameter, internal thread D1 | 0.0667 | 1.693 |
| Minor diameter, external UNR thread d3 | 0.0647 | 1.644 |
| 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 | 4.37° |
| Tensile stress area As | 0.00370 in² (2.39 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.
#2-56 UNC 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 0.0006 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 | 0.0854 | 0.0813 | 0.0041 |
| Pitch diameter | 0.0738 | 0.0717 | 0.0021 |
| Internal thread 2B | Max (in) | Min (in) | Tolerance (in) |
|---|---|---|---|
| Minor diameter | 0.0737 | 0.0667 | 0.0070 |
| Pitch diameter | 0.0772 | 0.0744 | 0.0028 |
Limits of size from ASME B1.1 Table 2.
#2-56 UNC tap drill size
Tap drill for #2-56 UNC: #50 (0.0700 in, 1.78 mm)
This leaves about 69% thread in the tapped hole.
#50 is the drill listed for #2-56 UNC in standard tap drill charts. It is 0.0160 in under the 0.0860 in major diameter.
The 2B minor diameter of the finished nut thread must lie between 0.067 and 0.074 in. The recommended drill falls inside that band.
Other drills that work for #2-56 UNC
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 |
|---|---|---|---|---|---|
| #51 | 1.70 | 0.0670 | 82% | yes | |
| #50recommended | 1.78 | 0.0700 | 69% | yes | |
| 1.8 mm | 1.80 | 0.0709 | 65% | yes | |
| #49 | 1.85 | 0.0730 | 56% | yes |
% thread = (major diameter − drill diameter) ÷ (1.299 × pitch). See the tap drill chart for how to choose a percentage.
Clearance hole for #2
The clearance hole depends only on the 0.0860 in diameter, not on the thread series. The normal fit, a #38 drill, is the usual choice for general assembly.
| Fit | Drill | Hole Ø (in) | Hole Ø (mm) | Clearance on Ø (in) |
|---|---|---|---|---|
| Close | 3/32 in | 0.0938 | 2.38 | 0.0078 |
| Normal | #38 | 0.1015 | 2.58 | 0.0155 |
| Loose | #32 | 0.1160 | 2.95 | 0.0300 |
Nominal drill sizes from ASME B18.2.8.
#2-56 UNC strength
SAE J429 starts at 1/4 in, so the SAE grades give no proof load for #2-56 UNC. For an estimate, multiply the tensile stress area of 0.00370 in² by the strength of the material: for example, 60,000 psi gives 222 lbf.
#2 pitches compared
All standard pitches on the #2 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 |
|---|---|---|---|---|---|
| #2-56 UNC | 56 | 0.0744 | 0.0667 | 0.00370 | #50 |
| #2-64 UNF | 64 | 0.0759 | 0.0691 | 0.00394 | #50 |
How the #2-56 UNC 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 = 0.0860 − 0.0116 = 0.0744 in
- D1 = d − 1.082532 × P = 0.0860 − 0.0193 = 0.0667 in
- d3 = d − 1.190785 × P = 0.0860 − 0.0213 = 0.0647 in (external root, 11H/16 deep)
- As = 0.7854 × (d − 0.9743 × P)² = 0.7854 × 0.0686² = 0.00370 in²
- Lead angle = arctan(P ÷ (π × d2)) = 4.37°
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.