---
type: "thread"
name: "#2-56 UNC"
designation: "#2-56 UNC"
series: "unc"
series_name: "Unified Coarse (UNC)"
system: "unified"
size_label: "#2"
tpi: 56
pitch_in: 0.017857
pitch_mm: 0.4536
basic_major_dia_in: 0.086
basic_major_dia_mm: 2.1844
basic_pitch_dia_in: 0.0744
basic_pitch_dia_mm: 1.8898
basic_minor_dia_in: 0.0667
basic_minor_dia_mm: 1.6942
tap_drill: "#50"
tap_drill_dia_in: 0.07
tap_drill_dia_mm: 1.778
tap_drill_thread_pct: 69
clearance_normal_drill: "#38"
tensile_stress_area_sqin: 0.003696
tensile_stress_area_sqmm: 2.385
internal_class_assumed: "2B"
standard: "ASME B1.1 (reference values based on the ASME B1.1 system; not a compliance claim)"
verified: "August 1, 2026"
url: https://shopmath.org/threads/unc/2-56
---

# #2-56 UNC Thread — Dimensions, Tap Drill & Clearance

The tap drill for #2-56 UNC is #50 — 0.0700 in (1.778 mm) — which leaves 69.0% thread engagement with a cut tap. Basic pitch diameter is 0.0744 in (1.890 mm) and basic minor diameter is 0.0667 in (1.694 mm). Tapped-hole figures on this page assume a class 2B thread; drill sizes for 50–85% engagement are in the tap drill table below.

- **Series**: Unified Coarse (UNC)
- **Designation**: #2-56 UNC
- **Threads per inch**: 56
- **Tap drill**: #50
- **Standard**: ASME B1.1 (reference values based on the ASME B1.1 system; not a compliance claim)

## Basic Thread Dimensions for #2-56 UNC

| Property | Inch | Millimetre |
| --- | --- | --- |
| Basic major diameter | 0.0860 | 2.184 |
| Threads per inch | 56 | — |
| Pitch | 0.0179 | 0.454 |
| Basic pitch diameter | 0.0744 | 1.890 |
| Basic minor diameter | 0.0667 | 1.694 |
| External thread root diameter | 0.0647 | 1.643 |
| Sharp V thread height (H) | 0.0155 | 0.394 |

_Basic dimensions are the theoretical values before any tolerance class is applied._

## Tap Drill Sizes for #2-56 UNC

Cut taps, from 50% to 85% thread engagement. The shop default is 75% — a #50 drill.

| Thread % | Target hole Ø (in) | Target hole Ø (mm) | Drill | Drill Ø (in) | Drill Ø (mm) | Actual thread % |
| --- | --- | --- | --- | --- | --- | --- |
| 50% | 0.0744 | 1.890 | #49 | 0.0730 | 1.854 | 56.0 |
| 60% | 0.0721 | 1.831 | #49 | 0.0730 | 1.854 | 56.0 |
| 65% | 0.0709 | 1.801 | #50 | 0.0700 | 1.778 | 69.0 |
| 70% | 0.0698 | 1.773 | #50 | 0.0700 | 1.778 | 69.0 |
| 75% (default) | 0.0686 | 1.742 | #50 | 0.0700 | 1.778 | 69.0 |
| 80% | 0.0674 | 1.712 | #51 | 0.0670 | 1.702 | 81.9 |
| 85% | 0.0663 | 1.684 | #51 | 0.0670 | 1.702 | 81.9 |

_Target hole Ø is the theoretical hole for the stated engagement. The drill column is the nearest size in the standard number/letter/fractional/metric index, and actual thread % is what that real drill produces._

## Clearance Hole Sizes for #2-56 UNC

The through-hole a #2-56 UNC fastener passes freely, not the tapped hole.

| Fit | Drill | Hole Ø (in) | Hole Ø (mm) | Use |
| --- | --- | --- | --- | --- |
| Close | 3/32 | 0.0945 | 2.400 | Located joints, dowelled assemblies |
| Normal | #38 | 0.1024 | 2.601 | General assembly — the default |
| Loose | #33 | 0.1142 | 2.901 | Slotted or adjustable joints, weldments |

_Clearance holes per ISO 273 (metric) and ASME B18.2.8 (inch)._

## Thread Class Limits for #2-56 UNC

Manufacturing limits for classes 2B, 3B, 2A, 3A. A gauge reading outside these bands is a reject, not a judgement call.

### External thread (bolt / screw) limits

| Limit | Class 2A (in) | Class 2A (mm) | Class 3A (in) | Class 3A (mm) |
| --- | --- | --- | --- | --- |
| Major Ø max | 0.0854 | 2.169 | 0.0860 | 2.184 |
| Major Ø min | 0.0813 | 2.065 | 0.0819 | 2.080 |
| Pitch Ø max | 0.0738 | 1.875 | 0.0744 | 1.890 |
| Pitch Ø min | 0.0716 | 1.819 | 0.0728 | 1.849 |
| Pitch Ø tolerance | 0.0021 | 0.053 | 0.0016 | 0.041 |
| Minor Ø max | 0.0622 | 1.580 | 0.0628 | 1.595 |
| Minor Ø min | 0.0600 | 1.524 | 0.0612 | 1.554 |
| Allowance | 0.0006 | 0.015 | 0.0000 | 0.000 |

### Internal thread (nut / tapped hole) limits

| Limit | Class 2B (in) | Class 2B (mm) | Class 3B (in) | Class 3B (mm) |
| --- | --- | --- | --- | --- |
| Major Ø max | 0.0913 | 2.319 | 0.0907 | 2.304 |
| Major Ø min | 0.0860 | 2.184 | 0.0860 | 2.184 |
| Pitch Ø max | 0.0772 | 1.961 | 0.0765 | 1.943 |
| Pitch Ø min | 0.0744 | 1.890 | 0.0744 | 1.890 |
| Pitch Ø tolerance | 0.0028 | 0.071 | 0.0021 | 0.053 |
| Minor Ø max | 0.0737 | 1.872 | 0.0737 | 1.872 |
| Minor Ø min | 0.0667 | 1.694 | 0.0667 | 1.694 |
| Allowance | 0.0000 | 0.000 | 0.0000 | 0.000 |

## Tensile Stress Area of #2-56 UNC

| Property | in² | mm² |
| --- | --- | --- |
| Tensile stress area | 0.00370 | 2.38 |

The effective cross-section used to size a #2-56 UNC fastener against a load. Multiply it by the material's allowable stress — the area itself carries no strength assumption.

## Other Thread Pitches in the Same Size as #2-56 UNC

A #2 fastener is not always #2-56 UNC. These are the other standard pitches cut on the same nominal size.

| Thread | Series | Pitch | Tap drill |
| --- | --- | --- | --- |
| [#2-64 UNF](https://shopmath.org/threads/unf/2-64.md) | Unified Fine (UNF) | 64 TPI | #50 |

## Sources and Method for #2-56 UNC

### Basic dimensions

- **Source**: NBS Handbook H28 (1969) Part I §§6-7 (public domain, 17 U.S.C. 105); normative modern reference ASME B1.1; series membership: Series membership: threadlib UIS_thread.csv (BSD-3-Clause); cross-checked against NBS H28 (1969) Part I Table 2.21; clearance holes: ISO 273 (metric) and ASME B18.2.8 (inch) via FreeCAD FeatureHole.cpp (LGPL-2.1+), cross-checked against bd_warehouse clearance_hole_sizes.csv (Apache-2.0)
- **Method**: computed
- **Verified against**: NBS H28 (1969) Part I, Table 2.21 (public domain); cross-checked against FreeCAD CAM thread tables (LGPL-2.1+) — 1064 checks passed at 0.0001 in

### Tap drill sizes

- **Source**: Machinery's Handbook %-thread formula (geometry-derived); nearest-standard-drill-to-75% rule verified at 36/39 published chart rows in docs/research/data-sourcing.md §3.4
- **Method**: computed
- **Verified against**: Wikipedia Unified Thread Standard, preferred cutting tap drill size columns (CC BY-SA 4.0) — 36/39 published UNC+UNF rows reproduced

### Class limits

- **Source**: NBS Handbook H28 (1969) Part I §§6-7 (public domain, 17 U.S.C. 105); normative modern reference ASME B1.1
- **Method**: computed
- **Verified against**: NBS H28 (1969) Part I, Table 2.21 (public domain); cross-checked against FreeCAD CAM thread tables (LGPL-2.1+) — 1064 checks passed at 0.0001 in

### Formulas

**Tap drill hole diameter, inch threads**

```text
hole Ø  =  D − (%thread ÷ 100) × (1.29904 ÷ n)

  D = basic major diameter (in)
  n = threads per inch
```

_1.29904 = 2 × 0.64952, twice the engaged thread height per side on the 60° Unified profile._

**Actual thread engagement of the chosen drill**

```text
%thread  =  (D − drill Ø) × n ÷ 1.29904 × 100
```

_The drill column rounds to a real drill; this is the engagement that drill actually leaves._

**Tensile stress area, Unified inch threads**

```text
At  =  0.7854 × (D − 0.9743 ÷ n)²
```

_ASME B1.1 / NBS H28 form. Reproduces the stored value exactly for this thread._

### Assumptions

- The headline tap drill assumes 75% thread engagement, the general shop default. Every engagement from 50% to 85% is in the tap drill table — change the assumption and the number moves.
- Tap drill figures are for cut taps. Drill diameters are nominal; a worn or run-out drill cuts oversize and reduces engagement further.
- Tapped-hole figures assume a class 2B internal thread, the general-purpose fit.
- Dataset scope: Le = 1D for UNC/UNF, 9P for UNEF — determined empirically against H28 Table 2.21, resolving data-sourcing.md open item 5. EXCLUDED: form (roll) tap drills, class 1A/1B limits, class 1AR allowances, constant-pitch UN series.

**Data last verified**: August 1, 2026

## Common Questions About #2-56 UNC

### What is the tap drill size for #2-56 UNC?

The tap drill for #2-56 UNC is #50 — 0.0700 in (1.778 mm). That leaves 69.0% thread engagement with a cut tap.

### What is the pitch diameter of #2-56 UNC?

The basic pitch diameter of #2-56 UNC is 0.0744 in (1.890 mm). For a class 2B thread it runs 0.0744 in (1.890 mm) to 0.0772 in (1.961 mm).

### What is the minor diameter of #2-56 UNC?

The basic minor diameter of #2-56 UNC is 0.0667 in (1.694 mm). That is the theoretical root of the internal thread, not the drilled hole size — see the tap drill table for the hole to drill.

### What is the thread pitch of #2-56 UNC?

#2-56 UNC has 56 threads per inch, a pitch of 0.0179 in (0.454 mm).

### What size clearance hole does a #2-56 UNC fastener need?

close fit 3/32 (0.0945 in (2.400 mm)), normal fit #38 (0.1024 in (2.601 mm)), loose fit #33 (0.1142 in (2.901 mm)). The normal fit is the general-assembly default.

### What is the tensile stress area of #2-56 UNC?

The tensile stress area of #2-56 UNC is 0.00370 in² (2.38 mm²). Multiply it by the fastener material's allowable stress to get a load.

## Related pages

- Smaller in Unified Coarse (UNC): [#1-64 UNC](https://shopmath.org/threads/unc/1-64.md) — tap drill #53
- Larger in Unified Coarse (UNC): [#3-48 UNC](https://shopmath.org/threads/unc/3-48.md) — tap drill #47
- All Unified Coarse (UNC) sizes: [Unified Coarse (UNC) thread chart](https://shopmath.org/threads/unc.md)
- Every thread series: [Screw thread reference](https://shopmath.org/threads.md)
- Cross-series chart: [Tap drill chart](https://shopmath.org/charts/tap-drill.md)

---

*Source: [ShopMath](https://shopmath.org/threads/unc/2-56) — data verified August 1, 2026. Reference data; verify against the governing standard before manufacture.*