---
type: "thread"
name: "2 1/2\"-4 UNC"
designation: "2 1/2-4 UNC"
series: "unc"
series_name: "Unified Coarse (UNC)"
system: "unified"
size_label: "2 1/2"
tpi: 4
pitch_in: 0.25
pitch_mm: 6.35
basic_major_dia_in: 2.5
basic_major_dia_mm: 63.5
basic_pitch_dia_in: 2.3376
basic_pitch_dia_mm: 59.375
basic_minor_dia_in: 2.2294
basic_minor_dia_mm: 56.6268
tap_drill: "1"
tap_drill_dia_in: 1
tap_drill_dia_mm: 25.4
tap_drill_thread_pct: 461.9
tensile_stress_area_sqin: 3.998828
tensile_stress_area_sqmm: 2579.884
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/2-1-2-4
---

# 2 1/2"-4 UNC Thread — Dimensions, Tap Drill & Clearance

The tap drill for 2 1/2"-4 UNC is 1 — 1.0000 in (25.400 mm) — which leaves 461.9% thread engagement with a cut tap. Basic pitch diameter is 2.3376 in (59.375 mm) and basic minor diameter is 2.2294 in (56.627 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 1/2-4 UNC
- **Threads per inch**: 4
- **Tap drill**: 1
- **Standard**: ASME B1.1 (reference values based on the ASME B1.1 system; not a compliance claim)

## Basic Thread Dimensions for 2 1/2"-4 UNC

| Property | Inch | Millimetre |
| --- | --- | --- |
| Basic major diameter | 2.5000 | 63.500 |
| Threads per inch | 4 | — |
| Pitch | 0.2500 | 6.350 |
| Basic pitch diameter | 2.3376 | 59.375 |
| Basic minor diameter | 2.2294 | 56.627 |
| External thread root diameter | 2.2023 | 55.938 |
| Sharp V thread height (H) | 0.2165 | 5.499 |

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

## Tap Drill Sizes for 2 1/2"-4 UNC

Cut taps, from 50% to 85% thread engagement. The shop default is 75% — a 1 drill.

| Thread % | Target hole Ø (in) | Target hole Ø (mm) | Drill | Drill Ø (in) | Drill Ø (mm) | Actual thread % |
| --- | --- | --- | --- | --- | --- | --- |
| 50% | 2.3376 | 59.375 | 1 | 1.0000 | 25.400 | 461.9 |
| 60% | 2.3051 | 58.550 | 1 | 1.0000 | 25.400 | 461.9 |
| 65% | 2.2889 | 58.138 | 1 | 1.0000 | 25.400 | 461.9 |
| 70% | 2.2727 | 57.727 | 1 | 1.0000 | 25.400 | 461.9 |
| 75% (default) | 2.2564 | 57.313 | 1 | 1.0000 | 25.400 | 461.9 |
| 80% | 2.2402 | 56.901 | 1 | 1.0000 | 25.400 | 461.9 |
| 85% | 2.2240 | 56.490 | 1 | 1.0000 | 25.400 | 461.9 |

_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._

## Thread Class Limits for 2 1/2"-4 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 | 2.4969 | 63.421 | 2.5000 | 63.500 |
| Major Ø min | 2.4731 | 62.817 | 2.4762 | 62.895 |
| Pitch Ø max | 2.3345 | 59.296 | 2.3376 | 59.375 |
| Pitch Ø min | 2.3242 | 59.035 | 2.3299 | 59.179 |
| Pitch Ø tolerance | 0.0104 | 0.264 | 0.0078 | 0.198 |
| Minor Ø max | 2.1721 | 55.171 | 2.1752 | 55.250 |
| Minor Ø min | 2.1618 | 54.910 | 2.1675 | 55.054 |
| Allowance | 0.0031 | 0.079 | 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 | 2.5496 | 64.760 | 2.5462 | 64.674 |
| Major Ø min | 2.5000 | 63.500 | 2.5000 | 63.500 |
| Pitch Ø max | 2.3511 | 59.718 | 2.3477 | 59.632 |
| Pitch Ø min | 2.3376 | 59.375 | 2.3376 | 59.375 |
| Pitch Ø tolerance | 0.0135 | 0.343 | 0.0101 | 0.257 |
| Minor Ø max | 2.2669 | 57.579 | 2.2594 | 57.389 |
| Minor Ø min | 2.2294 | 56.627 | 2.2294 | 56.627 |
| Allowance | 0.0000 | 0.000 | 0.0000 | 0.000 |

## Tensile Stress Area of 2 1/2"-4 UNC

| Property | in² | mm² |
| --- | --- | --- |
| Tensile stress area | 3.99883 | 2579.88 |

The effective cross-section used to size a 2 1/2"-4 UNC fastener against a load. Multiply it by the material's allowable stress — the area itself carries no strength assumption.

## Sources and Method for 2 1/2"-4 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 1/2"-4 UNC

### What is the tap drill size for 2 1/2"-4 UNC?

The tap drill for 2 1/2"-4 UNC is 1 — 1.0000 in (25.400 mm). That leaves 461.9% thread engagement with a cut tap.

### What is the pitch diameter of 2 1/2"-4 UNC?

The basic pitch diameter of 2 1/2"-4 UNC is 2.3376 in (59.375 mm). For a class 2B thread it runs 2.3376 in (59.375 mm) to 2.3511 in (59.718 mm).

### What is the minor diameter of 2 1/2"-4 UNC?

The basic minor diameter of 2 1/2"-4 UNC is 2.2294 in (56.627 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 1/2"-4 UNC?

2 1/2"-4 UNC has 4 threads per inch, a pitch of 0.2500 in (6.350 mm).

### What is the tensile stress area of 2 1/2"-4 UNC?

The tensile stress area of 2 1/2"-4 UNC is 3.99883 in² (2579.88 mm²). Multiply it by the fastener material's allowable stress to get a load.

## Related pages

- Smaller in Unified Coarse (UNC): [2 1/4"-4 UNC](https://shopmath.org/threads/unc/2-1-4-4.md) — tap drill 1
- Larger in Unified Coarse (UNC): [2 3/4"-4 UNC](https://shopmath.org/threads/unc/2-3-4-4.md) — tap drill 1
- 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/2-1-2-4) — data verified August 1, 2026. Reference data; verify against the governing standard before manufacture.*