---
type: "thread"
name: "1\"-8 UNC"
designation: "1-8 UNC"
series: "unc"
series_name: "Unified Coarse (UNC)"
system: "unified"
size_label: "1"
tpi: 8
pitch_in: 0.125
pitch_mm: 3.175
basic_major_dia_in: 1
basic_major_dia_mm: 25.4
basic_pitch_dia_in: 0.9188
basic_pitch_dia_mm: 23.3375
basic_minor_dia_in: 0.8647
basic_minor_dia_mm: 21.9634
tap_drill: "7/8"
tap_drill_dia_in: 0.875
tap_drill_dia_mm: 22.225
tap_drill_thread_pct: 77
clearance_normal_drill: "1"
tensile_stress_area_sqin: 0.605745
tensile_stress_area_sqmm: 390.802
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/1-8
---

# 1"-8 UNC Thread — Dimensions, Tap Drill & Clearance

The tap drill for 1"-8 UNC is 7/8 — 0.8750 in (22.225 mm) — which leaves 77.0% thread engagement with a cut tap. Basic pitch diameter is 0.9188 in (23.337 mm) and basic minor diameter is 0.8647 in (21.963 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**: 1-8 UNC
- **Threads per inch**: 8
- **Tap drill**: 7/8
- **Standard**: ASME B1.1 (reference values based on the ASME B1.1 system; not a compliance claim)

## Basic Thread Dimensions for 1"-8 UNC

| Property | Inch | Millimetre |
| --- | --- | --- |
| Basic major diameter | 1.0000 | 25.400 |
| Threads per inch | 8 | — |
| Pitch | 0.1250 | 3.175 |
| Basic pitch diameter | 0.9188 | 23.337 |
| Basic minor diameter | 0.8647 | 21.963 |
| External thread root diameter | 0.8512 | 21.620 |
| Sharp V thread height (H) | 0.1083 | 2.751 |

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

## Tap Drill Sizes for 1"-8 UNC

Cut taps, from 50% to 85% thread engagement. The shop default is 75% — a 7/8 drill.

| Thread % | Target hole Ø (in) | Target hole Ø (mm) | Drill | Drill Ø (in) | Drill Ø (mm) | Actual thread % |
| --- | --- | --- | --- | --- | --- | --- |
| 50% | 0.9188 | 23.337 | 59/64 | 0.9219 | 23.416 | 48.1 |
| 60% | 0.9026 | 22.926 | 29/32 | 0.9063 | 23.019 | 57.7 |
| 65% | 0.8945 | 22.720 | 57/64 | 0.8906 | 22.622 | 67.4 |
| 70% | 0.8863 | 22.512 | 57/64 | 0.8906 | 22.622 | 67.4 |
| 75% (default) | 0.8782 | 22.306 | 7/8 | 0.8750 | 22.225 | 77.0 |
| 80% | 0.8701 | 22.101 | 7/8 | 0.8750 | 22.225 | 77.0 |
| 85% | 0.8620 | 21.895 | 55/64 | 0.8594 | 21.828 | 86.6 |

_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 1"-8 UNC

The through-hole a 1"-8 UNC fastener passes freely, not the tapped hole.

| Fit | Drill | Hole Ø (in) | Hole Ø (mm) | Use |
| --- | --- | --- | --- | --- |
| Close | 1 | 1.0315 | 26.200 | Located joints, dowelled assemblies |
| Normal | 1 | 1.0945 | 27.800 | General assembly — the default |
| Loose | 1 | 1.1575 | 29.401 | Slotted or adjustable joints, weldments |

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

## Thread Class Limits for 1"-8 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 | 0.9980 | 25.349 | 1.0000 | 25.400 |
| Major Ø min | 0.9830 | 24.968 | 0.9850 | 25.019 |
| Pitch Ø max | 0.9168 | 23.287 | 0.9188 | 23.337 |
| Pitch Ø min | 0.9100 | 23.114 | 0.9137 | 23.208 |
| Pitch Ø tolerance | 0.0068 | 0.173 | 0.0051 | 0.130 |
| Minor Ø max | 0.8356 | 21.224 | 0.8376 | 21.275 |
| Minor Ø min | 0.8288 | 21.052 | 0.8326 | 21.148 |
| Allowance | 0.0020 | 0.051 | 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 | 1.0268 | 26.081 | 1.0246 | 26.025 |
| Major Ø min | 1.0000 | 25.400 | 1.0000 | 25.400 |
| Pitch Ø max | 0.9276 | 23.561 | 0.9254 | 23.505 |
| Pitch Ø min | 0.9188 | 23.337 | 0.9188 | 23.337 |
| Pitch Ø tolerance | 0.0088 | 0.224 | 0.0066 | 0.168 |
| Minor Ø max | 0.8897 | 22.598 | 0.8797 | 22.344 |
| Minor Ø min | 0.8647 | 21.963 | 0.8647 | 21.963 |
| Allowance | 0.0000 | 0.000 | 0.0000 | 0.000 |

## Tensile Stress Area of 1"-8 UNC

| Property | in² | mm² |
| --- | --- | --- |
| Tensile stress area | 0.60574 | 390.80 |

The effective cross-section used to size a 1"-8 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 1"-8 UNC

A 1 fastener is not always 1"-8 UNC. These are the other standard pitches cut on the same nominal size.

| Thread | Series | Pitch | Tap drill |
| --- | --- | --- | --- |
| [1"-20 UNEF](https://shopmath.org/threads/unef/1-20.md) | Unified Extra Fine (UNEF) | 20 TPI | 61/64 |
| [1"-12 UNF](https://shopmath.org/threads/unf/1-12.md) | Unified Fine (UNF) | 12 TPI | 59/64 |

## Sources and Method for 1"-8 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 1"-8 UNC

### What is the tap drill size for 1"-8 UNC?

The tap drill for 1"-8 UNC is 7/8 — 0.8750 in (22.225 mm). That leaves 77.0% thread engagement with a cut tap.

### What is the pitch diameter of 1"-8 UNC?

The basic pitch diameter of 1"-8 UNC is 0.9188 in (23.337 mm). For a class 2B thread it runs 0.9188 in (23.337 mm) to 0.9276 in (23.561 mm).

### What is the minor diameter of 1"-8 UNC?

The basic minor diameter of 1"-8 UNC is 0.8647 in (21.963 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 1"-8 UNC?

1"-8 UNC has 8 threads per inch, a pitch of 0.1250 in (3.175 mm).

### What size clearance hole does a 1"-8 UNC fastener need?

close fit 1 (1.0315 in (26.200 mm)), normal fit 1 (1.0945 in (27.800 mm)), loose fit 1 (1.1575 in (29.401 mm)). The normal fit is the general-assembly default.

### What is the tensile stress area of 1"-8 UNC?

The tensile stress area of 1"-8 UNC is 0.60574 in² (390.80 mm²). Multiply it by the fastener material's allowable stress to get a load.

## Related pages

- Smaller in Unified Coarse (UNC): [7/8"-9 UNC](https://shopmath.org/threads/unc/7-8-9.md) — tap drill 49/64
- Larger in Unified Coarse (UNC): [1 1/8"-7 UNC](https://shopmath.org/threads/unc/1-1-8-7.md) — tap drill 63/64
- 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/1-8) — data verified August 1, 2026. Reference data; verify against the governing standard before manufacture.*