---
type: "thread"
name: "#8-32 UNC"
designation: "#8-32 UNC"
series: "unc"
series_name: "Unified Coarse (UNC)"
system: "unified"
size_label: "#8"
tpi: 32
pitch_in: 0.03125
pitch_mm: 0.7938
basic_major_dia_in: 0.164
basic_major_dia_mm: 4.1656
basic_pitch_dia_in: 0.1437
basic_pitch_dia_mm: 3.65
basic_minor_dia_in: 0.1302
basic_minor_dia_mm: 3.3071
tap_drill: "#29"
tap_drill_dia_in: 0.136
tap_drill_dia_mm: 3.4544
tap_drill_thread_pct: 69
clearance_normal_drill: "#9"
tensile_stress_area_sqin: 0.014009
tensile_stress_area_sqmm: 9.038
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/8-32
---

# #8-32 UNC Thread — Dimensions, Tap Drill & Clearance

The tap drill for #8-32 UNC is #29 — 0.1360 in (3.454 mm) — which leaves 69.0% thread engagement with a cut tap. Basic pitch diameter is 0.1437 in (3.650 mm) and basic minor diameter is 0.1302 in (3.307 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**: #8-32 UNC
- **Threads per inch**: 32
- **Tap drill**: #29
- **Standard**: ASME B1.1 (reference values based on the ASME B1.1 system; not a compliance claim)

## Basic Thread Dimensions for #8-32 UNC

| Property | Inch | Millimetre |
| --- | --- | --- |
| Basic major diameter | 0.1640 | 4.166 |
| Threads per inch | 32 | — |
| Pitch | 0.0313 | 0.794 |
| Basic pitch diameter | 0.1437 | 3.650 |
| Basic minor diameter | 0.1302 | 3.307 |
| External thread root diameter | 0.1268 | 3.221 |
| Sharp V thread height (H) | 0.0271 | 0.688 |

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

## Tap Drill Sizes for #8-32 UNC

Cut taps, from 50% to 85% thread engagement. The shop default is 75% — a #29 drill.

| Thread % | Target hole Ø (in) | Target hole Ø (mm) | Drill | Drill Ø (in) | Drill Ø (mm) | Actual thread % |
| --- | --- | --- | --- | --- | --- | --- |
| 50% | 0.1437 | 3.650 | #27 | 0.1440 | 3.658 | 49.3 |
| 60% | 0.1396 | 3.546 | #28 | 0.1405 | 3.569 | 57.9 |
| 65% | 0.1376 | 3.495 | #29 | 0.1360 | 3.454 | 69.0 |
| 70% | 0.1356 | 3.444 | #29 | 0.1360 | 3.454 | 69.0 |
| 75% (default) | 0.1336 | 3.393 | #29 | 0.1360 | 3.454 | 69.0 |
| 80% | 0.1315 | 3.340 | #30 | 0.1285 | 3.264 | 87.4 |
| 85% | 0.1295 | 3.289 | #30 | 0.1285 | 3.264 | 87.4 |

_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 #8-32 UNC

The through-hole a #8-32 UNC fastener passes freely, not the tapped hole.

| Fit | Drill | Hole Ø (in) | Hole Ø (mm) | Use |
| --- | --- | --- | --- | --- |
| Close | #15 | 0.1811 | 4.600 | Located joints, dowelled assemblies |
| Normal | #9 | 0.1969 | 5.001 | General assembly — the default |
| Loose | #4 | 0.2126 | 5.400 | Slotted or adjustable joints, weldments |

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

## Thread Class Limits for #8-32 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 | 0.1631 | 4.143 | 0.1640 | 4.166 |
| Major Ø min | 0.1572 | 3.993 | 0.1580 | 4.013 |
| Pitch Ø max | 0.1428 | 3.627 | 0.1437 | 3.650 |
| Pitch Ø min | 0.1399 | 3.554 | 0.1415 | 3.594 |
| Pitch Ø tolerance | 0.0029 | 0.074 | 0.0022 | 0.056 |
| Minor Ø max | 0.1225 | 3.111 | 0.1234 | 3.134 |
| Minor Ø min | 0.1196 | 3.038 | 0.1212 | 3.079 |
| Allowance | 0.0009 | 0.023 | 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 | 0.1723 | 4.376 | 0.1714 | 4.354 |
| Major Ø min | 0.1640 | 4.166 | 0.1640 | 4.166 |
| Pitch Ø max | 0.1475 | 3.747 | 0.1465 | 3.721 |
| Pitch Ø min | 0.1437 | 3.650 | 0.1437 | 3.650 |
| Pitch Ø tolerance | 0.0038 | 0.097 | 0.0028 | 0.071 |
| Minor Ø max | 0.1388 | 3.526 | 0.1388 | 3.526 |
| Minor Ø min | 0.1302 | 3.307 | 0.1302 | 3.307 |
| Allowance | 0.0000 | 0.000 | 0.0000 | 0.000 |

## Tensile Stress Area of #8-32 UNC

| Property | in² | mm² |
| --- | --- | --- |
| Tensile stress area | 0.01401 | 9.04 |

The effective cross-section used to size a #8-32 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 #8-32 UNC

A #8 fastener is not always #8-32 UNC. These are the other standard pitches cut on the same nominal size.

| Thread | Series | Pitch | Tap drill |
| --- | --- | --- | --- |
| [#8-36 UNF](https://shopmath.org/threads/unf/8-36.md) | Unified Fine (UNF) | 36 TPI | #29 |

## Sources and Method for #8-32 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 #8-32 UNC

### What is the tap drill size for #8-32 UNC?

The tap drill for #8-32 UNC is #29 — 0.1360 in (3.454 mm). That leaves 69.0% thread engagement with a cut tap.

### What is the pitch diameter of #8-32 UNC?

The basic pitch diameter of #8-32 UNC is 0.1437 in (3.650 mm). For a class 2B thread it runs 0.1437 in (3.650 mm) to 0.1475 in (3.747 mm).

### What is the minor diameter of #8-32 UNC?

The basic minor diameter of #8-32 UNC is 0.1302 in (3.307 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 #8-32 UNC?

#8-32 UNC has 32 threads per inch, a pitch of 0.0313 in (0.794 mm).

### What size clearance hole does a #8-32 UNC fastener need?

close fit #15 (0.1811 in (4.600 mm)), normal fit #9 (0.1969 in (5.001 mm)), loose fit #4 (0.2126 in (5.400 mm)). The normal fit is the general-assembly default.

### What is the tensile stress area of #8-32 UNC?

The tensile stress area of #8-32 UNC is 0.01401 in² (9.04 mm²). Multiply it by the fastener material's allowable stress to get a load.

## Related pages

- Smaller in Unified Coarse (UNC): [#6-32 UNC](https://shopmath.org/threads/unc/6-32.md) — tap drill #36
- Larger in Unified Coarse (UNC): [#10-24 UNC](https://shopmath.org/threads/unc/10-24.md) — tap drill #25
- 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/8-32) — data verified August 1, 2026. Reference data; verify against the governing standard before manufacture.*