---
type: "thread"
name: "3/8\"-32 UNEF"
designation: "3/8-32 UNEF"
series: "unef"
series_name: "Unified Extra Fine (UNEF)"
system: "unified"
size_label: "3/8"
tpi: 32
pitch_in: 0.03125
pitch_mm: 0.7938
basic_major_dia_in: 0.375
basic_major_dia_mm: 9.525
basic_pitch_dia_in: 0.3547
basic_pitch_dia_mm: 9.0094
basic_minor_dia_in: 0.3412
basic_minor_dia_mm: 8.6665
tap_drill: "11/32"
tap_drill_dia_in: 0.34375
tap_drill_dia_mm: 8.7313
tap_drill_thread_pct: 77
clearance_normal_drill: "Y"
tensile_stress_area_sqin: 0.09324
tensile_stress_area_sqmm: 60.155
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/unef/3-8-32
---

# 3/8"-32 UNEF Thread — Dimensions, Tap Drill & Clearance

The tap drill for 3/8"-32 UNEF is 11/32 — 0.3438 in (8.731 mm) — which leaves 77.0% thread engagement with a cut tap. Basic pitch diameter is 0.3547 in (9.009 mm) and basic minor diameter is 0.3412 in (8.666 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 Extra Fine (UNEF)
- **Designation**: 3/8-32 UNEF
- **Threads per inch**: 32
- **Tap drill**: 11/32
- **Standard**: ASME B1.1 (reference values based on the ASME B1.1 system; not a compliance claim)

## Basic Thread Dimensions for 3/8"-32 UNEF

| Property | Inch | Millimetre |
| --- | --- | --- |
| Basic major diameter | 0.3750 | 9.525 |
| Threads per inch | 32 | — |
| Pitch | 0.0313 | 0.794 |
| Basic pitch diameter | 0.3547 | 9.009 |
| Basic minor diameter | 0.3412 | 8.666 |
| External thread root diameter | 0.3378 | 8.580 |
| 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 3/8"-32 UNEF

Cut taps, from 50% to 85% thread engagement. The shop default is 75% — a 11/32 drill.

| Thread % | Target hole Ø (in) | Target hole Ø (mm) | Drill | Drill Ø (in) | Drill Ø (mm) | Actual thread % |
| --- | --- | --- | --- | --- | --- | --- |
| 50% | 0.3547 | 9.009 | T | 0.3580 | 9.093 | 41.9 |
| 60% | 0.3506 | 8.905 | S | 0.3480 | 8.839 | 66.5 |
| 65% | 0.3486 | 8.854 | S | 0.3480 | 8.839 | 66.5 |
| 70% | 0.3466 | 8.804 | S | 0.3480 | 8.839 | 66.5 |
| 75% (default) | 0.3446 | 8.753 | 11/32 | 0.3438 | 8.731 | 77.0 |
| 80% | 0.3425 | 8.700 | 11/32 | 0.3438 | 8.731 | 77.0 |
| 85% | 0.3405 | 8.649 | R | 0.3390 | 8.611 | 88.7 |

_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 3/8"-32 UNEF

The through-hole a 3/8"-32 UNEF fastener passes freely, not the tapped hole.

| Fit | Drill | Hole Ø (in) | Hole Ø (mm) | Use |
| --- | --- | --- | --- | --- |
| Close | W | 0.3898 | 9.901 | Located joints, dowelled assemblies |
| Normal | Y | 0.4055 | 10.300 | General assembly — the default |
| Loose | Z | 0.4213 | 10.701 | Slotted or adjustable joints, weldments |

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

## Thread Class Limits for 3/8"-32 UNEF

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 | 0.3740 | 9.500 | 0.3750 | 9.525 |
| Major Ø min | 0.3680 | 9.347 | 0.3690 | 9.373 |
| Pitch Ø max | 0.3537 | 8.984 | 0.3547 | 9.009 |
| Pitch Ø min | 0.3503 | 8.898 | 0.3522 | 8.946 |
| Pitch Ø tolerance | 0.0034 | 0.086 | 0.0025 | 0.064 |
| Minor Ø max | 0.3334 | 8.468 | 0.3344 | 8.494 |
| Minor Ø min | 0.3300 | 8.382 | 0.3319 | 8.430 |
| Allowance | 0.0010 | 0.025 | 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 | 0.3839 | 9.751 | 0.3828 | 9.723 |
| Major Ø min | 0.3750 | 9.525 | 0.3750 | 9.525 |
| Pitch Ø max | 0.3591 | 9.121 | 0.3580 | 9.093 |
| Pitch Ø min | 0.3547 | 9.009 | 0.3547 | 9.009 |
| Pitch Ø tolerance | 0.0044 | 0.112 | 0.0033 | 0.084 |
| Minor Ø max | 0.3486 | 8.854 | 0.3469 | 8.811 |
| Minor Ø min | 0.3412 | 8.666 | 0.3412 | 8.666 |
| Allowance | 0.0000 | 0.000 | 0.0000 | 0.000 |

## Tensile Stress Area of 3/8"-32 UNEF

| Property | in² | mm² |
| --- | --- | --- |
| Tensile stress area | 0.09324 | 60.16 |

The effective cross-section used to size a 3/8"-32 UNEF 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 3/8"-32 UNEF

A 3/8 fastener is not always 3/8"-32 UNEF. These are the other standard pitches cut on the same nominal size.

| Thread | Series | Pitch | Tap drill |
| --- | --- | --- | --- |
| [3/8"-24 UNF](https://shopmath.org/threads/unf/3-8-24.md) | Unified Fine (UNF) | 24 TPI | Q |
| [3/8"-16 UNC](https://shopmath.org/threads/unc/3-8-16.md) | Unified Coarse (UNC) | 16 TPI | 5/16 |

## Sources and Method for 3/8"-32 UNEF

### 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 3/8"-32 UNEF

### What is the tap drill size for 3/8"-32 UNEF?

The tap drill for 3/8"-32 UNEF is 11/32 — 0.3438 in (8.731 mm). That leaves 77.0% thread engagement with a cut tap.

### What is the pitch diameter of 3/8"-32 UNEF?

The basic pitch diameter of 3/8"-32 UNEF is 0.3547 in (9.009 mm). For a class 2B thread it runs 0.3547 in (9.009 mm) to 0.3591 in (9.121 mm).

### What is the minor diameter of 3/8"-32 UNEF?

The basic minor diameter of 3/8"-32 UNEF is 0.3412 in (8.666 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 3/8"-32 UNEF?

3/8"-32 UNEF has 32 threads per inch, a pitch of 0.0313 in (0.794 mm).

### What size clearance hole does a 3/8"-32 UNEF fastener need?

close fit W (0.3898 in (9.901 mm)), normal fit Y (0.4055 in (10.300 mm)), loose fit Z (0.4213 in (10.701 mm)). The normal fit is the general-assembly default.

### What is the tensile stress area of 3/8"-32 UNEF?

The tensile stress area of 3/8"-32 UNEF is 0.09324 in² (60.16 mm²). Multiply it by the fastener material's allowable stress to get a load.

## Related pages

- Smaller in Unified Extra Fine (UNEF): [5/16"-32 UNEF](https://shopmath.org/threads/unef/5-16-32.md) — tap drill 9/32
- Larger in Unified Extra Fine (UNEF): [7/16"-28 UNEF](https://shopmath.org/threads/unef/7-16-28.md) — tap drill Y
- All Unified Extra Fine (UNEF) sizes: [Unified Extra Fine (UNEF) thread chart](https://shopmath.org/threads/unef.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/unef/3-8-32) — data verified August 1, 2026. Reference data; verify against the governing standard before manufacture.*