---
type: "thread"
name: "1 3/8\"-6 UNC"
designation: "1 3/8-6 UNC"
series: "unc"
series_name: "Unified Coarse (UNC)"
system: "unified"
size_label: "1 3/8"
tpi: 6
pitch_in: 0.166667
pitch_mm: 4.2333
basic_major_dia_in: 1.375
basic_major_dia_mm: 34.925
basic_pitch_dia_in: 1.2667
basic_pitch_dia_mm: 32.1742
basic_minor_dia_in: 1.1946
basic_minor_dia_mm: 30.3428
tap_drill: "1"
tap_drill_dia_in: 1
tap_drill_dia_mm: 25.4
tap_drill_thread_pct: 173.2
clearance_normal_drill: "1"
tensile_stress_area_sqin: 1.154883
tensile_stress_area_sqmm: 745.084
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/1-3-8-6
---

# 1 3/8"-6 UNC Thread — Dimensions, Tap Drill & Clearance

The tap drill for 1 3/8"-6 UNC is 1 — 1.0000 in (25.400 mm) — which leaves 173.2% thread engagement with a cut tap. Basic pitch diameter is 1.2667 in (32.174 mm) and basic minor diameter is 1.1946 in (30.343 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 3/8-6 UNC
- **Threads per inch**: 6
- **Tap drill**: 1
- **Standard**: ASME B1.1 (reference values based on the ASME B1.1 system; not a compliance claim)

## Basic Thread Dimensions for 1 3/8"-6 UNC

| Property | Inch | Millimetre |
| --- | --- | --- |
| Basic major diameter | 1.3750 | 34.925 |
| Threads per inch | 6 | — |
| Pitch | 0.1667 | 4.233 |
| Basic pitch diameter | 1.2667 | 32.174 |
| Basic minor diameter | 1.1946 | 30.343 |
| External thread root diameter | 1.1765 | 29.883 |
| Sharp V thread height (H) | 0.1443 | 3.665 |

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

## Tap Drill Sizes for 1 3/8"-6 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% | 1.2667 | 32.174 | 1 | 1.0000 | 25.400 | 173.2 |
| 60% | 1.2451 | 31.625 | 1 | 1.0000 | 25.400 | 173.2 |
| 65% | 1.2343 | 31.351 | 1 | 1.0000 | 25.400 | 173.2 |
| 70% | 1.2234 | 31.074 | 1 | 1.0000 | 25.400 | 173.2 |
| 75% (default) | 1.2126 | 30.800 | 1 | 1.0000 | 25.400 | 173.2 |
| 80% | 1.2018 | 30.526 | 1 | 1.0000 | 25.400 | 173.2 |
| 85% | 1.1910 | 30.251 | 1 | 1.0000 | 25.400 | 173.2 |

_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 3/8"-6 UNC

The through-hole a 1 3/8"-6 UNC fastener passes freely, not the tapped hole.

| Fit | Drill | Hole Ø (in) | Hole Ø (mm) | Use |
| --- | --- | --- | --- | --- |
| Close | 1 | 1.4370 | 36.500 | Located joints, dowelled assemblies |
| Normal | 1 | 1.5000 | 38.100 | General assembly — the default |
| Loose | 1 | 1.6102 | 40.899 | Slotted or adjustable joints, weldments |

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

## Thread Class Limits for 1 3/8"-6 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 | 1.3726 | 34.864 | 1.3750 | 34.925 |
| Major Ø min | 1.3544 | 34.402 | 1.3568 | 34.463 |
| Pitch Ø max | 1.2644 | 32.116 | 1.2667 | 32.174 |
| Pitch Ø min | 1.2564 | 31.913 | 1.2608 | 32.024 |
| Pitch Ø tolerance | 0.0080 | 0.203 | 0.0060 | 0.152 |
| Minor Ø max | 1.1561 | 29.365 | 1.1585 | 29.426 |
| Minor Ø min | 1.1481 | 29.162 | 1.1525 | 29.273 |
| Allowance | 0.0024 | 0.061 | 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 | 1.4094 | 35.799 | 1.4068 | 35.733 |
| Major Ø min | 1.3750 | 34.925 | 1.3750 | 34.925 |
| Pitch Ø max | 1.2771 | 32.438 | 1.2745 | 32.372 |
| Pitch Ø min | 1.2667 | 32.174 | 1.2667 | 32.174 |
| Pitch Ø tolerance | 0.0104 | 0.264 | 0.0078 | 0.198 |
| Minor Ø max | 1.2251 | 31.117 | 1.2146 | 30.851 |
| Minor Ø min | 1.1946 | 30.343 | 1.1946 | 30.343 |
| Allowance | 0.0000 | 0.000 | 0.0000 | 0.000 |

## Tensile Stress Area of 1 3/8"-6 UNC

| Property | in² | mm² |
| --- | --- | --- |
| Tensile stress area | 1.15488 | 745.08 |

The effective cross-section used to size a 1 3/8"-6 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 3/8"-6 UNC

A 1 3/8 fastener is not always 1 3/8"-6 UNC. These are the other standard pitches cut on the same nominal size.

| Thread | Series | Pitch | Tap drill |
| --- | --- | --- | --- |
| [1 3/8"-18 UNEF](https://shopmath.org/threads/unef/1-3-8-18.md) | Unified Extra Fine (UNEF) | 18 TPI | 1 |
| [1 3/8"-12 UNF](https://shopmath.org/threads/unf/1-3-8-12.md) | Unified Fine (UNF) | 12 TPI | 1 |

## Sources and Method for 1 3/8"-6 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 3/8"-6 UNC

### What is the tap drill size for 1 3/8"-6 UNC?

The tap drill for 1 3/8"-6 UNC is 1 — 1.0000 in (25.400 mm). That leaves 173.2% thread engagement with a cut tap.

### What is the pitch diameter of 1 3/8"-6 UNC?

The basic pitch diameter of 1 3/8"-6 UNC is 1.2667 in (32.174 mm). For a class 2B thread it runs 1.2667 in (32.174 mm) to 1.2771 in (32.438 mm).

### What is the minor diameter of 1 3/8"-6 UNC?

The basic minor diameter of 1 3/8"-6 UNC is 1.1946 in (30.343 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 3/8"-6 UNC?

1 3/8"-6 UNC has 6 threads per inch, a pitch of 0.1667 in (4.233 mm).

### What size clearance hole does a 1 3/8"-6 UNC fastener need?

close fit 1 (1.4370 in (36.500 mm)), normal fit 1 (1.5000 in (38.100 mm)), loose fit 1 (1.6102 in (40.899 mm)). The normal fit is the general-assembly default.

### What is the tensile stress area of 1 3/8"-6 UNC?

The tensile stress area of 1 3/8"-6 UNC is 1.15488 in² (745.08 mm²). Multiply it by the fastener material's allowable stress to get a load.

## Related pages

- Smaller in Unified Coarse (UNC): [1 1/4"-7 UNC](https://shopmath.org/threads/unc/1-1-4-7.md) — tap drill 1
- Larger in Unified Coarse (UNC): [1 1/2"-6 UNC](https://shopmath.org/threads/unc/1-1-2-6.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/1-3-8-6) — data verified August 1, 2026. Reference data; verify against the governing standard before manufacture.*