---
type: "thread"
name: "1 1/2\"-6 UNC"
designation: "1 1/2-6 UNC"
series: "unc"
series_name: "Unified Coarse (UNC)"
system: "unified"
size_label: "1 1/2"
tpi: 6
pitch_in: 0.166667
pitch_mm: 4.2333
basic_major_dia_in: 1.5
basic_major_dia_mm: 38.1
basic_pitch_dia_in: 1.3917
basic_pitch_dia_mm: 35.3492
basic_minor_dia_in: 1.3196
basic_minor_dia_mm: 33.5178
tap_drill: "1"
tap_drill_dia_in: 1
tap_drill_dia_mm: 25.4
tap_drill_thread_pct: 230.9
clearance_normal_drill: "1"
tensile_stress_area_sqin: 1.405252
tensile_stress_area_sqmm: 906.612
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/1-1-2-6
---

# 1 1/2"-6 UNC Thread — Dimensions, Tap Drill & Clearance

The tap drill for 1 1/2"-6 UNC is 1 — 1.0000 in (25.400 mm) — which leaves 230.9% thread engagement with a cut tap. Basic pitch diameter is 1.3917 in (35.349 mm) and basic minor diameter is 1.3196 in (33.518 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 1/2-6 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 1/2"-6 UNC

| Property | Inch | Millimetre |
| --- | --- | --- |
| Basic major diameter | 1.5000 | 38.100 |
| Threads per inch | 6 | — |
| Pitch | 0.1667 | 4.233 |
| Basic pitch diameter | 1.3917 | 35.349 |
| Basic minor diameter | 1.3196 | 33.518 |
| External thread root diameter | 1.3015 | 33.058 |
| 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 1/2"-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% | 1.3917 | 35.349 | 1 | 1.0000 | 25.400 | 230.9 |
| 60% | 1.3701 | 34.800 | 1 | 1.0000 | 25.400 | 230.9 |
| 65% | 1.3593 | 34.526 | 1 | 1.0000 | 25.400 | 230.9 |
| 70% | 1.3484 | 34.249 | 1 | 1.0000 | 25.400 | 230.9 |
| 75% (default) | 1.3376 | 33.975 | 1 | 1.0000 | 25.400 | 230.9 |
| 80% | 1.3268 | 33.701 | 1 | 1.0000 | 25.400 | 230.9 |
| 85% | 1.3160 | 33.426 | 1 | 1.0000 | 25.400 | 230.9 |

_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._

## Clearance Hole Sizes for 1 1/2"-6 UNC

The through-hole a 1 1/2"-6 UNC fastener passes freely, not the tapped hole.

| Fit | Drill | Hole Ø (in) | Hole Ø (mm) | Use |
| --- | --- | --- | --- | --- |
| Close | 1 | 1.5630 | 39.700 | Located joints, dowelled assemblies |
| Normal | 1 | 1.6260 | 41.300 | General assembly — the default |
| Loose | 1 | 1.7323 | 44.000 | Slotted or adjustable joints, weldments |

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

## Thread Class Limits for 1 1/2"-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 | 1.4976 | 38.039 | 1.5000 | 38.100 |
| Major Ø min | 1.4794 | 37.577 | 1.4818 | 37.638 |
| Pitch Ø max | 1.3893 | 35.288 | 1.3917 | 35.349 |
| Pitch Ø min | 1.3812 | 35.083 | 1.3857 | 35.197 |
| Pitch Ø tolerance | 0.0081 | 0.206 | 0.0061 | 0.155 |
| Minor Ø max | 1.2811 | 32.540 | 1.2835 | 32.601 |
| Minor Ø min | 1.2730 | 32.334 | 1.2774 | 32.446 |
| 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 | 1.5346 | 38.979 | 1.5320 | 38.913 |
| Major Ø min | 1.5000 | 38.100 | 1.5000 | 38.100 |
| Pitch Ø max | 1.4023 | 35.618 | 1.3996 | 35.550 |
| Pitch Ø min | 1.3917 | 35.349 | 1.3917 | 35.349 |
| Pitch Ø tolerance | 0.0105 | 0.267 | 0.0079 | 0.201 |
| Minor Ø max | 1.3501 | 34.292 | 1.3396 | 34.026 |
| Minor Ø min | 1.3196 | 33.518 | 1.3196 | 33.518 |
| Allowance | 0.0000 | 0.000 | 0.0000 | 0.000 |

## Tensile Stress Area of 1 1/2"-6 UNC

| Property | in² | mm² |
| --- | --- | --- |
| Tensile stress area | 1.40525 | 906.61 |

The effective cross-section used to size a 1 1/2"-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 1/2"-6 UNC

A 1 1/2 fastener is not always 1 1/2"-6 UNC. These are the other standard pitches cut on the same nominal size.

| Thread | Series | Pitch | Tap drill |
| --- | --- | --- | --- |
| [1 1/2"-18 UNEF](https://shopmath.org/threads/unef/1-1-2-18.md) | Unified Extra Fine (UNEF) | 18 TPI | 1 |
| [1 1/2"-12 UNF](https://shopmath.org/threads/unf/1-1-2-12.md) | Unified Fine (UNF) | 12 TPI | 1 |

## Sources and Method for 1 1/2"-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 1/2"-6 UNC

### What is the tap drill size for 1 1/2"-6 UNC?

The tap drill for 1 1/2"-6 UNC is 1 — 1.0000 in (25.400 mm). That leaves 230.9% thread engagement with a cut tap.

### What is the pitch diameter of 1 1/2"-6 UNC?

The basic pitch diameter of 1 1/2"-6 UNC is 1.3917 in (35.349 mm). For a class 2B thread it runs 1.3917 in (35.349 mm) to 1.4023 in (35.618 mm).

### What is the minor diameter of 1 1/2"-6 UNC?

The basic minor diameter of 1 1/2"-6 UNC is 1.3196 in (33.518 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 1/2"-6 UNC?

1 1/2"-6 UNC has 6 threads per inch, a pitch of 0.1667 in (4.233 mm).

### What size clearance hole does a 1 1/2"-6 UNC fastener need?

close fit 1 (1.5630 in (39.700 mm)), normal fit 1 (1.6260 in (41.300 mm)), loose fit 1 (1.7323 in (44.000 mm)). The normal fit is the general-assembly default.

### What is the tensile stress area of 1 1/2"-6 UNC?

The tensile stress area of 1 1/2"-6 UNC is 1.40525 in² (906.61 mm²). Multiply it by the fastener material's allowable stress to get a load.

## Related pages

- Smaller in Unified Coarse (UNC): [1 3/8"-6 UNC](https://shopmath.org/threads/unc/1-3-8-6.md) — tap drill 1
- Larger in Unified Coarse (UNC): [1 3/4"-5 UNC](https://shopmath.org/threads/unc/1-3-4-5.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/1-1-2-6) — data verified August 1, 2026. Reference data; verify against the governing standard before manufacture.*