---
type: "thread"
name: "1 11/16\"-18 UNEF"
designation: "1 11/16-18 UNEF"
series: "unef"
series_name: "Unified Extra Fine (UNEF)"
system: "unified"
size_label: "1 11/16"
tpi: 18
pitch_in: 0.055556
pitch_mm: 1.4111
basic_major_dia_in: 1.6875
basic_major_dia_mm: 42.8625
basic_pitch_dia_in: 1.6514
basic_pitch_dia_mm: 41.9456
basic_minor_dia_in: 1.6274
basic_minor_dia_mm: 41.336
tap_drill: "1"
tap_drill_dia_in: 1
tap_drill_dia_mm: 25.4
tap_drill_thread_pct: 952.6
tensile_stress_area_sqin: 2.095372
tensile_stress_area_sqmm: 1351.85
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/1-11-16-18
---

# 1 11/16"-18 UNEF Thread — Dimensions, Tap Drill & Clearance

The tap drill for 1 11/16"-18 UNEF is 1 — 1.0000 in (25.400 mm) — which leaves 952.6% thread engagement with a cut tap. Basic pitch diameter is 1.6514 in (41.946 mm) and basic minor diameter is 1.6274 in (41.336 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**: 1 11/16-18 UNEF
- **Threads per inch**: 18
- **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 11/16"-18 UNEF

| Property | Inch | Millimetre |
| --- | --- | --- |
| Basic major diameter | 1.6875 | 42.862 |
| Threads per inch | 18 | — |
| Pitch | 0.0556 | 1.411 |
| Basic pitch diameter | 1.6514 | 41.946 |
| Basic minor diameter | 1.6274 | 41.336 |
| External thread root diameter | 1.6213 | 41.181 |
| Sharp V thread height (H) | 0.0481 | 1.222 |

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

## Tap Drill Sizes for 1 11/16"-18 UNEF

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% | 1.6514 | 41.946 | 1 | 1.0000 | 25.400 | 952.6 |
| 60% | 1.6442 | 41.763 | 1 | 1.0000 | 25.400 | 952.6 |
| 65% | 1.6406 | 41.671 | 1 | 1.0000 | 25.400 | 952.6 |
| 70% | 1.6370 | 41.580 | 1 | 1.0000 | 25.400 | 952.6 |
| 75% (default) | 1.6334 | 41.488 | 1 | 1.0000 | 25.400 | 952.6 |
| 80% | 1.6298 | 41.397 | 1 | 1.0000 | 25.400 | 952.6 |
| 85% | 1.6262 | 41.306 | 1 | 1.0000 | 25.400 | 952.6 |

_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._

## Thread Class Limits for 1 11/16"-18 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 | 1.6860 | 42.824 | 1.6875 | 42.862 |
| Major Ø min | 1.6773 | 42.603 | 1.6788 | 42.642 |
| Pitch Ø max | 1.6499 | 41.907 | 1.6514 | 41.946 |
| Pitch Ø min | 1.6449 | 41.781 | 1.6476 | 41.849 |
| Pitch Ø tolerance | 0.0050 | 0.127 | 0.0038 | 0.097 |
| Minor Ø max | 1.6138 | 40.990 | 1.6153 | 41.029 |
| Minor Ø min | 1.6088 | 40.864 | 1.6116 | 40.935 |
| Allowance | 0.0015 | 0.038 | 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 | 1.7021 | 43.233 | 1.7004 | 43.190 |
| Major Ø min | 1.6875 | 42.862 | 1.6875 | 42.862 |
| Pitch Ø max | 1.6580 | 42.113 | 1.6563 | 42.070 |
| Pitch Ø min | 1.6514 | 41.946 | 1.6514 | 41.946 |
| Pitch Ø tolerance | 0.0065 | 0.165 | 0.0049 | 0.124 |
| Minor Ø max | 1.6400 | 41.656 | 1.6355 | 41.542 |
| Minor Ø min | 1.6274 | 41.336 | 1.6274 | 41.336 |
| Allowance | 0.0000 | 0.000 | 0.0000 | 0.000 |

## Tensile Stress Area of 1 11/16"-18 UNEF

| Property | in² | mm² |
| --- | --- | --- |
| Tensile stress area | 2.09537 | 1351.85 |

The effective cross-section used to size a 1 11/16"-18 UNEF fastener against a load. Multiply it by the material's allowable stress — the area itself carries no strength assumption.

## Sources and Method for 1 11/16"-18 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 1 11/16"-18 UNEF

### What is the tap drill size for 1 11/16"-18 UNEF?

The tap drill for 1 11/16"-18 UNEF is 1 — 1.0000 in (25.400 mm). That leaves 952.6% thread engagement with a cut tap.

### What is the pitch diameter of 1 11/16"-18 UNEF?

The basic pitch diameter of 1 11/16"-18 UNEF is 1.6514 in (41.946 mm). For a class 2B thread it runs 1.6514 in (41.946 mm) to 1.6580 in (42.113 mm).

### What is the minor diameter of 1 11/16"-18 UNEF?

The basic minor diameter of 1 11/16"-18 UNEF is 1.6274 in (41.336 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 11/16"-18 UNEF?

1 11/16"-18 UNEF has 18 threads per inch, a pitch of 0.0556 in (1.411 mm).

### What is the tensile stress area of 1 11/16"-18 UNEF?

The tensile stress area of 1 11/16"-18 UNEF is 2.09537 in² (1351.85 mm²). Multiply it by the fastener material's allowable stress to get a load.

## Related pages

- Smaller in Unified Extra Fine (UNEF): [1 5/8"-18 UNEF](https://shopmath.org/threads/unef/1-5-8-18.md) — tap drill 1
- 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/1-11-16-18) — data verified August 1, 2026. Reference data; verify against the governing standard before manufacture.*