---
type: "thread"
name: "13/16\"-20 UNEF"
designation: "13/16-20 UNEF"
series: "unef"
series_name: "Unified Extra Fine (UNEF)"
system: "unified"
size_label: "13/16"
tpi: 20
pitch_in: 0.05
pitch_mm: 1.27
basic_major_dia_in: 0.8125
basic_major_dia_mm: 20.6375
basic_pitch_dia_in: 0.78
basic_pitch_dia_mm: 19.812
basic_minor_dia_in: 0.7584
basic_minor_dia_mm: 19.2634
tap_drill: "49/64"
tap_drill_dia_in: 0.765625
tap_drill_dia_mm: 19.4469
tap_drill_thread_pct: 72.2
tensile_stress_area_sqin: 0.458177
tensile_stress_area_sqmm: 295.597
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/13-16-20
---

# 13/16"-20 UNEF Thread — Dimensions, Tap Drill & Clearance

The tap drill for 13/16"-20 UNEF is 49/64 — 0.7656 in (19.447 mm) — which leaves 72.2% thread engagement with a cut tap. Basic pitch diameter is 0.7800 in (19.812 mm) and basic minor diameter is 0.7584 in (19.263 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**: 13/16-20 UNEF
- **Threads per inch**: 20
- **Tap drill**: 49/64
- **Standard**: ASME B1.1 (reference values based on the ASME B1.1 system; not a compliance claim)

## Basic Thread Dimensions for 13/16"-20 UNEF

| Property | Inch | Millimetre |
| --- | --- | --- |
| Basic major diameter | 0.8125 | 20.637 |
| Threads per inch | 20 | — |
| Pitch | 0.0500 | 1.270 |
| Basic pitch diameter | 0.7800 | 19.812 |
| Basic minor diameter | 0.7584 | 19.263 |
| External thread root diameter | 0.7530 | 19.126 |
| Sharp V thread height (H) | 0.0433 | 1.100 |

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

## Tap Drill Sizes for 13/16"-20 UNEF

Cut taps, from 50% to 85% thread engagement. The shop default is 75% — a 49/64 drill.

| Thread % | Target hole Ø (in) | Target hole Ø (mm) | Drill | Drill Ø (in) | Drill Ø (mm) | Actual thread % |
| --- | --- | --- | --- | --- | --- | --- |
| 50% | 0.7800 | 19.812 | 25/32 | 0.7813 | 19.844 | 48.1 |
| 60% | 0.7735 | 19.647 | 25/32 | 0.7813 | 19.844 | 48.1 |
| 65% | 0.7703 | 19.566 | 49/64 | 0.7656 | 19.447 | 72.2 |
| 70% | 0.7670 | 19.482 | 49/64 | 0.7656 | 19.447 | 72.2 |
| 75% (default) | 0.7638 | 19.401 | 49/64 | 0.7656 | 19.447 | 72.2 |
| 80% | 0.7605 | 19.317 | 49/64 | 0.7656 | 19.447 | 72.2 |
| 85% | 0.7573 | 19.235 | 3/4 | 0.7500 | 19.050 | 96.2 |

_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._

## Thread Class Limits for 13/16"-20 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 | 0.8112 | 20.605 | 0.8125 | 20.637 |
| Major Ø min | 0.8030 | 20.396 | 0.8044 | 20.432 |
| Pitch Ø max | 0.7787 | 19.779 | 0.7800 | 19.812 |
| Pitch Ø min | 0.7742 | 19.665 | 0.7767 | 19.728 |
| Pitch Ø tolerance | 0.0044 | 0.112 | 0.0033 | 0.084 |
| Minor Ø max | 0.7462 | 18.953 | 0.7475 | 18.986 |
| Minor Ø min | 0.7418 | 18.842 | 0.7442 | 18.903 |
| Allowance | 0.0013 | 0.033 | 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 | 0.8255 | 20.968 | 0.8240 | 20.930 |
| Major Ø min | 0.8125 | 20.637 | 0.8125 | 20.637 |
| Pitch Ø max | 0.7858 | 19.959 | 0.7844 | 19.924 |
| Pitch Ø min | 0.7800 | 19.812 | 0.7800 | 19.812 |
| Pitch Ø tolerance | 0.0058 | 0.147 | 0.0043 | 0.109 |
| Minor Ø max | 0.7699 | 19.555 | 0.7661 | 19.459 |
| Minor Ø min | 0.7584 | 19.263 | 0.7584 | 19.263 |
| Allowance | 0.0000 | 0.000 | 0.0000 | 0.000 |

## Tensile Stress Area of 13/16"-20 UNEF

| Property | in² | mm² |
| --- | --- | --- |
| Tensile stress area | 0.45818 | 295.60 |

The effective cross-section used to size a 13/16"-20 UNEF fastener against a load. Multiply it by the material's allowable stress — the area itself carries no strength assumption.

## Sources and Method for 13/16"-20 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 13/16"-20 UNEF

### What is the tap drill size for 13/16"-20 UNEF?

The tap drill for 13/16"-20 UNEF is 49/64 — 0.7656 in (19.447 mm). That leaves 72.2% thread engagement with a cut tap.

### What is the pitch diameter of 13/16"-20 UNEF?

The basic pitch diameter of 13/16"-20 UNEF is 0.7800 in (19.812 mm). For a class 2B thread it runs 0.7800 in (19.812 mm) to 0.7858 in (19.959 mm).

### What is the minor diameter of 13/16"-20 UNEF?

The basic minor diameter of 13/16"-20 UNEF is 0.7584 in (19.263 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 13/16"-20 UNEF?

13/16"-20 UNEF has 20 threads per inch, a pitch of 0.0500 in (1.270 mm).

### What is the tensile stress area of 13/16"-20 UNEF?

The tensile stress area of 13/16"-20 UNEF is 0.45818 in² (295.60 mm²). Multiply it by the fastener material's allowable stress to get a load.

## Related pages

- Smaller in Unified Extra Fine (UNEF): [3/4"-20 UNEF](https://shopmath.org/threads/unef/3-4-20.md) — tap drill 45/64
- Larger in Unified Extra Fine (UNEF): [7/8"-20 UNEF](https://shopmath.org/threads/unef/7-8-20.md) — tap drill 53/64
- 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/13-16-20) — data verified August 1, 2026. Reference data; verify against the governing standard before manufacture.*