---
type: "thread"
name: "3/4\"-20 UNEF"
designation: "3/4-20 UNEF"
series: "unef"
series_name: "Unified Extra Fine (UNEF)"
system: "unified"
size_label: "3/4"
tpi: 20
pitch_in: 0.05
pitch_mm: 1.27
basic_major_dia_in: 0.75
basic_major_dia_mm: 19.05
basic_pitch_dia_in: 0.7175
basic_pitch_dia_mm: 18.2245
basic_minor_dia_in: 0.6959
basic_minor_dia_mm: 17.6759
tap_drill: "45/64"
tap_drill_dia_in: 0.703125
tap_drill_dia_mm: 17.8594
tap_drill_thread_pct: 72.2
clearance_normal_drill: "51/64"
tensile_stress_area_sqin: 0.38626
tensile_stress_area_sqmm: 249.2
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/3-4-20
---

# 3/4"-20 UNEF Thread — Dimensions, Tap Drill & Clearance

The tap drill for 3/4"-20 UNEF is 45/64 — 0.7031 in (17.859 mm) — which leaves 72.2% thread engagement with a cut tap. Basic pitch diameter is 0.7175 in (18.224 mm) and basic minor diameter is 0.6959 in (17.676 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**: 3/4-20 UNEF
- **Threads per inch**: 20
- **Tap drill**: 45/64
- **Standard**: ASME B1.1 (reference values based on the ASME B1.1 system; not a compliance claim)

## Basic Thread Dimensions for 3/4"-20 UNEF

| Property | Inch | Millimetre |
| --- | --- | --- |
| Basic major diameter | 0.7500 | 19.050 |
| Threads per inch | 20 | — |
| Pitch | 0.0500 | 1.270 |
| Basic pitch diameter | 0.7175 | 18.224 |
| Basic minor diameter | 0.6959 | 17.676 |
| External thread root diameter | 0.6905 | 17.539 |
| 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 3/4"-20 UNEF

Cut taps, from 50% to 85% thread engagement. The shop default is 75% — a 45/64 drill.

| Thread % | Target hole Ø (in) | Target hole Ø (mm) | Drill | Drill Ø (in) | Drill Ø (mm) | Actual thread % |
| --- | --- | --- | --- | --- | --- | --- |
| 50% | 0.7175 | 18.224 | 23/32 | 0.7188 | 18.256 | 48.1 |
| 60% | 0.7110 | 18.059 | 23/32 | 0.7188 | 18.256 | 48.1 |
| 65% | 0.7078 | 17.978 | 45/64 | 0.7031 | 17.859 | 72.2 |
| 70% | 0.7045 | 17.894 | 45/64 | 0.7031 | 17.859 | 72.2 |
| 75% (default) | 0.7013 | 17.813 | 45/64 | 0.7031 | 17.859 | 72.2 |
| 80% | 0.6980 | 17.729 | 45/64 | 0.7031 | 17.859 | 72.2 |
| 85% | 0.6948 | 17.648 | 11/16 | 0.6875 | 17.462 | 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._

## Clearance Hole Sizes for 3/4"-20 UNEF

The through-hole a 3/4"-20 UNEF fastener passes freely, not the tapped hole.

| Fit | Drill | Hole Ø (in) | Hole Ø (mm) | Use |
| --- | --- | --- | --- | --- |
| Close | 49/64 | 0.7795 | 19.799 | Located joints, dowelled assemblies |
| Normal | 51/64 | 0.8110 | 20.599 | General assembly — the default |
| Loose | 57/64 | 0.9055 | 23.000 | Slotted or adjustable joints, weldments |

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

## Thread Class Limits for 3/4"-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 | 0.7487 | 19.017 | 0.7500 | 19.050 |
| Major Ø min | 0.7405 | 18.809 | 0.7419 | 18.844 |
| Pitch Ø max | 0.7162 | 18.192 | 0.7175 | 18.224 |
| Pitch Ø min | 0.7118 | 18.080 | 0.7142 | 18.141 |
| Pitch Ø tolerance | 0.0044 | 0.112 | 0.0033 | 0.084 |
| Minor Ø max | 0.6837 | 17.366 | 0.6850 | 17.399 |
| Minor Ø min | 0.6793 | 17.254 | 0.6817 | 17.315 |
| 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 | 0.7629 | 19.378 | 0.7615 | 19.342 |
| Major Ø min | 0.7500 | 19.050 | 0.7500 | 19.050 |
| Pitch Ø max | 0.7233 | 18.372 | 0.7218 | 18.334 |
| Pitch Ø min | 0.7175 | 18.224 | 0.7175 | 18.224 |
| Pitch Ø tolerance | 0.0057 | 0.145 | 0.0043 | 0.109 |
| Minor Ø max | 0.7074 | 17.968 | 0.7036 | 17.871 |
| Minor Ø min | 0.6959 | 17.676 | 0.6959 | 17.676 |
| Allowance | 0.0000 | 0.000 | 0.0000 | 0.000 |

## Tensile Stress Area of 3/4"-20 UNEF

| Property | in² | mm² |
| --- | --- | --- |
| Tensile stress area | 0.38626 | 249.20 |

The effective cross-section used to size a 3/4"-20 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 3/4"-20 UNEF

A 3/4 fastener is not always 3/4"-20 UNEF. These are the other standard pitches cut on the same nominal size.

| Thread | Series | Pitch | Tap drill |
| --- | --- | --- | --- |
| [3/4"-16 UNF](https://shopmath.org/threads/unf/3-4-16.md) | Unified Fine (UNF) | 16 TPI | 11/16 |
| [3/4"-10 UNC](https://shopmath.org/threads/unc/3-4-10.md) | Unified Coarse (UNC) | 10 TPI | 21/32 |

## Sources and Method for 3/4"-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 3/4"-20 UNEF

### What is the tap drill size for 3/4"-20 UNEF?

The tap drill for 3/4"-20 UNEF is 45/64 — 0.7031 in (17.859 mm). That leaves 72.2% thread engagement with a cut tap.

### What is the pitch diameter of 3/4"-20 UNEF?

The basic pitch diameter of 3/4"-20 UNEF is 0.7175 in (18.224 mm). For a class 2B thread it runs 0.7175 in (18.224 mm) to 0.7233 in (18.372 mm).

### What is the minor diameter of 3/4"-20 UNEF?

The basic minor diameter of 3/4"-20 UNEF is 0.6959 in (17.676 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 3/4"-20 UNEF?

3/4"-20 UNEF has 20 threads per inch, a pitch of 0.0500 in (1.270 mm).

### What size clearance hole does a 3/4"-20 UNEF fastener need?

close fit 49/64 (0.7795 in (19.799 mm)), normal fit 51/64 (0.8110 in (20.599 mm)), loose fit 57/64 (0.9055 in (23.000 mm)). The normal fit is the general-assembly default.

### What is the tensile stress area of 3/4"-20 UNEF?

The tensile stress area of 3/4"-20 UNEF is 0.38626 in² (249.20 mm²). Multiply it by the fastener material's allowable stress to get a load.

## Related pages

- Smaller in Unified Extra Fine (UNEF): [11/16"-24 UNEF](https://shopmath.org/threads/unef/11-16-24.md) — tap drill 41/64
- Larger in Unified Extra Fine (UNEF): [13/16"-20 UNEF](https://shopmath.org/threads/unef/13-16-20.md) — tap drill 49/64
- 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/3-4-20) — data verified August 1, 2026. Reference data; verify against the governing standard before manufacture.*