---
type: "thread"
name: "7/8\"-9 UNC"
designation: "7/8-9 UNC"
series: "unc"
series_name: "Unified Coarse (UNC)"
system: "unified"
size_label: "7/8"
tpi: 9
pitch_in: 0.111111
pitch_mm: 2.8222
basic_major_dia_in: 0.875
basic_major_dia_mm: 22.225
basic_pitch_dia_in: 0.8028
basic_pitch_dia_mm: 20.3911
basic_minor_dia_in: 0.7547
basic_minor_dia_mm: 19.1694
tap_drill: "49/64"
tap_drill_dia_in: 0.765625
tap_drill_dia_mm: 19.4469
tap_drill_thread_pct: 75.8
clearance_normal_drill: "59/64"
tensile_stress_area_sqin: 0.461734
tensile_stress_area_sqmm: 297.892
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/7-8-9
---

# 7/8"-9 UNC Thread — Dimensions, Tap Drill & Clearance

The tap drill for 7/8"-9 UNC is 49/64 — 0.7656 in (19.447 mm) — which leaves 75.8% thread engagement with a cut tap. Basic pitch diameter is 0.8028 in (20.391 mm) and basic minor diameter is 0.7547 in (19.169 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**: 7/8-9 UNC
- **Threads per inch**: 9
- **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 7/8"-9 UNC

| Property | Inch | Millimetre |
| --- | --- | --- |
| Basic major diameter | 0.8750 | 22.225 |
| Threads per inch | 9 | — |
| Pitch | 0.1111 | 2.822 |
| Basic pitch diameter | 0.8028 | 20.391 |
| Basic minor diameter | 0.7547 | 19.169 |
| External thread root diameter | 0.7427 | 18.865 |
| Sharp V thread height (H) | 0.0962 | 2.443 |

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

## Tap Drill Sizes for 7/8"-9 UNC

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% | 0.8028 | 20.391 | 51/64 | 0.7969 | 20.241 | 54.1 |
| 60% | 0.7884 | 20.025 | 25/32 | 0.7813 | 19.844 | 65.0 |
| 65% | 0.7812 | 19.843 | 25/32 | 0.7813 | 19.844 | 65.0 |
| 70% | 0.7740 | 19.660 | 25/32 | 0.7813 | 19.844 | 65.0 |
| 75% (default) | 0.7667 | 19.474 | 49/64 | 0.7656 | 19.447 | 75.8 |
| 80% | 0.7595 | 19.291 | 49/64 | 0.7656 | 19.447 | 75.8 |
| 85% | 0.7523 | 19.108 | 3/4 | 0.7500 | 19.050 | 86.6 |

_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._

## Clearance Hole Sizes for 7/8"-9 UNC

The through-hole a 7/8"-9 UNC fastener passes freely, not the tapped hole.

| Fit | Drill | Hole Ø (in) | Hole Ø (mm) | Use |
| --- | --- | --- | --- | --- |
| Close | 57/64 | 0.9055 | 23.000 | Located joints, dowelled assemblies |
| Normal | 59/64 | 0.9370 | 23.800 | General assembly — the default |
| Loose | 1 | 1.0315 | 26.200 | Slotted or adjustable joints, weldments |

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

## Thread Class Limits for 7/8"-9 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 | 0.8731 | 22.177 | 0.8750 | 22.225 |
| Major Ø min | 0.8592 | 21.824 | 0.8611 | 21.872 |
| Pitch Ø max | 0.8009 | 20.343 | 0.8028 | 20.391 |
| Pitch Ø min | 0.7946 | 20.183 | 0.7981 | 20.272 |
| Pitch Ø tolerance | 0.0063 | 0.160 | 0.0047 | 0.119 |
| Minor Ø max | 0.7288 | 18.512 | 0.7307 | 18.560 |
| Minor Ø min | 0.7225 | 18.352 | 0.7259 | 18.438 |
| Allowance | 0.0019 | 0.048 | 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 | 0.8992 | 22.840 | 0.8972 | 22.789 |
| Major Ø min | 0.8750 | 22.225 | 0.8750 | 22.225 |
| Pitch Ø max | 0.8110 | 20.599 | 0.8090 | 20.549 |
| Pitch Ø min | 0.8028 | 20.391 | 0.8028 | 20.391 |
| Pitch Ø tolerance | 0.0082 | 0.208 | 0.0061 | 0.155 |
| Minor Ø max | 0.7776 | 19.751 | 0.7681 | 19.510 |
| Minor Ø min | 0.7547 | 19.169 | 0.7547 | 19.169 |
| Allowance | 0.0000 | 0.000 | 0.0000 | 0.000 |

## Tensile Stress Area of 7/8"-9 UNC

| Property | in² | mm² |
| --- | --- | --- |
| Tensile stress area | 0.46173 | 297.89 |

The effective cross-section used to size a 7/8"-9 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 7/8"-9 UNC

A 7/8 fastener is not always 7/8"-9 UNC. These are the other standard pitches cut on the same nominal size.

| Thread | Series | Pitch | Tap drill |
| --- | --- | --- | --- |
| [7/8"-20 UNEF](https://shopmath.org/threads/unef/7-8-20.md) | Unified Extra Fine (UNEF) | 20 TPI | 53/64 |
| [7/8"-14 UNF](https://shopmath.org/threads/unf/7-8-14.md) | Unified Fine (UNF) | 14 TPI | 13/16 |

## Sources and Method for 7/8"-9 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 7/8"-9 UNC

### What is the tap drill size for 7/8"-9 UNC?

The tap drill for 7/8"-9 UNC is 49/64 — 0.7656 in (19.447 mm). That leaves 75.8% thread engagement with a cut tap.

### What is the pitch diameter of 7/8"-9 UNC?

The basic pitch diameter of 7/8"-9 UNC is 0.8028 in (20.391 mm). For a class 2B thread it runs 0.8028 in (20.391 mm) to 0.8110 in (20.599 mm).

### What is the minor diameter of 7/8"-9 UNC?

The basic minor diameter of 7/8"-9 UNC is 0.7547 in (19.169 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 7/8"-9 UNC?

7/8"-9 UNC has 9 threads per inch, a pitch of 0.1111 in (2.822 mm).

### What size clearance hole does a 7/8"-9 UNC fastener need?

close fit 57/64 (0.9055 in (23.000 mm)), normal fit 59/64 (0.9370 in (23.800 mm)), loose fit 1 (1.0315 in (26.200 mm)). The normal fit is the general-assembly default.

### What is the tensile stress area of 7/8"-9 UNC?

The tensile stress area of 7/8"-9 UNC is 0.46173 in² (297.89 mm²). Multiply it by the fastener material's allowable stress to get a load.

## Related pages

- Smaller in Unified Coarse (UNC): [3/4"-10 UNC](https://shopmath.org/threads/unc/3-4-10.md) — tap drill 21/32
- Larger in Unified Coarse (UNC): [1"-8 UNC](https://shopmath.org/threads/unc/1-8.md) — tap drill 7/8
- 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/7-8-9) — data verified August 1, 2026. Reference data; verify against the governing standard before manufacture.*