---
title: "7/8\"-9 UNC tap drill 49/64\", clearance 23.02 mm"
description: "7/8\"-9 UNC: tap drill 49/64\", clearance hole 23.02 mm, 33.34 mm hex. Full thread dimensions, how to identify it, and torque."
url: https://atoneplace.net/fastener/seven-eighths-9/
site: "At One Place"
---

# 7/8"-9 UNC thread

**7/8"-9 UNC tap drill: 49/64"**

Clearance hole 23.02 mm · 33.34 mm hex

7/8"-9 UNC is a coarse Unified inch thread with a 22.225 mm (0.875") major diameter and 9 threads per inch (2.822 mm pitch). Tap it with a 49/64" drill; drill 23.02 mm to pass a bolt through.

## The numbers

- **Designation:** 7/8"-9 UNC
- **Major diameter:** 22.225 mm / 0.875"
- **Minor diameter:** 19.17 mm
- **Pitch:** 9 threads per inch (2.822 mm pitch)
- **Tap drill (75% thread):** 49/64" — 19.45 mm
- **Clearance hole (close fit):** 23.02 mm / 0.906"
- **Hex across flats:** 33.34 mm

## How to measure what you have

1. Measure across the thread crests with calipers. A used 7/8"-9 UNC reads a little under 22.225 mm — the crests are truncated by the standard, then worn and plated, so anything from about 21.8 mm up is consistent with this size.
2. Count the threads crossing one inch. This one gives 9.
3. A thread pitch gauge settles it in seconds and costs very little. Guessing between a coarse and a fine thread of the same diameter is the usual way a thread gets stripped.

## Tightening torque

**Typical torque: 465–640 N·m**

343.0–472.1 lb·ft

> Plain steel property class 8.8 or SAE grade 5, dry threads, tightened to about 65% of yield. Lubricated threads reach the same preload at 20-30% less torque. Where the manufacturer publishes a figure — spark plugs, wheel nuts, cylinder heads — use theirs.

## Where this size turns up

- Heavy structural anchor bolts
- Large machine base and press bolting

## Other threads at this diameter

*Same diameter, different pitch. Forcing the wrong one strips the thread in a turn or two.*

| Thread | Pitch | Tap drill | Clearance |
| --- | --- | --- | --- |
| 7/8"-14 UNF | 14 tpi | 13/16" | 23.02 mm |

## Common questions

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

49/64" (19.45 mm), which gives about 75% thread engagement.

### What clearance hole for 7/8"-9 UNC?

23.02 mm for a close fit — enough to pass the bolt without slop.

### What spanner fits 7/8"-9 UNC?

33.34 mm across the flats on a standard hex head.

## Related pages

- [7/8"-14 UNF](https://atoneplace.net/fastener/seven-eighths-14/)
- [1"-8 UNC](https://atoneplace.net/fastener/one-inch-8/)
- [1"-12 UNF](https://atoneplace.net/fastener/one-inch-12/)
- [3/4"-10 UNC](https://atoneplace.net/fastener/three-quarters-10/)
- [3/4"-16 UNF](https://atoneplace.net/fastener/three-quarters-16/)
- [3/4"-20 UNEF](https://atoneplace.net/fastener/three-quarters-20/)
- [5/8"-11 UNC](https://atoneplace.net/fastener/five-eighths-11/)
- [5/8"-18 UNF](https://atoneplace.net/fastener/five-eighths-18/)
- [5/8"-27 UNS](https://atoneplace.net/fastener/five-eighths-27/)
- [9/16"-12 UNC](https://atoneplace.net/fastener/nine-sixteenths-12/)
- [9/16"-18 UNF](https://atoneplace.net/fastener/nine-sixteenths-18/)

## Sources

- Official vendor product documentation — At One Place (/about/)
- Typical range compiled from manufacturer specifications — At One Place (/about/)