---
title: "1\"-8 UNC tap drill 7/8\", clearance 26.19 mm"
description: "1\"-8 UNC: tap drill 7/8\", clearance hole 26.19 mm, 38.1 mm hex. Full thread dimensions, how to identify it, and torque."
url: https://atoneplace.net/fastener/one-inch-8/
site: "At One Place"
---

# 1"-8 UNC thread

**1"-8 UNC tap drill: 7/8"**

Clearance hole 26.19 mm · 38.1 mm hex

1"-8 UNC is a coarse Unified inch thread with a 25.4 mm (1.000") major diameter and 8 threads per inch (3.175 mm pitch). Tap it with a 7/8" drill; drill 26.19 mm to pass a bolt through.

## The numbers

- **Designation:** 1"-8 UNC
- **Major diameter:** 25.4 mm / 1.000"
- **Minor diameter:** 21.963 mm
- **Pitch:** 8 threads per inch (3.175 mm pitch)
- **Tap drill (75% thread):** 7/8" — 22.22 mm
- **Clearance hole (close fit):** 26.19 mm / 1.031"
- **Hex across flats:** 38.1 mm

## How to measure what you have

1. Measure across the thread crests with calipers. A used 1"-8 UNC reads a little under 25.4 mm — the crests are truncated by the standard, then worn and plated, so anything from about 25.0 mm up is consistent with this size.
2. Count the threads crossing one inch. This one gives 8.
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: 700–960 N·m**

516.4–708.2 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

- Structural anchor rods in foundations
- Heavy machine bases and press frames
- Large flange and tower connections

## 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 |
| --- | --- | --- | --- |
| 1"-12 UNF | 12 tpi | 59/64" | 26.19 mm |

## Common questions

### What size drill for an 1"-8 UNC tap?

7/8" (22.22 mm), which gives about 75% thread engagement.

### What clearance hole for 1"-8 UNC?

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

### What spanner fits 1"-8 UNC?

38.1 mm across the flats on a standard hex head.

## Related pages

- [1"-12 UNF](https://atoneplace.net/fastener/one-inch-12/)
- [7/8"-9 UNC](https://atoneplace.net/fastener/seven-eighths-9/)
- [7/8"-14 UNF](https://atoneplace.net/fastener/seven-eighths-14/)
- [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/)