---
title: "3/8\"-24 UNF tap drill Q, clearance 10.08 mm"
description: "3/8\"-24 UNF: tap drill Q, clearance hole 10.08 mm, 14.29 mm hex. Full thread dimensions, how to identify it, and torque."
url: https://atoneplace.net/fastener/three-eighths-24/
site: "At One Place"
---

# 3/8"-24 UNF thread

**3/8"-24 UNF tap drill: Q**

Clearance hole 10.08 mm · 14.29 mm hex

3/8"-24 UNF is a fine Unified inch thread with a 9.525 mm (0.375") major diameter and 24 threads per inch (1.058 mm pitch). Tap it with a Q drill; drill 10.08 mm to pass a bolt through.

## The numbers

- **Designation:** 3/8"-24 UNF
- **Major diameter:** 9.525 mm / 0.375"
- **Minor diameter:** 8.379 mm
- **Pitch:** 24 threads per inch (1.058 mm pitch)
- **Tap drill (75% thread):** Q — 8.43 mm
- **Clearance hole (close fit):** 10.08 mm / 0.397"
- **Hex across flats:** 14.29 mm

## How to measure what you have

1. Measure across the thread crests with calipers. A used 3/8"-24 UNF reads a little under 9.525 mm — the crests are truncated by the standard, then worn and plated, so anything from about 9.1 mm up is consistent with this size.
2. Count the threads crossing one inch. This one gives 24.
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: 38–52 N·m**

28.0–38.4 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

- US inverted-flare brake line fittings for 3/16-inch tube
- Automotive fine-thread bolts and studs
- Aircraft hardware

> Also the thread on US inverted-flare brake line fittings for 3/16-inch tube, which is why a brake union and a bolt can look interchangeable and are not.

## 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 |
| --- | --- | --- | --- |
| 3/8"-16 UNC | 16 tpi | 5/16" | 10.08 mm |
| 3/8"-32 UNEF | 32 tpi | 11/32" | 10.08 mm |

## Common questions

### What size drill for an 3/8"-24 UNF tap?

Q (8.43 mm), which gives about 75% thread engagement.

### What clearance hole for 3/8"-24 UNF?

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

### What spanner fits 3/8"-24 UNF?

14.29 mm across the flats on a standard hex head.

## Related pages

- [3/8"-16 UNC](https://atoneplace.net/fastener/three-eighths-16/)
- [3/8"-32 UNEF](https://atoneplace.net/fastener/three-eighths-32/)
- [7/16"-14 UNC](https://atoneplace.net/fastener/seven-sixteenths-14/)
- [7/16"-20 UNF](https://atoneplace.net/fastener/seven-sixteenths-20/)
- [5/16"-18 UNC](https://atoneplace.net/fastener/five-sixteenths-18/)
- [5/16"-24 UNF](https://atoneplace.net/fastener/five-sixteenths-24/)
- [1/2"-13 UNC](https://atoneplace.net/fastener/half-13/)
- [1/2"-20 UNF](https://atoneplace.net/fastener/half-20/)
- [1/2"-28 UNEF](https://atoneplace.net/fastener/half-28/)
- [1/4"-20 UNC](https://atoneplace.net/fastener/quarter-20/)
- [1/4"-28 UNF](https://atoneplace.net/fastener/quarter-28/)
- [1/4"-32 UNEF](https://atoneplace.net/fastener/quarter-32/)

## Sources

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