---
title: "M18×2.5 tap drill 15.5 mm, clearance 19 mm"
description: "M18×2.5: tap drill 15.5 mm, clearance hole 19 mm, 27 mm hex. Full thread dimensions, how to identify it, and torque."
url: https://atoneplace.net/fastener/m18-2-5/
site: "At One Place"
---

# M18×2.5 thread

**M18×2.5 tap drill: 15.5 mm**

Clearance hole 19 mm · 27 mm hex

M18×2.5 is a coarse metric thread with a 18 mm (0.709") major diameter and 2.5 mm pitch. Tap it with a 15.5 mm drill; drill 19 mm to pass a bolt through.

## The numbers

- **Designation:** M18×2.5
- **Major diameter:** 18 mm / 0.709"
- **Minor diameter:** 15.294 mm
- **Pitch:** 2.5 mm pitch
- **Tap drill (75% thread):** 15.5 mm — 15.5 mm
- **Clearance hole (close fit):** 19 mm / 0.748"
- **Hex across flats:** 27 mm

## How to measure what you have

1. Measure across the thread crests with calipers. A used M18×2.5 reads a little under 18 mm — the crests are truncated by the standard, then worn and plated, so anything from about 17.6 mm up is consistent with this size.
2. Measure the pitch: put a rule against the threads and count the distance over ten crests, then divide by ten. This one should give 2.5 mm.
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: 255–340 N·m**

188.1–250.8 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 plant and excavator pin retention
- Large gearbox and bearing housings
- Crane and lifting-gear fittings

## 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 |
| --- | --- | --- | --- |
| M18×1.5 | 1.5 mm | 16.5 mm | 19 mm |

## Common questions

### What size drill for an M18×2.5 tap?

15.5 mm (15.5 mm), which gives about 75% thread engagement.

### What clearance hole for M18×2.5?

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

### What spanner fits M18×2.5?

27 mm across the flats on a standard hex head.

## Related pages

- [M18×1.5](https://atoneplace.net/fastener/m18-1-5/)
- [M16×2](https://atoneplace.net/fastener/m16-2/)
- [M20×2.5](https://atoneplace.net/fastener/m20-2-5/)
- [M16×1.5](https://atoneplace.net/fastener/m16-1-5/)
- [M20×1.5](https://atoneplace.net/fastener/m20-1-5/)
- [M20×2](https://atoneplace.net/fastener/m20-2/)
- [M14×2](https://atoneplace.net/fastener/m14-2/)
- [M22×2.5](https://atoneplace.net/fastener/m22-2-5/)
- [M14×1.5](https://atoneplace.net/fastener/m14-1-5/)
- [M14×1.25](https://atoneplace.net/fastener/m14-1-25/)
- [M22×1.5](https://atoneplace.net/fastener/m22-1-5/)

## Sources

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