M18×1.5 thread

M18×1.5 is a fine metric thread with a 18 mm (0.709") major diameter and 1.5 mm pitch. Tap it with a 16.5 mm drill; drill 19 mm to pass a bolt through.

The numbers

Designation

M18×1.5

Major diameter

18 mm / 0.709"

Minor diameter

16.376 mm

Pitch

1.5 mm pitch

Tap drill (75% thread)

16.5 mm — 16.5 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 M18×1.5 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 1.5 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

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

• Oxygen and lambda sensor bosses in exhaust manifolds
• Older 18 mm spark plugs
• Hydraulic port threads

The oxygen (lambda) sensor thread on virtually every petrol car. Sensors are torqued to about 40-50 Nm, far below the bolt figure quoted here.

Other threads at this diameter