M14×1.25 thread

M14×1.25 is a fine metric thread with a 14 mm (0.551") major diameter and 1.25 mm pitch. Tap it with a 12.8 mm drill; drill 15 mm to pass a bolt through.

The numbers

Designation

M14×1.25

Major diameter

14 mm / 0.551"

Minor diameter

12.647 mm

Pitch

1.25 mm pitch

Tap drill (75% thread)

12.8 mm — 12.8 mm

Clearance hole (close fit)

15 mm / 0.591"

Hex across flats

21 mm

How to measure what you have

1. Measure across the thread crests with calipers. A used M14×1.25 reads a little under 14 mm — the crests are truncated by the standard, then worn and plated, so anything from about 13.6 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 1.25 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

• Standard 14 mm automotive spark plugs
• Wheel studs on some Porsche and older European cars
• Glow plugs on some diesels

The classic 14 mm automotive spark plug thread. Plugs are torqued to the plug maker’s figure, typically 20-30 Nm, not to the bolt figure quoted here.

Other threads at this diameter