M8×1.25 thread

M8×1.25 is a coarse metric thread with a 8 mm (0.315") major diameter and 1.25 mm pitch. Tap it with a 6.8 mm drill; drill 8.4 mm to pass a bolt through.

The numbers

Designation

M8×1.25

Major diameter

8 mm / 0.315"

Minor diameter

6.647 mm

Pitch

1.25 mm pitch

Tap drill (75% thread)

6.8 mm — 6.8 mm

Clearance hole (close fit)

8.4 mm / 0.331"

Hex across flats

13 mm

How to measure what you have

1. Measure across the thread crests with calipers. A used M8×1.25 reads a little under 8 mm — the crests are truncated by the standard, then worn and plated, so anything from about 7.6 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

• Bed frame and flat-pack furniture bolts
• Motorcycle engine covers and mirror mounts
• Automotive bracket, cover and heat-shield bolts
• Garden machinery and mower deck hardware

Nominal minus pitch is 6.75 mm. Every tap drill chart rounds it to the stocked 6.8 mm drill, which is what a tap set contains and what to buy.

Other threads at this diameter