M8×1 thread

M8×1 is a fine metric thread with a 8 mm (0.315") major diameter and 1 mm pitch. Tap it with a 7 mm drill; drill 8.4 mm to pass a bolt through.

The numbers

Designation

M8×1

Major diameter

8 mm / 0.315"

Minor diameter

6.917 mm

Pitch

1 mm pitch

Tap drill (75% thread)

7 mm — 7 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 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 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

• Banjo bolts on motorcycle and car brake and fuel lines
• Engine oil drain plugs on some Japanese engines
• Tie-rod and ball-joint stud threads

Other threads at this diameter