M42×4.5 thread

M42×4.5 is a coarse metric thread with a 42 mm (1.654") major diameter and 4.5 mm pitch. Tap it with a 37.5 mm drill; drill 43 mm to pass a bolt through.

The numbers

Designation

M42×4.5

Major diameter

42 mm / 1.654"

Minor diameter

37.129 mm

Pitch

4.5 mm pitch

Tap drill (75% thread)

37.5 mm — 37.5 mm

Clearance hole (close fit)

43 mm / 1.693"

Hex across flats

65 mm

How to measure what you have

1. Measure across the thread crests with calipers. A used M42×4.5 reads a little under 42 mm — the crests are truncated by the standard, then worn and plated, so anything from about 41.6 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 4.5 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

• Wind turbine tower and foundation flange bolts
• Bridge bearing and expansion-joint anchors
• Heavy press and rolling mill frames