Metric vs imperial screws — the sizes that fool everyone
Metric and imperial screws share a lot of near-identical diameters. Near-identical is exactly the problem: the screw starts, then destroys the hole.
The dangerous look-alikes
These pairs are close in diameter but have different thread pitches, so they never truly fit.
- M2 (2.00 mm) vs #2-56 (2.18 mm)
- M2.5 (2.50 mm) vs #4-40 (2.84 mm) — the most common mix-up in PC building
- M3 (3.00 mm) vs #5-40 (3.18 mm)
- M4 (4.00 mm) vs #8-32 (4.17 mm) — the classic server rack confusion
- M5 (5.00 mm) vs #10-32 (4.83 mm) — also a rack thread
- M6 (6.00 mm) vs 1/4in-20 (6.35 mm) — camera and tripod hardware
How to tell them apart quickly
Compare thread count over 10 mm against a known metric screw. Imperial threads on these sizes are usually coarser or finer by an obvious amount.
A thread pitch gauge costs very little and removes all doubt.
If a screw threads in freely for two full turns and then binds hard, back it out. Do not power through.
Where mixing actually happens
Server racks: both M6 and #10-32 rails exist, and the cage nuts are not interchangeable.
PC cases: motherboard standoffs are #6-32, drive screws are often M3.
Camera gear: 1/4-20 and 3/8-16 tripod threads sit alongside M3, M4 and M6 accessory holes on the same cage.
Common questions
Can I force a #4-40 into an M3 hole?
It will bite and feel tight, but it is cutting new threads and ruining the hole. Take it out.
Is M6 the same as 1/4 inch?
No. 1/4 inch is 6.35 mm and has 20 threads per inch. M6 is 6.00 mm with a 1.0 mm pitch.
How do I know which rack thread I have?
Look at the rail holes: square holes take cage nuts (M6 or #10-32 depending on the kit), round threaded holes are usually #10-32 in North America and M6 elsewhere.