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.
Keep reading
Tools and screws for this job
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Metric screw assortment kit
One box covering the sizes in this guide
Precision driver and bit set
The right bit is what stops rounded heads
Digital caliper
Measure before you buy anything