Screws, Bolts, Nuts and Fasteners

Metric or unified threads: reading the size code

A size such as M3x6 means a 3 mm thread, 6 mm long, in the metric system. A code such as #6-32 means gauge number 6 with 32 threads per inch, from the Unified Thread Standard. They are close but do not interchange: #6 is about 3.5 mm across, M3 is 3 mm. In computers, 2.5 inch drives and many accessories use M3, M.2 mounting uses smaller M2, and many cases and drive bays use 6-32.

Rack equipment has its own threads. Cage nuts clip into the square holes of a rack rail and give a threaded point for a rack screw. Common sizes are 10-32, 12-24 and M6, and the rail has to match the screws, so look at the rails before choosing. Square holes take cage nuts, tapped holes take screws directly, and plain round holes need a nut or clip nut behind the screw.

Head style, drive and finish choices

Head shape decides how the fastener sits. Flat or countersunk heads finish flush and need a countersunk hole, pan and truss heads sit proud and spread the load over a wider area, and thumb screws turn by hand without tools. Length is measured under the head on most screws but overall on flat heads, which include the head height. Drive is Phillips or hex, so keep a matching driver or key.

Finish matters for looks and corrosion. Black gloss or black oxide screws suit visible build work, and gold or colored thumb screws are cosmetic. Stainless steel resists rust, but common stainless grades are less strong than hardened carbon-steel bolts, and stainless threads running in stainless can seize unless lubricated. Do not overtighten small screws: M2 and M3 threads strip easily, especially in aluminum.

Questions about Screws, Bolts, Nuts and Fasteners

How can I identify an unknown screw?

Measure the thread diameter with calipers and check the pitch with a thread gauge. About 3 mm points to M3 and about 3.5 mm to #6. Then measure the length and note the head type. Test-fit it in a matching nut or tapped hole before driving it into a part.

Can I use a longer screw if the right length is not available?

A longer screw works only if there is clear depth behind the hole. One that bottoms out or presses on a circuit board or drive can crack or damage it. A screw that is too short holds on too few threads, so aim for at least one screw diameter of engagement.

Are fastener kits worth buying over single packs?

A kit helps when you service a variety of equipment and want common sizes on hand. For one repair, a pack of the exact thread and length is better, since a kit may not include your pitch. Check the kit's thread standard and size range before choosing.