I've never seen 4-40 thread standoffs (that is the size of the hold down screws for DVI, VGA, serial, etc.), but definitely have seen both 6-32 (aka hard disc screws) and M3 (aka CD-ROM screws ...

The major thread diameter of a shoulder screw must not exceed the shoulder diameter. Commercial-grade shoulder screws have UNC class 3A threads. So, a commercial-grade screw with a ¼-in.-diameter ...

The size of a screw is the size that would have been 100% mesh so your 1/4 inch screw will fit into a 1/4 inch hole with a small gap. There are 3 common standards for machine screws.

Matching a replacement screw or bolt can be a headache given the number of sizes and threading pitches you can find in even a modest hardware store. Mate the old and new for a perfect match ...

They're available in different sizes, but the most useful ones are similar in diameter and length to a 16d nail. For example, Spax makes a No. 9 screw that's 3¼ inches long.

Sellers called his talk, "On a Uniform Standard of Screw Threads". He complained that American nuts, bolts and screws were all individually made by hand, and that they still had to be hand-matched ...