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DB9 female port often have female slots for two screws to secure the male plug onto it. What is the type of this screw?

Word 'screw' is mentioned 7 times in Wikipedia DB-9 article but not the type.

Same port is a COM port on computer. Old school mouse port too.

At least Farnell, parts distributor have these screws categorized under: d-sub-jack-screws, and many items say M3 that being the type of the thread and the size both, 3 millimeters.

However, I have an unknown screw that fits into the mentioned female slot, and I have a legit M3 type screw, and the threads are way denser on the standard M3 screw. They have the same thickness tho.

Two leftmost ones fit, and are of the type of which I want to know the designation, the one on the right is M3

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3 Answers 3

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4 - 40 UNC ASME/ANSI (US customary units rather than Imperial)

Thread Specifications

The final two digits are the number of turns per inch, so the pitch is 0.025" (US: 25 mils, UK: 25 thou).

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  • $\begingroup$ Useful if minimal answer. If you could link to a site which specifies the dimensions for a 4-40 thread, it would improve the answer. $\endgroup$
    – Eric S
    Commented Feb 18, 2020 at 20:25
  • $\begingroup$ @EricShain Added. $\endgroup$
    – Graham Nye
    Commented Feb 18, 2020 at 20:59
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At least 3 sizes of threads seem to be in common use on DE-9/DB9 connectors:

#4-40 UNC

M2.6 x 0.45

M3 x 0.5

And, the screws that go into the attachment blocks are about 5.7mm long, but it is a specialty screw that isn't threaded all the way up to its head.

https://omronfs.omron.com/en_US/ecb/products/pdf/en-xm3_l_xm2_l_xm4k_xm4l.pdf

https://www.datasheets.com/en/part-details/jey-9s-1a2b-j-s-t-mfg-co--ltd-34474395

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It says this in one of the screw definitions:

Thread Size - Imperial : 4 - 40

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  • $\begingroup$ I was just thinking when I saw the question they must be imperial - DB connectors predate metric screw threads and have never changed. $\endgroup$
    – PeterJ
    Commented Feb 18, 2020 at 10:06
  • $\begingroup$ @PeterJ: DB connectors (1952) don't predate metric screw threads (1947) (I think). $\endgroup$
    – Transistor
    Commented Feb 18, 2020 at 19:48

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