It's a UNC thread and below 1/4" they use numbers rather than diameter much like wood screws 

What sort of head are you after?

Edited By JasonB on 16/02/2022 20:30:12

It has diameter 0.125". Often used in US designs.

**LINK**

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Presumably sarcasm … but why the need ?

MichaelG.

I guess switching to numbers avoids silly complicated fractions on smaller sizes.

Same reason for BA being numbered I guess.

Naming and identification systems need to be clear and not easily mis-understood. Not slaves to theoretical rules.

Metric way is nicely tied to rules but in practice it can be less than clear. Decimal points are easily missed or mis-read.

Simple M for standard metric coarse then M diameter … x … pitch for fine or constant pitch switches horses in a bad, confusing way.

Should have followed the old American convention of NC, NF, NS diameter … x … tpi so MC diameter for standard coarse, MF for standard fine and MS diameter … x … pitch for anything non standard such as the various permitted constant pitch sizes.

Much clearer and its obvious when you have a weird one to deal with.

Clive

Several UK suppliers come up on Google, I don't know your exact requirements.

Tony

Colin,

if you have trouble sourcing the 5-40 fasteners, there is also the 1/8" x 40 TPI ME (Model Engineer) thread, if this could be incorporated in your application. The ME threads use 55 degrees thread angle rather than the 60 degrees in the case of UNC.

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Always worthwhile having a look at Accu