Why do screw thread diameters vary from their nominal values?

Question

I'm trying to identify several screws. Guides online are fairly straightforward - measure the diameter of the thread part, and pitch if possible. However, none of my screw measurements make any sense. I've ruled out my caliper as the cause.
Here's some of my measurements (Suspected screw: Expected -> Actual):

Known M2.5: 2.5mm -> 2.43 mm
M3: 3mm -> 3.38 mm, 0.5mm pitch
M4: 4mm -> 3.88 mm
Known 5/16": 0.3125" -> 0.305", 18 threads/inch

My measurements seem to be off my 2% to 10%. Am I missing something? Is my M3 screw actually M3.5 or M4? I can't find, say, 3.38mm on any screw charts (example)

isherwood · Accepted Answer

It stands to reason that most threads will measure slightly below nominal. That's much less of a compatibility problem than if they're too large. It's probably due to threads being cut or cast using tooling for the actual shank diameter, where slightly too much material is removed either intentionally or due to manufacturing tolerances.
To that point, I'd be willing to bet that your M3 is actually an M3.5. It being so would put it in line with the negative variance of all your other examples.

Ecnerwal · Answer

For machine screws:
You can either purchase a screw identifying plate (with labeled threaded and sometimes clearance holes), or you can purchase known nuts that you keep track of and thread "suspect" screws into them. There are also dedicated thread pitch gauges which are far more accurate than trying to "eyeball count" threads against a ruler.
Aside from your caliper appearing to read a bit on the small side, the most likely cause of confusion is trying to force a metric fastener to be "english" or an "english" fastener to be Metric. This leads to misreading your measurements slightly to fit the bias (and that can be where a thread gauge that simply won't allow you to fake it can help get you on track.)
Depending how deep in the rabbit hole you go, there's also the "Actual 'English' issue" of getting into Whitworth threads rather than SAE "inch & Number" threads. They are a different thread shape, more an issue with antiques than current production now.

MonkeyZeus · Answer

Quite frankly, you need a thread gauge:

https://www.amazon.com/ChgImposs-Imperial-Whitworth-Industrial-Measurement/dp/B07J9V9JTK/

Chris H · Answer

Metric screw diameters are always below nominal.
The nominal size is as if the thread came to a sharp point, and it doesn't.  For the exact amount see Wikipedia, from where I lifted this drawing (thanks to user Inductiveload).

Note the P/8 (i.e pitch/8) flat on the outside (top) of the external thread.  This means the outside diameter (roughly doing the trigonometry) is about 0.2×pitch less than nominal.  Taking your M4, with  a pitch of 0.7, you'd expect an OD of 4-0.2*0.7=3.86, an almost perfect match to your 3.88.  Similarly for you M2.5.
So your 3.38mm OD screw isn't M3.  The diameter is good for M3.5, but that has a 0.6mm pitch.  I don't know how you measured the pitch, but that's a fairly easy error.  Without a thread gauge I'd measure the length of 10 (or more) turns, and divide to get the pitch.
M3.5 is rare, but common in British and some European electrical hardware, usually slotted (e.g. it holds our sockets and light switches to the back boxes).  I have seen socket head M3.5 on a metric version of an American product, where a UK engineer would have used M4

Why do screw thread diameters vary from their nominal values?

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