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)
Best 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.