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On the sizing of nuts and bolts? Answered

When a hex nut is referred to as "X"mm, is that measured to the ridges of the thread (the green line on the image, or into the troughs of the thread (the extra red parts of the line)?


Metric fastners always refer to the stud the nut FITS, so its the red bits here, theMAJOR diameter of the stud/

I think the measures are taken with the same criteria, whether metric or inch. I agree with James R Patrick.

The original rod from which an M8 (for example) bolt is formed is 8mm in diameter. The threads cut into that rod don't slim it down at all (well, not by any intentionally-appreciable amount, anyway)*. Therefore, ITS peaks/tips/ridges are still 8mm overall, so the troughs of the nut are also 8mm, to accommodate them.

*Obviously, the cutters on the machines that make the nuts and bolts do wear down - often by different amounts - which is why you'll sometimes get nuts that are looser/tighter on the same bolt (or bolts that are looser/tighter on the same nut, of course).

What is the connection between the shaft size and the spanner size then - I don't know. But an M8 bolt needs an M10 spanner.

The size of a bolt refers to the hole that it would fit into, so the tips of the threads. A matching nut would have the same "size" and the measurement would refer to the troughs of the nut threads.