how to find out a metric male thread size without a measuring tool

[Theo]

If you need to know the size of a metric male thread in this situation:
• you don't have measuring tools,
• you don't have female threads of known size to test
• you have sockets
which can happen when you lose a fastener from a vehicle and you need to buy a spare one to get home,
just slide the male thread into some sockets for hexagonal heads until you find one that matches. This works because you can inscribe a circle having a certain diameter into a hexagon as wide as that diameter and as far as I've seen the outer diameters of male threads are sligthly smaller than the nominal values and hexagonal sockets are slightly wider than the nominal value.
So for example if a male thread doesn't fit into a 9 mm socket but does in a 10 mm, then it's an M10 thread.
I guess it can work with imperial tools, too.

 

[Erwin P]

If the head is 10 or 8 with a flance it's probably m6. 13-10 for m8. 17 for m10. 19 for m12 etc. m5 is normally 6 but on bikes often a flanced 8mm, that's the only kind of confusing one.

 

[Philip]

If you need to know the size of a metric male thread in this situation:
• you don't have measuring tools,
• you don't have female threads of known size to test
• you have sockets
which can happen when you lose a fastener from a vehicle and you need to buy a spare one to get home,
just slide the male thread into some sockets for hexagonal heads until you find one that matches. This works because you can inscribe a circle having a certain diameter into a hexagon as wide as that diameter and as far as I've seen the outer diameters of male threads are sligthly smaller than the nominal values and hexagonal sockets are slightly wider than the nominal value.
So for example if a male thread doesn't fit into a 9 mm socket but does in a 10 mm, then it's an M10 thread.
I guess it can work with imperial tools, too.

This might work. Still, you need to check the pitch with a ruler. Many automotive fasteners come in multiple and sometimes weird pitch sizes (0.75, 0.8, 1.0, 1.25, 1.5, 1.75mm)

 

[Theo]

This might work. Still, you need to check the pitch with a ruler. Many automotive fasteners come in multiple and sometimes weird pitch sizes (0.75, 0.8, 1.0, 1.25, 1.5, 1.75mm)

One way to estimate the pitch is to count how many coils fit into the width of a socket. For example, if you suspect that the pitch of an M8 screw is the standard 1.25 mm, since 1.25 • 4 = 5, you can verify that 4 coils fit into the width of a 5mm hexagonal socket. Actually in this case it would be better, if you have a long enough thread and a 15 mm socket, to verify that in 15 mm you have 12 coils since the higher the coil count, the smaller the measuring error becomes. That's why I count more coils even when measuring with a ruler. My rulers are graduated in mm and sockets utilized as rulers are still available in integer mm sizes. 1 mm error measuring 1.25 mm is huge, while 1 mm error measuring 15 mm is less than 10% and there is only a limited list of possible pitch sizes to choose from; it can't be 0.83 mm.
It's better to choose a coil count that is convenient: 0.8? then try 10 coils in an 8 mm hexagon. If it seems to be off you can try 0.75, then check whether 8 coils fit in a 6 mm hexagon. That's still the same approach that I use with rulers.

And I've just asked an AI what it would do in that situation and it hasn't found this solution!