Specifications of International Thread Standards

Help with the determination of threads

I often get inquiries regarding the determination of threads:

"I've got a screw here with an outer diameter of approx. 8.32 mm and about 10 threads over 12 mm. Which type of thread is that?"

My question about the flank angle remains unanswered more often than not.

As, unfortunately, my "crystal ball" is not equipped with an interior lighting, I am also in the dark here and cannot answer the question.

Another question which sometimes reaches me is whether it would be possible for me to put the values into a data base in order to enable a search for the thread. In theory this is possible, only the result would look like this:

With a diameter of 12.700 mm (1/2") you will get the following hits:
1/2" 16 - UN, 60° flank angle
1/2" 10 - ACME, 29° flank angle
BSB 1/2" - 26, 55° flank angle
1/2" 10 - STUB ACME, 29° flank angle
ME 1/2" - 32, 55° flank angle
V 1/2" - 12, 60° flank angle
Sellers 1/2" - 13, 60° flank angle
1/2" - 13 UNC, 60° flank angle
1/2" - 20 UNF, 60° flank angle
1/2" - 28 UNEF, 60° flank angle
1/2" - 12 UNS, 60° flank angle
BSF 1/2" - 16, 55° flank angle
W 1/2" - 12, 55° flank angle

If you only have a short piece of screw, pipe, nut oder fitting available for measurement, the counting of threads for 25.4 mm (1 Inch) turns into an ordeal. A determination of turns over a certain length and a projection to 1 inch would be a jammy shot - and this doesn't even take measuring errors due to a rotten measuring instrument into account.

Here, a thread gauge and a vernier caliper can provide a remedy. In case you have to deal with unknown threads more frequently, these tools are definitely worth buying.


For the most common threads, thread templates with flank angles of 55° and 60° are available which enable you to get the pitch.

The following questions can serve as a rough guide to at least find an approach and simplify the search:

1. From which country is the object?
2. How old is the object?
3. Screw and nut or pipe and fitting?

1. In most cases threads from GB have a flank angle of 55°, the ones from the USA have 60°.
Threads still in use (manufactured later than approx. 1930) can be found in the navigation. The ones from the USA are shown in this colour, the one from GB in this colour.

2. As a proper standardisation has only been developed much later, the determination of a thread of an older year of manufacture is extremely difficult. Threads which rank among the exotics are shown in the following colour.

3. If the subject matter is a pipe joint, one should start the search in the navigation with the pipe threads from the presumed manufacturer's country. A search for screw joints will be more difficult as everything below a diameter of 10 mm could end in chaos:

A search for an outer diameter of 5.3 mm provides approx. 14 hits, which is not particularly helpful without the number of turns and the flank angle.

Anyway, I wish you success!