Its worth reading this:
http://skgrimes.com/library/faq/how-to-measure-threads but older threads can be worn so a bit of adjustment or tolerance has to be applied.
I have a couple of screw pitch thread gauges (search eBay with this and you will find some and they are not expensive - the threads on flanges I have look as though they are of Whitworth form - 55 degree thread angle). You also need a fairly powerful magnifying glass to view the gauge and thread. When I first tried I found it difficult, but going back several times seems to help and eventually you settle on the thread (tip = threads per inch) which you repeatedly decide is right. I'll post my spares and wants in the for sale or trade section but it would be useful to have a 'sticky' thread there about flanges. They are a real bugbear and expensive to get made new.
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