Screw threads are often problems with out proper tools to measure-actually if your eyes are still good, a six inch steel rule will work fine for counting threads, and to give you a diameter either internal or external. The link below is a good site that provides a nice table, and to the right of the page, a wealth of information on various mechanical subjects and issues. Would suggest a hard copy-and plastic laminate for durability for your shop-that is, if you do not have a standard Machinery Handbook available.

http://www.engineersedge.com/screw_threads_chart.htm

Thanks DF, this will come in handy.

Always handy and relevant; thanks Df.