According to Shigley and Mischke’s Mechanical Engineering Design (McGraw-Hill 1989) experiments show that the tensile stress supported by a threaded rod equals that of a rod with diameter that is the mean of pitch and minor diameters of the threads.  Thus, the tensile stress area of a threaded fastener is computed from the average of its pitch and minor diameters.

• Here is a link to Instock Fasteners which provides ANSI/ASME screw thread sizes:

http://www.instockfasteners.com/Tools/ScrewThreadTables.ASP

• Marine Fasteners lists the thread sizes and provides sizes for tap and clearance holes:

http://www.marfas.com/machinescrewtapping.shtml

This site also shows the various types of machine screw heads.

• Engineers Edge provides a table of thread sizes at:

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

• And head dimensions can be found at SmithFast:

http://www.smithfast.com/msflathead.html