There are two basic thread pitches for every commonly-used bolt diameter. Once we get into metrics, it's different, but the only place you'll find metric threads on a Stovebolt is spark plug threads.

U.N.C.- - -Unified National Coarse

U.N.F.- - -Unified National Fine (Same as SAE threads)

1/4" UNC---20 Threads per inch
1/4" UNF---28 TPI

5/16" UNC---18 TPI
5/16" UNF---24 TPI

3/8" UNC----16 TPI
3/8" UNF----24 TPI

7/16" UNC---14 TPI
7/16" UNF---20 TPI

1/2" UNC----13 TPI
1/2" UNF----20 TPI

Metric bolts and nuts are measured in millimeters of diameter, and the distance from one thread to the next in millimeters. For instance: 6 X .75 X 20: 6MM diameter, .75 MM between threads, 20 MM long. An inch-size bolt might be 1/4" X 20 X 1 1/2": 1/4" diameter, 20 threads per inch, 1 1/2" long.

Bolt head size is usually standardized, also, at least for fractional bolts:

1/4" bolt: 7/16" head
5/16" bolt: 1/2" head
3/8" bolt: 9/16" head
7/16" bolt: 5/8" head
1/2" bolt, 3/4" head

Hope this helps!
Jerry

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