SAE Thread Specifications

The Society of Automotive Engineers or SAE has standards for the number of threads per inch that should be on machine screws of different sizes.

Coarse and Fine

Machine screws in the U.S. conform to SAE standards of threads per inch.Machine screws in the U.S. conform to SAE standards of threads per inch.
A machine screw is one that accepts a nut or goes into a threaded pipe or hole. These standards apply only to American, or standard, bolts and screws. British and metric systems have their own measurements for threads. There are also SAE thread standards for pipes, ports and flange ports.

There are two grades of threads per inch: coarse and fine. For each specific bolt size, there are two possible thread types. One bolt --- or machine screw --- has more threads per inch, which is the fine option, and the other has fewer threads per inch, which is the coarse option.

UN Thread Types

Although metric, British and American systems for describing threading are different, there is a UN --- or unified --- system that uses a threads per inch count. There are variations on the UN measurements. For example, UNJ threads are slightly larger than the UN diameter. The UNF is a finer thread, with more threads per inch, and the UNEF is extra fine thread, with even higher counts per inch. For someone visiting a home repair or automotive repair store, UN threading is the standard.

Common Sizes

Some common sizes of bolts and threads include #00 and #0, which come in fine thread only, 90 and 80 threads per inch respectively. Size #1 threads per inch is 64 in coarse and 72 in fine, while size #3 is 48 in coarse and 56 in fine. For #5 fasteners, threads per inch are 40 in coarse and 44 in fine, while #8 is 32 in coarse and 36 in fine. A 1/4-inch fastener has 20 threads per in in coarse and 28 in fine, while a 3/8 fastener has 16 in coarse and 24 in fine.

About the Author

Rachel Murdock published her first article in "The Asheville Citizen Times" in 1982. Her work has been published in the "American Fork Citizen" and "Cincinnati Enquirer" as well as on corporate websites and in other online publications. She earned a Bachelor of Arts in journalism at Brigham Young University and a Master of Arts in mass communication at Miami University of Ohio.