Guys,
Since 1948, both SAE and Unified threads are cut at 60 degrees. It's the BSW
& BSF threads that are cut at 55 degrees. In November 1948, NATO issued a
new standard for threads, the Unified National system. American bolts had
flat-bottomed groves between threads, which interfered with British
round-topped threads. Likewise, British bolts wouldn't fit American nuts.
The Unified system uses a round-bottom grove to fit the British threads, and
a flat-topped thread to fit the American threads, so it not only fit itself,
but both existing systems. For more information, check these out:
http://www.enginehistory.org/british_fasteners.htm
http://www.enginehistory.org/BSFasteners/BSReference_3.pdf
http://www.colinusher.info/Livesteam/mewdata.html
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