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The system of measurement is called "Imperial" in the former British Empire but the Unified thread standards are international and replaced the SAE, USS, BSW, BSF and several other thread specifications. What used to be called SAE is now UNF, which also replaced the BSF thread standards. The old USS is now UNC, which also replaced the BSW thread series. They are pretty interchangeable for
the coarse threads with the exception of ½"-13 UNC - the BSW series had 12 threads per inch. The BSF series is much less interchangeable because they used 26 and 30 threads per inch where UNC mostly uses 20, 24, 28 and 32 threads per inch. The difference in thread angle (Whitworth = 55°, Unified = 60°) is usually not a problem if the threads match in other respects. It is worthy of consideration in very highly loaded joints and those prone to fatigue with cyclic stresses.
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George Downs, Bartlesville, Heart of the USA!
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