What is the difference between BSF/BSW and UNF/UNC?

Both of these thread types are use for imperial thread.
BSF and BSW are Britisch measurement systems, while UNF and UNC are American measurement systems.
BSF and UNF are the fine thread systems, while BSW and UNC are the coarse thread systems.

The difference between BSF/BSW and UNF/UNC is dual:

  • The thread angle
    The BSF/BSW have a thread angle of 55°
  • BSF-BSW thread angle

    BSF-BSW thread angle

    The UNF/UNC have a thread angle of 60

    UNF-UNC thread angle

    UNF-UNC thread angle

    • The thread profile 
      The BSF/BSW have a wave like thread profile.

    BSF-BSW thread profile

    BSF-BSW thread profile

    The UNF/UNC have theorically a flattend crest and root in the thread profile, however to aid the manufacturing proces they are slightly rounded inside a maximum outline.

    UNF-UNC theoretical thread profile

    UNF-UNC theoretical thread profile

    UNF-UNC practical thread profile

    UNF-UNC practical thread profile


Be aware that although they look similar, they should not be used as full replacements or be mixed with each other!

  • UNF and BSF have completly different thread per inch value. So they don't fit.
  • UNC and BSW have the same thread per inch value, accept for the 1/2" size. Theoretical it would be possible to interchange them as they have the same amount of threads per inch. However due to the difference in thread angle, the connection would not be as strong as when using the proper combinations.


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We received this the question from a customer:


What is the intended application of BSF and BSW threads?
How can it be that both have been replaced by a single Metric thread?


They are standards of threads, and are not replaced, they are just not popular in use anymore, since the imperial empire of England is not controlling the world anymore.
BSF = british standard fine
BSW = british standard withworth

parts are still available, such as with:

BSF and BSW recoil kits

To repair such threads.

Current applications:

Stage lighting suspension bolts are standard with 3/8 and 1/2 BSW. in the past, these got converted to metric threads but then converted back, after complaints that the finer metric threads increased the time and difficulty of setup, which often takes place at the top of a ladder.  Fixings for garden gates traditionally use Whitworth carriage bolts, these are still the standard used in the UK.