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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.

Auto-Lok Nuts offer both threads and metric.

More info:

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

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How can it be that both have been replaced by a single Metric thread?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



We can offer parts to repair such threads, just get in contact with us!!

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.