However whilst there is a 5 degree difference, it is not noticeable to the eye - and both threads will still screw into each other.

The reason that almost all BSW and UNC threads can be interchanged is that they have the same thread per inch count (TPI). This term means the number of threads present within a radial measurement along the fastener of one inch.

This is because in this ½” thread size only, BSW has a thread count of 12 TPI whilst UNC has a thread count of 13 TPI. Frustratingly close but enough to make them non-matching!

The angle of the thread is slightly different, with BSW having a 55 degree angle and UNC having a 60 degree angle. The thread angle is the term given to the name for the angle between two thread flanks.

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If you are struggling to find either a BSW thread (British Standard Whitworth) or a UNC thread (Unified National Coarse), it is useful to know that in almost all cases the two threads are interchangeable in practical terms.