I've been buying and selling imperial based tools for over thirty years.
 
These charts help compare and explain the various imperial standards (Whitworth/BSF,BA and AF) with metric and list all spanner/socket sizes from 10BA to 60mm at bottom of page.
 
Enthusiasts working and repairing older machinery soon encounter Whitworth and BSF (British Standard Fine) or Imperial AF (SAE) fixings along with BA (British Association) and possibly BSC (British Standard Cycle) threads. Metric tools aren't made to fit these,the correct imperial or inch based tools are available  I prepared the tables on this much copied page to help explain these Imperial Standards and what tools and sizes are needed to fit them. 
 
 

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