In most cases the British gun trade used three basic thread series; British Standard Whitworth [BSW], British standard fine[BSF] and British Association [BA].they tended to use off standard sizes for example BSF 9/32 or 3BA.By using off standard sizes,it was in my opinion an attempt to limit the ability of those outside the trade to repair or supply parts. This use of off-standard threads is very apparent in the thread series used on British cleaning rods and associated bore cleaning tools.
As stated by others due to thread geometry American and Unified threads will not interchange with same diameter/pitch Whitworth thread series.
