Whitworth (BSW) & BSF

If you’ve ever wondered, whilst working on your 1950’s or 60’s car, why some of the heads are rounded on the bolts, maybe it’s a because there are a combination of bolt types used on the vehicle. Whilst everyone working on a 50’s or 60’s classic car will have AF spanners, how many of us own a good set of Whitworth? . Some people may have noticed that their spanners show both BSF and Whitworth for the jaw sizes, which can cause real confusion!

British standard fine (BSF) is a screw thread form as a “fine” alternative to BSW or British standard Whitworth. BSF and BSW were common prior to the introduction of “unified” and later metric threads. Whilst Whitworth and BSF became obsolete after 1948, in practice they continued to be used well into the 1970’s. Indeed, because of the cost of moving from Whitworth to Unified, some components used both types of fasteners on the same unit. However, in practice, the changes took a long time to feed through, with the transition taking a long time to complete. Most British cars, motorbikes and planes, built into the 1970’s were still using Whitworth threads.

It seems the impact of using differentthread sizes caused huge problems during WW2, particularly with the construction of the Rolls Royce Merlin engine by American Factories. Whilst the US car manufacturers were used to making V8 engines, manufacturing the merlin, with its BSF and Whitworth threads were such an issue that Ford refused to make them. It was then down to Packard to make the switch, which cased significant complexity for the US firm. No wonder they wanted to “unify” threads once the war was over.

1959 francis falcon barnett

Whilst many Fasteners businesses have moved completely to Metric, Leyton Fasteners have continued to stock BSW and BSF alongside unified threads UNF and UNC. This is particularly important if you are repairing machinery from the pre 1970 period, where there can be a mixture of unified, BSF and Whitworth.


If you need BSF give us a call or email, we have plentiful stocks in many sizes.