Whitworth Thread System

In Britain, road vehicles, aeroplanes and machinery until the 1970s used British Standard thread forms. The following families of threads existed;

• miniature sizes known as British Association (BA). BA size numbers are in reverse. The lower the number, the larger the bolt size, so 0 BA is the largest and 16 BA is the smallest. BA sizes are mainly used in electrical equipment.
• coarse threads known as British Standard Whitworth (BSW)
• fine threads known as British Standard Fine (BSF)
• standards for pipes and tapered pipes

The US equivalents are the Unified Thread System formed by ANSI/NATO in 1948. These standards were agreed between Britain, Canada and the US and were based on the previous American National standard, except with rounded roots and optionally rounded crests. This composed the following:

• miniature sizes below ¼ inch (UNM), with roots from an old American Society of Mechanical Engineers (ASME) standard
• coarse threads (UNC)
• fine threads (UNF), with roots from old Society of Automotive Engineers (SAE) and ASME standards

Whitworth threads are used for most mechanical equipment in the RLH.

With the UNF/UNC system the number stamped on the wrench or socket is the dimension of the hexagonal bolt head measured across the flats (AF = Across Flats); so the wrench required for a ¼ UNC bolt is stamped ⁷⁄₁₆. Whitworth wrench sizes, however, indicate the size of the bolt, i.e., a wrench required for the equivalent ¼ BSW bolt is stamped ¼W.

Whitworth heads are NOT the same size as the equivalent UNC/UNF - they are larger. For this reason, it is imperative that a set of Whitworth wrenches are purchased before working on any antique British machinery. Otherwise, expect rounded-off heads and busted knuckles, trademarks of the careless craftsman.

Whitworth wrenches and sockets are available through clubs that cater to British cars. It's also a good idea to get a set of taps and dies to fix the attempts of those who have gone before you in the vain attempt to convert, intentionally or otherwise, to SAE format.

Another area that causes confusion is the fact that some BSW/BSF nuts can be screwed on UNF/UNC bolts and vice-versa. Under NO circumstances should this be practised. Most of the coarse threads share the same threads per inch, which means BSW nuts can be screwed onto UNC bolts and vice-versa (the exception to this being ½ inch, of which the BSW size has 12 threads per inch whereas the UNC size has 13). Whitworth and UNC/UNF thread forms differ greatly, the primary difference being the thread angle. Consequently, if these fasteners are interchanged, considerable loss of holding force, fatigue resistance and strength will result.

Some suppliers in the US have been known to sell so-called Whitworth hardware, which has SAE sized hex heads mixed with Whitworth thread forms. This really only serves to cause confusion and increase the risk of the wrong equipment being used in future. Fully correct Whitworth fasteners are available in the US.

SOCKETS and WRENCHES (SPANNERS): Key to Nut & Bolt Sizes

The information on this page, concerning Whitworth threads, may be freely copied or re-published elsewhere without special permission. Last updated 2020.