What's a whit worth?

A look at the man British motorcycle mechanics love to hate

May 20 - If you know the name Whitworth at all, you probably remember him as the guy who forced you to buy all new tools when you got that British bike.

After all, Brit Sir Joseph Whitworth's off-size nuts and bolts are not quite metric, not quite SAE, and they can be maddening to discover when you're elbow-deep in a Triumph Bonneville or a Royal Enfield Bullet.

But instead of cursing Sir Joe, you should be thanking him. Why? Ironically, he's the man who made it possible for you to interchange threaded fasteners between different pieces of machinery.

To find out how, we need to take a trip back in time to mid-nineteenth century England. The industrial revolution was in full-swing, railroads had begun to crisscross the land and factories were increasingly mechanized. There was just one small problem - the nuts and bolts that held everything together weren't standardized at all. Manufacturers had their own proprietary sizes, and fasteners were often made specifically for each application, making repairs difficult and costly.

Enter Sir Joseph Whitworth (left), a brilliant engineer and inventor. Realizing that the myriad of screw sizes then in use created a logistical and production nightmare, he set about creating a set of standards for threaded fasteners. Based on the earlier work of Joseph Clement, who established the concept of a standard thread pitch, Whitworth proposed a system where thread pitch and depth would be based on the diameter of the fastener. Big bolts would have larger, coarser threads, while small fasteners would have correspondingly smaller and finer threads. To further standardize things, all threads in his Whitworth system would share an internal angle of 55 degrees. Introduced in 1841, this revolutionary new system quickly became the standard throughout England and the British Empire.

Though his standard wasn't adopted much outside of the U.K., it did serve to wave the flag for standardized fasteners, an idea whose time had definitely come. In 1918, the United States Society of Automotive Engineers (SAE, of course) established the National Fine (NF) and National Coarse (NC) thread forms, which became the standard on this side of the pond, while the rest of the world adopted the metric system in one form or another.

The Whitworth system was not without its problems. While it worked well where large, coarse threads were needed - like the cast-iron boilers on steam engines, it didn't work so well on smaller, more delicate pieces of machinery. This led to the development of new standards for different applications: British Standard Fine (BSF) for uses requiring a finer thread pitch; British Association (BA) for the small screws on instruments and electrical equipment; British Standard Cycle (BSC) for cycles and motorcycles; and British Standard Pipe (BSP) for the self-sealing threads on plumbing fittings. Things were starting to get a little confusing, (a feeling that's not uncommon to British motorcycle mechanics).

The problem of differing standards in the U.S. and Britain came to a head during World War II, when a flood of U.S. lend-lease equipment arrived in Europe with incompatible fasteners. Clearly something needed to be done.

Soon after the war, Britain, Canada and the United States agreed upon a unified standard based on the American NC and NF systems, which were renamed Unified National Coarse (UNC) and Unified National Fine (UNF). Many small British firms – motorcycle manufacturers, for example – had too much money invested in tooling to immediately make the switch to the new UNC and UNF systems, and continued to use the older British standards until stocks ran out or new tooling was needed. That's why it's not uncommon to find three or even four different types of thread forms on British bikes and cars of the `50s and `60s.

Britain eventually saw the writing on the wall, and in 1965 the British Standards Institute finally declared the Whitworth system obsolete in favor of the International Organization for Standardization (ISO) metric system.

Today, Whitworth sizes pose more of a problem than a solution to the home mechanic. There is almost no overlap between the Whitworth, Metric, and SAE systems (click here for table), making a set of BSW (British Standard Whitworth) wrenches a mandatory purchase for anyone with a classic British bike or Madras-built Royal Enfield. But before you curse at the thought of buying another set of tools, remember Sir Joseph and his revolutionary thread system – without Whitworth things could be a lot worse.

© 2002, American Motorcyclist Association