For FAH volunteers who have worked on one or more of the collection’s Stanleys, it is learned early on that some of the screw and bolt fasteners used on various parts of the engine and body are not standard thread sizes found in common use today. In checking the author’s copy of List of Parts Used to Make a Complete Model 735 Car,  issued by the Stanley Motor Carriage Co. of Newton, Massachusetts, on April 1, 1918, we find various lengths of #5-32 Round Head Machine Screws and ¼”-30 Fillister Head Machine Screws listed (the first number is the diameter, the second number is the threads per inch). Today, the common sizes in use would be #5-40 or #5-44 and ¼”-20 or ¼”-28.

Pictured below is a Craftsman Vanadium-series open-end wrench or spanner, as it is referenced in Britain. Sears Roebuck & Company (owned by Stanley Black & Decker since 2017) contracted with various tool manufacturers to produce the Craftsman line sold through Sears-branded catalogs beginning in 1927. In 1932, Craftsman introduced “Vanadium Steel” wrenches that tested 50% lighter yet 200% stronger than their previously offered wrenches.

Note that the open-end wrench pictured below is marked 3/8 W and 7/16 W. A quick check using a ruler reveals the openings physically measure 11/16″ and 13/16″ wide, respectively. While the wrench and others in the set were produced long after Stanley steam car production ceased, what sort of fastener is the wrench designed for?

In the 1800s, as the Industrial Revolution was taking hold worldwide, screw and bolt fasteners were manufactured by any number of ironworking shops. One can only imagine the diversity of screws and bolts produced. The Franklin Institute in Philadelphia, at the urging of Congress, released a report in 1861 defining a set of fastener standards based on the work of William Sellers that the U.S. Congress adopted as the “United States Thread” standard. The U.S. Navy, as well as many of this nation’s railroads, converted to using only United States Thread-compliant screws, bolts, and nuts by 1870. The great advantage being that compliance ensured screws and bolts, and their associated nuts, were interchangeable between manufacturers.

After World War I, it was realized that while a standard existed, there were issues that needed addressing. The National Screw Thread Commission was established by an Act of Congress in 1918 and by 1921 issued a preliminary report of the Commission’s recommendations. As a result, the “American National Standard” form of screw thread was born based on the existing standard. The American National Standard includes two series: National Coarse-Thread and National Fine-Thread, both in use today. In 1948 the National Standard Thread was changed to the Unified National Standard Thread, when the U.S. standard was adopted by Britain and Canada for use on war equipment.

In developing the National Standard screw thread, the Commission adopted many of the principals and specifications of the Whitworth Thread, which was used by British railways and industry. The Whitworth system, defined by Joseph Whitworth in 1841, became the world’s first screw thread standard and was the British standard until replaced with British Association Standard Thread. A difference not adopted was maintaining Sellers’s 60-degree thread angle instead of the 55-degrees used on Whitworth threads. Whitworth rounded the peaks and valleys of the thread, which Sellers left flat. The rounded edges and corners reduce stress points where fractures often originate providing Whitworth threads superior fatigue strength. The Whitworth thread was a preferred choice for many American locomotive builders for firebox stay bolts in the 1800s and early 1900s due to strength advantages while Sellers threads were extensively found throughout the rest of the locomotive.

The wrench pictured in our question is for Whitworth bolts! The Whitworth system defines a bolt based on the diameter of the bolt shaft and not the dimensions across the flats of the bolt’s head or matching nut, as is current practice in the United States. Thus, the wrench pictured will work for either a 3/8″ or 7/16″ diameter Whitworth Thread bolt. The U.S. National Standard would use a 9/16″ wrench (the distance across the flats of the bolt’s head or nut) for a 3/8″ diameter bolt with either National Coarse or National Fine threads.