Industry-Standard Numbers for Open-End Wrenches

Open-end wrenches were one of the first tools to be offered in standardized sizes, and from an early date these tools were assigned "standard" model numbers. These numbers actually evolved from the more-or-less sequential model numbers assigned by J.H. Williams & Company, generally credited as being the first tool maker to offer wrenches in standard sizes. As the system proved to be useful, other manufacturers started adopting the same numbering, and soon the wrench models were considered as "Trade Numbers" or "Industry-Standard Numbers".

By the time of the 1912 15th Edition of the Williams catalog, open-end wrenches could still be covered by two-digit model numbers. However, additional size combinations were needed within a few years, and the system was expanded by adding a "7" prefix to some numbers, plus an alphabetic suffix (A, B, or C) to others.

Not all manufacturers adopted the Williams system, at least not at first. Billings & Spencer was probably the most significant manufacturer with a separate numbering system, and their 11xx model series offered even a few more size combinations than Williams.

Williams eventually extended the numbering system to cover box-end wrenches as well, by adding a 4-digit offset to indicate the design. For example, the 8000-series indicated a standard-length offset box wrenches, and the 9000-series indicated a short offset box wrenches.

By the 1930s, some manufacturers (notably Blue Point/Snap-on and Vlchek) had started using the opening sizes in 32nds to encode the model number for their tools. In this more rational numbering system, the venerable 725B wrench became a (Blue Point Supreme) S1618 or a (Vlchek) W1618 wrench.

Standards for Opening Sizes

The wrench model number specified a combination of opening sizes, but the means of describing the sizes added further confusion. In the late 19th and early 20th centuries, wrench sizes were commonly named by the nominal size of the bolts whose nuts they fit, rather than the size (across the flats) of the nuts themselves. For example, a "1/2" opening might fit a 7/8 (across the flats) nut threaded on a 1/2 inch bolt.

To further compound the confusion, there were six different standard systems for nuts and bolts in common use during the early 20th century. These standards all worked in a similar way to set the size of the nuts used for a bolt of a given diameter.

• U.S.S. or U.S. Standard
• A.L.A.M. Standard
• S.A.E. Standard
• Manufacturer's Standard for Hex Head Bolts
• Hex Head Cap Screw Standard
• Hex Head Coach Screw Standard

The A.L.A.M. standard was defined by the Association of Licensed Automobile Manufacturers, an early trade group. The A.L.A.M. standard was later superseded by the S.A.E. (Society of Automotive Engineers) standard, but there are some differences between the two.

Of these six, the three major standards were the U.S.S., S.A.E., and Hex Cap Screw standards, and these three were most commonly marked on wrenches. The latter convention was often indicated by a hexagon symbol followed by "C" or "CAP".

These multiple standards meant that some wrench openings might be marked with three size indications. For example, a number 23 wrench would be marked "3/16 U.S.S." for the small opening, and "1/4 U.S.S.", "5/16 Hex Cap", and "5/16 S.A.E." on the larger end. Also, the U.S.S. size was sometimes marked as "NUT".

This confusing system of multiple size standards persisted until around 1927, when it was phased out in favor of the across-flats size.

The across-flats nut sizes corresponding to the bolt sizes in each standard are shown in Table 1 below. The data for the A.L.A.M. Standard, Manufacturer's Standard, and Hex Head Coach Screw Standard were reverse-engineered from a table in the 1915 Mossberg catalog.

Specification Table for Industry-Standard Single-Open Wrenches

Single-Open Wrenches in 1901 J.H. Williams Catalog

Fig. 1. 1901 Catalog Listing for Williams Single-Ended Engineers' Wrenches.

Fig. 1 shows a listing of U.S. Standard single-ended engineers' wrenches, as published on page 11 of the 1901 Williams 10th edition catalog.

The 25 sizes cover a range of openings from 5/16 up to 5-3/8(!) inches.

Specification Table for Industry-Standard Double-Open Wrenches

The table below shows the specifications for the industry-standard double-open wrench models, with the Billings & Spencer 11xx model numbers included as well. The opening sizes are shown in each of applicable sizing systems, along with the nominal milled opening and decimal equivalent.