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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 prefix (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 they offered even a few more size combinations in their 11xx model series.


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 at least three different standard systems for nuts and bolts:

  • The U.S.S. or U.S. Standard,
  • The S.A.E. or Society of Automotive Engineers standard, and
  • The Hex Cap screw standard used by hardware manufacturers.

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.

This confusing system of multiple size standards persisted until the late 1920s, when it was phased out in favor of the across-flats size.


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.


Table 1. Specifications for Industry-Standard Double-Open Wrenches with B. & S. Models
Standard
No.
Billings
No.
U.S.S.
Sizes
Hex Cap
Sizes
S.A.E.
Sizes
Milled
Opening
Decimal
Equiv.
721 1100 1/8- 1/8 - 3/16 ... 5/16 - 3/8 0.313 - 0.375
21 1101 1/8 - 3/16 1/8 - ... 5/16 - 13/32 0.313 - 0.406
722 1102 1/8 - 1/8 - 1/4 - 1/4 5/16 - 7/16 0.313 - 0.438
22 1103 1/8 - 1/4 1/8 - 5/16 - 5/16 5/16 - 1/2 0.313 - 0.500
723 1104 ... 3/16 - 1/4 -1/4 3/8 - 7/16 0.375 - 0.438
723A 1105 - 1/4 3/16 - 5/16 -5/16 3/8 - 1/2 0.375 - 0.500
724 1106 ... 5/16 - 3/8 - 3/8 3/8 - 9/16 0.375 - 0.563
23 1107 3/16 - 1/4 - 5/16 - 5/16 13/32 - 1/2 0.406 - 0.500
24 1108 3/16 - 5/16 ... ... 13/32 - 19/32 0.406 - 0.594
725 1109 - 1/4 1/4 - 5/16 1/4 - 5/16 7/16 - 1/2 0.438 - 0.500
725A 1110 ... 1/4 - 3/8 1/4 - 3/8 7/16 - 9/16 0.438 - 0.563
725B 1111 1/4 - 5/16 - 3/8 5/16 - 3/8 1/2 - 9/16 0.500 - 0.563
25 1112 1/4 - 5/16 5/16 - 5/16 - 1/2 - 19/32 0.500 - 0.594
726 1113 1/4 - 5/16 - 7/16 5/16 - 7/16 1/2 - 5/8 0.500 - 0.625
26 1114 1/4 - 3/8 5/16 - 5/16 - 1/2 - 11/16 0.500 - 0.688
... 1115 - 1/4 5/16 - 1/2 5/16 - 1/2 1/2 - 3/4 0.500 - 0.750
727 1116 ... 3/8 - 7/16 3/8 - 7/16 9/16 - 5/8 0.563 - 0.625
27C 1118X - 3/8 3/8 - 3/8 - 9/16 - 11/16 0.563 - 0.688
728 1117 ... 3/8 - 1/2 3/8 - 1/2 9/16 - 3/4 0.563 - 0.750
27 1118 5/16 - 3/8 ... ... 19/32 - 11/16 0.594 - 0.688
28 1119 5/16 - 7/16 ... ... 19/32 - 25/32 0.594 - 0.781
... 1119X - 3/8 7/16 - 7/16 - 5/8 - 11/16 0.625 - 0.688
729 1120 ... 7/16 - 1/2 7/16 - 1/2 5/8 - 3/4 0.625 - 0.750
28S 1120X - 7/16 7/16 - 7/16 - 5/8 - 25/32 0.625 - 0.781
730 1121 ... 7/16 - 9/16 7/16 - 5/8 - 13/16 0.625 - 0.813
730A 1122 - 1/2 7/16 - 5/8 7/16 - 9/16 5/8 - 7/8 0.625 - 0.875
... 1123X 3/8 - - 1/2 - 1/2 11/16 - 3/4 0.688 - 0.750
29 1123 3/8 - 7/16 ... ... 11/16 - 25/32 0.688 - 0.781
30 1124 3/8 - 1/2 - 5/8 - 9/16 11/16 - 7/8 0.688 - 0.875
731 1125 ... 1/2 - 9/16 1/2 - 3/4 - 13/16 0.750 - 0.813
731A 1126 - 1/2 1/2 - 5/8 1/2 - 9/16 3/4 - 7/8 0.750 - 0.875
31 1128 7/16 - 1/2 - 5/8 - 9/16 25/32 - 7/8 0.781 - 0.875
32 1129 7/16 - 9/16 ... ... 25/32 - 31/32 0.781 - 0.969
731B 1130 - 1/2 9/16 - 5/8 - 9/16 13/16 - 7/8 0.813 - 0.875
732 1131 ... 9/16 - 3/4 - 11/16 13/16 - 1 0.813 - 1.000
33 1132 1/2 - 9/16 5/8 - 9/16 - 7/8 - 31/32 0.875 - 0.969
733 1133 1/2 - 5/8 - 3/4 9/16 - 11/16 7/8 - 1 0.875 - 1.000
34 1134 1/2 - 5/8 5/8 - 9/16 - 3/4 7/8 - 1-1/16 0.875 - 1.063
734 1135 1/2 - 5/8 - 7/8 9/16 - 7/8 - 1-1/8 0.875 - 1.125
33C 1132X ... - 3/4 5/8 - 11/16 15/16 - 1 0.938 - 1.000
35 1136 9/16 - 5/8 ... - 3/4 31/32 - 1-1/16 0.969 - 1.063
36 1137 9/16 - 3/4 - 1 - 7/8 31/32 - 1-1/4 0.969 - 1.250
735 1138 ... 3/4 - 7/8 11/16 - 1 - 1-1/8 1.000 - 1.125
736 1139 - 3/4 3/4 - 1 11/16 - 7/8 1 - 1-1/4 1.000 - 1.250
37 1140 5/8 - 3/4 - 1 3/4 - 7/8 1-1/16 - 1-1/4 1.063 - 1.250
38 1141 5/8 - 7/8 ... 3/4 - 1 1-1/16 - 1-7/16 1.063 - 1.438
737 1140X - 3/4 7/8 - 1 - 7/8 1-1/8 - 1-1/4 1.125 - 1.250
739 1145 3/4 - 1 - 1-1/8 7/8 - 1-1/4 - 1-3/8 1.250 - 1.375
39 1146 3/4 - 7/8 1 - 7/8 - 1 1-1/4 - 1-7/16 1.250 - 1.438
40 1148 3/4 - 1 1 - 7/8 - 1-1/8 1-1/4 - 1-5/8 1.250 - 1.625
41 1150 7/8 - 1 ... 1 - 1-1/8 1-7/16 - 1-5/8 1.438 - 1.625
42 1151 7/8 - 1-1/8 ... 1 - 1-1/4 1-7/16 - 1-13/16 1.438 - 1.813
43 1153 1 - 1-1/8 ... 1-1/8 - 1-1/4 1-5/8 - 1-13/16 1.625 - 1.813
44 1154 1 - 1-1/4 ... 1-1/8 - 1-3/8 1-5/8 - 2 1.625 - 2.000
45 1155 1-1/8 - 1-1/4 ... 1-1/4 - 1-3/8 1-13/16 - 2 1.813 - 2.000
46 1156 1-1/8 - 1-3/8 ... 1-1/4 - 1-1/2 1-13/16 - 2-3/16 1.813 - 2.188
47 1157 1-1/4 - 1-3/8 ... 1-3/8 - 1-1/2 2 - 2-3/16 2.000 - 2.188
48 1158 1-1/4 - 1-1/2 ... 1-3/8 - 2 - 2-3/8 2.000 - 2.375
49 1159 1-3/8 - 1-1/2 ... 1-1/2 - 2-1/16 - 2-3/8 2.063 - 2.375

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