“You can use an eraser on the drafting table or a sledgehammer on the construction site.” - Frank Lloyd Wright
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5 - Size Standard MarkingsNote: In the United States prior to 1929 the sizes stamped on wrenches usually referred to the diameter of the bolt not the actual opening size. Thus a wrench stamped ½ U. S. would actually have a 7/8" wrench opening size as a nut for a ½" diameter U. S. Standard bolt would measure 13/16" across the flats and allowing for 1/16 clearance would require a 7/8" wrench opening. This same size wrench would also fit nuts for 5/8" hex cap screw and bolt and nuts for 9/16" S.A.E Standard Cap Screws and thus would be marked ½ U.S, 5/8 Hex Cap, and 9/16 SAE. Carl Bolt's website page on tool standards pictures 17 wrenches with 19 different identifications and sizes all marked 1/2"! This could get pretty confusing so circa 1929 the American Engineering Standards Committee specified that wrenches should be marked with the actual width across the flats of the nut or bolt head they would fitAF Initials: AF (used with size in inches of nut or bolt head i.e. I/2 AF)Across Flats (British?)Size as measured across the flat sides of nut or bolt head or actual wrench opening sizeOC Hex logo and initial C (used with size in inches of bolt diameter i.e. I/2 OC)Hex Cap ScrewFound on older forged wrenches before 1920 standard want into effect (see note above)SAE Initials: SAE (used with size in inches of bolt diameter i.e. I/2 SAE)Society of Automotive Engineers (fine threaded nuts and bolts)Found on older forged wrenches before the 1929 standard want into effect (see note above)USS Initials: USS (used with size in inches of bolt diameter i.e. I/2 USS)United States Standard.Found on older forged wrenches before the 1929 standard want into effect (see note above)
