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Measuring Between the Flats 1.Lay the nut on a flat surface so that the hole lays flush. 2.Place the end of a measuring tape on one of the flat sides of the nut's outer perimeter. 3. Stretch the tape measure across the nut's diameter to the flat-side directly across from the one your pulling from. If the nut is metric, count the number of lines on the tape measure to find the measurement. If you count 9 lines, the size is a 9-mm (millimeter) nut. If the nut is US standard, count the smallest lines on the tape measure in sixteenths. Add the sixteenths together to find the size. If you count 11 lines, the size of the nut is 11/16. You can also measure from point-to-point to find the "Whitworth system" measurement. Instead of measuring from flat to flat, you measure from point-to-point on the hexagon. These two measurement methods help you know what size wrench or socket to use on a nut. Measuring for Diameter 1.Lay the nut down as you did in Section 1. Set the end of the tape measure at one side of the inner threads of the nut. 2. Stretch the tape measure to the other side of the inner threads of the nut. 3.Count the lines to find the diameter of the threaded hole. Measure across the widest opening the threads make. This measurement will help you match the thread diameter of the nut with the thread diameter of the bolt. Measuring for Thread Coarseness or Fineness 1.Count the number of threads along the inside thickness of the nut. Use a magnifying glass and a pin to keep track of the threads. 2.Write the total down on a piece of paper. 3.Figure the threads per inch of nut thickness. A nut with 16 threads per inch has coarse threads, while a nut with twenty threads per inch is fine thread. If the nut is 1/4-inch thick and that 1/4-inch has 4 threads, the nut is coarse-threaded. If the nut has five threads in that 1/4-inch thickness, the nut is fine-threaded. To measure nut thickness, hold the nut between your fingers and measure the perimeter parallel to the nut's hole.

Ancient Screws Hex-head nuts and bolts are part of a fastening system that uses screw threads. Screw threads date back to Assyrian King Sennacherib in the seventh century BCE. Sennacherib used screws as part of the pumps that fed the water systems for the Hanging Gardens of Babylon. The Greek mathematician, Archytas of Tarentum, described wooden screw threads in the third century BCE, and by the first century BCE wooden screws were widely used in oil and wind presses. These were usually attached to some sort of permanent handle as a turning device. Metal Screws Metal screws and bolts first appeared in Europe in the 1400s, but did not become a common fastener until the 18th century, when machine tools were developed that could manufacture them. Between 1770 and 1798, British instrument maker Jesse Ramsden, British engineer Henry Maudslay and U.S. inventor David Wilkins all patented screw-cutting lathes for making threaded rods. Early screws tended to be custom made with square bolt heads. Replacement bolts were all custom made and therefore not available in large enough quantities to become widely used. Standardization Square-head bolts were common in early applications because they were easier to make with the tools, metals and techniques of the time. Square heads require less accurate tolerances, so that a wrench that might not be the exact size of a bolt but be near enough to turn a hand-machined square bolt head. Square heads, however are large and require more room to turn. By 1841, British toolmaker Joseph Whitworth and his American counterpart, William Sellers of the Franklin Institute had proposed creating a system of standardized screw threads. Standardized bolts and nuts soon followed as toolmakers developed new techniques for making them in quantity. Bessemer Between 1856 and 1876, British Metallurgist Sir Henry Bessemer developed the Bessemer process, a way to mass produce cheap mild steel. When machinists used cast iron and cruder forms of steel, square bolt heads were easier to make. As machinery became smaller and more compact, however, the hex-head bolt evolved to meet the need for more compact bolt heads. Mass Production In 1830 James Nasmyth, an assistant to Henry Maudslay, designed a pioneering milling attachment for Maudslay's bench lathe to make a large batch of hex-head bolts for a scale model they were building for the London Science Museum. By the 1840s, cold-heading machines became available for stamping metal. It took until the 1880s, when Bessemer steel mills began producing the new mild steel in accurate thicknesses and quantity, before cold-heading machines began punching out hex nuts. This innovation meant that nuts stamped from flat metal stock and machined to exact tolerances could be screwed onto bolts made by the new screw-making machines in mills anywhere in the country. Larger hex nuts quickly replaced square bolt heads in heavy industrial applications.

Hex nuts and bolts join parts together and are mated in size (metric or standard), hole diameter and thread pitch as well as width and thickness. The hex nut corner angles prevent rounding off during the tightening process. Size Hex nut sizes available in most American hardware stores include standard sizes, ranging from 8 and 10 (less than 1/4 inch) to fractions of an inch (from 1/4 inch to 3 inches), indicating their thread hole diameters. Hex nut thread holes must match the hex bolt post diameter and thread pitch in order to tighten properly. Width Across Flats Hex nut measurements from one flat edge to the opposite flat edge range from an average 11/32 inch (size 8) to 4.5 inches (size 3), with slight variations depending on manufacturer, material (brass, steel) and oxidation-preventive coating. Width Across Corners Hex nuts range from a minimum 3/8 inch (size 8) to maximum 4.959 inch (size 3) when measured from one of the six corners to the opposite corner. Standard Hex Nut Thickness Standard hex nut thickness runs from a minimum 3/16 inch (size 8) to a maximum 2.654 inches (size 3). Hex Jam Nut Thickness Hex jam nuts, used to tighten against standard hex nuts and lock them in place, run from an average 5/32 inch (size 1/4) to an average 1 19/32 inch (size 3) in thickness.

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