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Hex nuts are one of the most common nuts available and are used with anchors, bolts, screws, studs, threaded rods and on any other fastener that has machine threads. Hex is short for hexagon, which means they have six sides. Hex nuts are also known as:
finished hex nuts, hex full nuts.
Lock washers may used with hex nuts.
Threads are standard right-hand and Unified inch coarse series (UNC, Unified National Coarse) or Unified inch fine (UNF, Unified National Fine). Left-hand threaded hex nuts are also available—see Left Hand Nuts.
Hex nut size refers to its nominal thread diameter. Typically, sizes range from 1/4" to about 2 1/2". Size is specified in inches, usually fractional rather than decimal. Table 1 lists width across flats and across corners, and thickness.
Width across flats, which is wrench size, ranges from 1.50 to 1.75 times the nominal hex nut size and thickness is 0.850 to 0.875 times the size. Nuts that are 5/8" and smaller are double (top and bottom) chamfered (beveled); larger sizes may be double chamfered or have a washer face bearing (bottom) surface and a chamfered top.
To ensure full thread engagement with the hex nut, bolts/screws should be long enough to allow at least two full threads to extend beyond the nut face after tightening. Conversely, there should be two full threads exposed on the head side of the nut to make sure the nut can be properly tightened.
Unlike materials such as stainless steel and brass, steel hex nuts are available in different strength "grades" as designated by the SAE (Society of Automotive Engineers). Grade 2 is the lowest, in terms of both strength and cost; Grade 5 offers medium strength; and Grade 8 is the highest SAE grade and is usually priced the highest as well. Never replace a graded hex nut with a lower grade or lower strength nut. Surface markings identify the grade.
Common materials include steel (unplated and plated—see more about finishes below), stainless steel, brass, silicon bronze, aluminum and nylon. Steel remains the least costly material followed by stainless steel; copper alloys (such as brass and silicon bronze) are the most expensive.
Common finishes for steel are zinc plating and hot dip galvanizing. Zinc, the most popular and least expensive commercial plating, offers moderate corrosion resistance. Hot dip galvanized is a thick coating of zinc that protects against corrosion in harsh environments. Stainless steel, though, is a better choice when corrosion is of concern except when submerged in salt water without free oxygen where it can suffer from severe pitting corrosion. Unplated and uncoated steel hex nuts—referred to as plain finish—may also be available and are susceptible to rust. Not all types are available in all materials and finishes.
Always follow the manufacturer's recommendations regarding nut reuse, thread lubricants/locking compounds and torque values.
It is advisable to match materials and finishes of hex nuts, washers and bolts/screws. When using hot dipped galvanized bolts, always use hot dipped galvanized nuts, which are overtapped (threaded larger than normal) to accommodate the thick zinc coating on the bolts (using a galvanized nut on a non-galvanized bolt will result in an unacceptably loose fit).
Stainless steel hex nuts and bolts/screws used together are susceptible to thread galling and seizing. While it may not be completely preventable, it can be substantially reduced. A thread lubricant is the most effective method. Alternatively, stainless steel alloys having different hardnesses—like a 316 nut and a 304 bolt—have less tendency to gall.
If you need a thinner nut, choose a jam nut. For smaller sizes—diameter, width and thickness—look to machine screw nuts. For a wider and thicker nut, consider a heavy hex nut.
Refer to ASME B18.2.2 for specifications relating to hex nuts.
Hex Nut Dimensions
Table 1. Hex Nut Dimensions. Nut size, basic
width across flats, average width across corners,
and thickness, in inches. (Note: Average size is
calculated and rounded to the nearest 64th.)