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Heavy Hex Nuts

Heavy hex nuts are thicker and wider than (finished) hex nuts of the same size, which makes them the strongest of comparably-graded nuts. Their additional strength comes from increased thread engagement, which is due to their thickness, and greater resistance to dilation (expansion or stretching) because of their larger width. The heavy hex nut's increased size yields a larger bearing surface and improved wrenchability. Hex is short for hexagon, which means they have six sides. They are also known as: 

heavy nuts.

  Lock washers may used with heavy 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 heavy hex nuts are also available—see Left Hand Nuts.

Heavy hex nut size refers to its nominal thread diameter. Typically, sizes range from 1/4" to about 3 1/2". Size is specified in inches, usually fractional rather than decimal. Table 1 lists width across flats and across corners, and thickness. Not all types are available in all sizes.

Width across flats, which is wrench size, ranges from about 1.53 to 2.00 times the nominal heavy hex nut size and height is 0.938 to 0.986 times the size. Nuts that are 7/16" 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 heavy hex nut, the bolt/screw 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 heavy hex nuts are available in different strength "grades." Grades 2 and 2H and ASTM A194 and A563-C are typically available. Never replace a graded heavy 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 and brass. Steel remains the least costly material followed by stainless steel; copper alloys (such as brass) 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 heavy 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 heavy hex nuts, washers and bolts/screws. When using hot dipped galvanized bolts, always use hot dipped galvanized nuts, which are tapped oversize (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 and using a plain finish or zinc plated nut on a hot dip galvanized bolt will result in a fit that is much too tight).

Stainless steel heavy 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/screw—have less tendency to gall.

If you need a thinner nut of the same width, choose a heavy hex jam nut. For a less wide, less thick nut, consider a hex nut.

Refer to ASME B18.2.2 for specifications relating to heavy hex nuts.

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