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Jam nuts are thin hex nuts usually used as a second nut that is "jammed" (tightened) against a full size nut or alone as a thin nut. They are used to "lock" turnbuckles and control linkages, like steering tie rods, after adjustment. Hex is short for hexagon, which means they have six sides. Jam nuts are also known as:
half height nuts, hex thin nuts
(even though they are referred to as half height, they are really two-thirds height). Jam nuts are used without lock washers when jammed against another nut, and may be used with lock washers when installed as a single nut.
When used in a "jam" configuration, the jam nut is installed first and contacts the mating surface. The full size nut is installed second and contacts the jam nut. Although this may be the generally accepted practice, there is still controversy as to which nut should be installed first. In addition, there is concern that the two nuts will not be equally torqued so one may yield before the other. If tension load is critical, one source suggests using a single lock nut instead.
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 jam nuts are also available—see Left Hand Nuts.
Jam 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. Not all types are available in all sizes.
Width across flats, which is wrench size, ranges from 1.50 to 1.75 times the nominal jam nut size and thickness is 0.542 to 0.625 times the size, which is two-thirds the thickness of a standard nut. 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.
Unlike materials such as stainless steel and brass, steel jam 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; and Grade 8 is the highest SAE grade and is usually priced the highest as well. Never replace a graded jam 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.
A common finish for steel is zinc plating. Zinc, the most popular and least expensive commercial plating, offers moderate corrosion resistance. 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 jam nuts—referred to as plain finish—may also be available and are susceptible to rust.
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 jam nuts, washers and bolts/screws.
Stainless steel jam 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 thicker nut, choose a hex nut. For a wider and thicker nut, consider a heavy hex nut.
Refer to ASME B18.2.2 for specifications relating to jam nuts.
Jam Nut Dimensions
Table 1. Jam 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.)