Hex head bolts and cap screws are used in a variety of applications with various grades, materials, and finishes to choose from. Here is some general info for Grade 2, Grade 5, Grade 8, and stainless hex bolts. Hex head bolts and fasteners are used in a range of applications and industries. They are used for fastening wood, metal, and other construction materials as well as being used in all types of manufacturing. They are ideal for docks, bridges, highway structures, and more. Hex head bolts and screws can be used with most everyday tools, making them perfect for applications that need versatility.
The following is some general information on sizing. The size and length of inch series fasteners—also known as English, Imperial and U.S. Customary fasteners—is specified in inches, usually fractional rather than decimal.
Size refers to the nominal diameter of the bolt while length is just that—how long the bolt is.
Hex bolts range in size from 1/4″ to 4″ in diameter; 1/4″ to 1 1/2″ sizes are commonly available.
Lengths range from 3/8″ to 24″; 1/2″ to 10″ is typical. From 3/8″ to 1″, lengths are in 1/8″ increments. Between 1″ and 4″, 1/4″ steps are common. From 4″ to 8″, lengths usually increase by 1/2″. Longer than 8″, expect 2″ steps. Length is measured from the underhead bearing surface to the extreme point (threaded end) of the bolt.
Larger sizes are not manufactured in short lengths, just as smaller sizes are not available in long lengths. Not all hex bolts are readily available in all sizes and lengths.
The bolt should be long enough to allow at least two full threads to extend beyond the nut face after tightening, which ensures full thread engagement with the nut. Conversely, there should be two full threads exposed on the head side of the nut to make sure the nut can be properly tightened. More exposed threads within the grip (the area between the head and nut) will result in a “springier” bolt. It is also recommended that threads not be in the shear planes.
There are several grades and materials of hex head bolts to consider. Here are a few of the most common. Grade 2 bolts ( A307) are a standard hardware grade steel. This is the least expensive and most common. They have a tensile strength of 60,000 PSI.
Grade 5 bolts are a heat treated carbon bolt. Hardened to increase its strength, it’s most commonly found in automotive applications. Grade 5 bolts have 3 evenly spaced radial lines … resembling an airplane propeller. They have a tensile strength of 120,000 PSI
Grade 8 bolts are a heat treated alloy steel that are hardened more than the Grade 5. Thus, they are stronger and can be used in more demanding situations. Grade 8 bolts have 6 evenly spaced radial lines. Grade 8 bolts have a tensile strength of 150,000 PSI
For Stainless Hex Bolts there are different grades but the most common are bolts made in the 303 and 304 series of stainless. Many people think stainless bolts are very hard. This is not true. 300 series stainless is just a tad under a Grade 5 in strength. Usually a tensile strength in the range of 112,000 to 116,000 PSI.
Finally, how a hex bolt is finished or the coating on its exterior will depend on the application it’s being used for. Here is some information on common finishes for hex bolts. Unplated or uncoated steel, referred to as “plain finish,” has not been surface treated to prevent rust other than a light coating of oil for temporary protection. Consequently, common finishes for steel are zinc plating and hot dip galvanizing. Zinc, the most popular and least expensive commercial plating, offers moderate corrosion resistance. A surface coating of clear or bluish chromate helps prevent the formation of “white rust,” a dull white corrosion caused by moisture; zinc with clear chromate is usually a silver color and is referred to as “zinc plated” or simply “zinc.” Yellow chromate, however, is more effective in protecting against white oxidation and is yellow in color; “zinc yellow plated,” “zinc yellow,” “yellow zinc plated” and “yellow zinc” are the terms used to describe this finish. Hot dip galvanized is a thick coating of zinc that protects against corrosion in harsh environments and has a dull gray appearance. Silicon bronze and stainless steel, though, are the better choices when corrosion is of concern. (More information about finishes can be found in our Tech Data section.)
Hot dip galvanized, stainless steel and silicon bronze are usually recommended if the bolts will be used with pressure preservative treated wood such as “ACQ” (Alkaline Copper Quaternary)—check local building codes and contact your lumber supplier for recommendations. In marine environments, “stainless steels are subject to potentially severe pitting-corrosion attack when immersed in salt water, without free-oxygen,” which makes silicon bronze a preferred material because stainless steel needs oxygen to create its self-healing, corrosion-resisting chromium oxide film.
In summary, here are the key designations to consider when you’re thinking about what type of bolt to use for your application and also how to correctly order them from your vendor. Knowing how to refer to the item you’re ordering is often critical to ensuring you are getting exactly what you want and need for your application. Hex bolts are typically described as follows:
- Nominal Size (a diameter from 1/4″ to 4″, usually specified in fractional inches)
- Threads per Inch (20 to 6 for UNC and 28 to 12 for UNF)
- Bolt Length (in inches, measured from the underhead bearing surface to the extreme point)
- Drive Style (Hex Head)
- Fastener Name (Bolt)
- Grade (for steel bolts only: ASTM Grade A307, SAE Grade 2, 5, 8; or non-standard Grade 9)
- Material (steel, stainless steel, brass, silicon bronze, nylon; if not specified, steel is assumed)
- Finish (for steel bolts only: plain, zinc plated or hot dip galvanized)
- Country of Origin (imported, North America [Canada], USA; if not specified, imported is assumed)