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Hex bolts are used to fasten two or more parts together to form an assembly either because it cannot be manufactured as a single part or to allow for maintenance and repair disassembly. By definition "A Bolt is a headed and externally threaded mechanical device designed for insertion through holes in assembled parts to mate with a nut and is normally intended to be tightened or released by turning that nut." When used with a preformed internally threaded (tapped) hole, the hex bolt's head is turned to tighten, which technically makes it a screw (see our Tech Data section for a discussion regarding the differences between bolts and screws). Hex bolts are also known as: finished hex bolts, hex head bolts, cap screws, hex cap screws, hex head cap screws and, if fully threaded, tap bolts, hex tap bolts and hex head tap bolts.
THREADS AND POINT
Hex bolts have either cut or rolled screw threads that comply with the Unified system: UNC (Unified National Coarse) and UNF (Unified National Fine). Coarse threaded fasteners have fewer threads per inch than fine threaded fasteners because coarse threads are farther apart. For example, a coarse threaded 1/4" hex bolt has 20 threads per inch whereas a 1/4" fine threaded bolt has 28 threads per inch. Consequently, the larger the number of threads per inch, the finer the thread. Also, smaller sizes have more threads per inch than larger sizes—a 1 1/2" UNC bolt, for example, has just 6 threads per inch. Coarse bolts are more common, and there are advantages and disadvantages to both coarse and fine threads. (For more information about threads, visit our Tech Data section.) Short lengths are fully threaded while longer lengths follow a standard formula: for bolts 6" and shorter, the threaded length is two times the basic thread diameter plus 1/4", and for lengths greater than 6", two times the diameter plus 1/2". This, however, is the minimum threaded length and manufacturers may thread the bolt substantially more. In addition, some longer bolts are fully threaded and are often called tap bolts. Table 1 summarizes the number of threads per inch for the various sizes. Right-hand threads are standard (turn clockwise to tighten). The point of hex bolts is chamfered or rounded to protect the first thread from damage and to facilitate entry into an internally threaded part such as a nut or tapped hole.
SIZES AND LENGTHS
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 nominal diameter of the bolt while length is just that—how long the bolt is.
Hex bolts range in size from 1/4" to 1 1/2" in diameter; 1/4" to 1 1/4" sizes are common.
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 washer 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.
DRIVE AND HEAD STYLE
As the name suggests, hex bolts have hex (short for "hexagon") heads along with an underhead washer bearing surface. Hex heads have six flat sides and are also called "6 point." The size of the head varies with the size of the bolt. Since it is externally wrenched, sufficient space around the head must be allowed for a wrench or socket. Refer to Table 1 for head width (width across flats), which is wrench size, and width across corners for each bolt size.
Hex Bolt Dimensions, Sizes, Threads per Inch, Head Dimensions & Minimum Threaded Length
Table 1. Hex bolt dimensions including nominal size in fractional and decimal inches, threads per inch for UNC and UNF types, head width and height, and minimum threaded length for bolt lengths of 6 inches and less (<6") and more than 6 inches (>6"). (Note: Head Width Across Corners is average size rounded to nearest 64th.) Click on link above to see table.
Unlike other materials such as stainless steel and brass, steel hex bolts are available in different strength "grades" as designated by the SAE (Society of Automotive Engineers), which is the most common strength grade system for externally threaded carbon steel fasteners. There is no direct relationship between the grade number and its strength, except that a higher grade (larger number) represents a higher tensile strength. 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 bolt with a lower grade or lower strength—do not substitute a stainless steel or silicon bronze bolt for a Grade 5 or 8 bolt, or a brass bolt for a Grade 2, 5 or 8 bolt. Head markings identify the grades: no lines is Grade 2, three radial lines 120° apart identifies the fastener as Grade 5, and six lines at 60° intervals are used for Grade 8. (An easy way to remember these SAE bolt grades is to add "2" to the number of lines: no lines + 2 = Grade 2; 3 lines + 2 = Grade 5; and 6 lines + 2 = Grade 8.) The manufacturer will also include its own identifying head mark. Details of the three common SAE grades and identification markings are shown in Table 2.
SAE Strength Grades of Hex Bolts
Table 2. Grade designation; nominal size; material and treatment; proof, yield and tensile strengths; hardness; and grade identification markings of SAE graded hex bolts. Click on link above to see table.
Grade 9 hex bolts are generally accepted to be about 20% stronger than SAE Grade 8 bolts. However, Grade 9 is not an SAE grade nor are its chemical and physical properties standardized. Head marking is often one or more letters and the number "9."
Bolts specified as AN (Air Force-Navy), MS (Military Specification) and NAS (National Aerospace Standard) are said to be manufactured to more stringent standards. They are usually available from aircraft and military parts suppliers.
Common materials are steel (unplated and plated; see Finishes below), stainless steel, brass, silicon bronze, and nylon. Grade 2 hex bolts are made from low or medium carbon steel, cold worked; Grade 5 is from medium carbon steel, quenched and tempered; and Grade 8 is manufactured from medium carbon alloy steel, quenched and tempered. Stainless steel, such as 304 and 316, cannot be heat treated and brass and bronze do not respond to heat treatment. Steel fasteners are magnetic, stainless steel may be slightly magnetic, brass and silicon bronze are not made of iron (nonferrous) so they are nonmagnetic, and nylon is nonmetallic and nonmagnetic. Steel remains the least costly material followed by stainless steel; the copper alloys (such as brass and silicon bronze) are the most expensive. (See More Information below for details about matching materials and stainless steel thread galling and seizing, and refer to our Tech Data section for more about materials.)
Nylon is lightweight, has excellent elasticity, resists chemical solvent attacks, is electrically insulating and self-extinguishes if subjected to fire. At the same time, it is low strength, hygroscopic (absorbs moisture), which causes a change in dimensions, and can become brittle in hot, sunny environments. Table 3 lists these characteristics for all material types.
COUNTRY OF ORIGIN
Unlike other types of fasteners, which mainly come from offshore manufacturers, you have a choice with Grade 5 and 8 hex bolts: imported (often from Asia), Canada and domestic (USA). Imported fasteners tend to be the least expensive, Canada is priced a little higher, and domestic may be the most expensive.
Hex Bolt Material Characteristics
Table 3. Characteristics of materials commonly used for hex bolts. Click on link above to see table.
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; "yellow zinc plated," "yellow zinc," "zinc yellow plated" and "zinc yellow" 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.
Hex bolts are typically described as follows:
- Nominal Size (a diameter from 1/4" to 1 1/2", 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 washer to the extreme point)
- Drive Style (Hex Head)
- Fastener Name (Bolt)
- Grade (for steel bolts only: 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, Canada, USA; if not specified, imported is assumed)
- 5/16-18 x 7/8" Hex Bolt, Grade 2, Plain Finish
- 3/8-16 x 3 3/4" Hex Bolt, Grade 8, Yellow Zinc Plated, USA
- 1/4-20 x 1 1/2" Hex Bolt, Silicon Bronze
NUTS AND WASHERS
When selecting a nut for use with a graded hex bolt, it is very important that the nut be the same grade as the bolt; one grade higher is also acceptable as one source states. A bolted joint will be no stronger than its lowest graded component, so a Grade 2 nut and a Grade 8 bolt will create a Grade 2 joint at best. Since it is preferable for the bolt to break before the threads strip—because a broken bolt is more easily discovered than diminishing clamping force as threads gradually strip—the nut should never be a lower grade than the hex bolt.
Flat washers are used to increase the bearing surface of the bolt's head or nut thus spreading the clamping force over a larger area. Grade 2 flat washers should only be used with Grade 2 hex bolts. Use hardened flat washers with bolt Grades 5 and 8. Because Grade 2 flat washers are made of soft, low carbon steel, they will "yield" (compress, cup, bend, etc.) under the higher torque values normally associated with Grade 5 and 8 bolts. As a result, there will be a reduction in clamping force as the washer yields. Also, use only SAE pattern flat washers with Grade 5 and 8 bolts because, as it is explained, the oversized hole in USS washers is too large to support properly the bearing surface of the bolt's head and one washer size smaller will be too tight and may cause the head to fail. When a hardened flat washer is placed under the head, be certain the "rounded" side is against the head—the "sharp" side of the washer can damage the bolt's underhead fillet (the concave junction where the head and shank meet), which can lead to bolt failure.
As for lock washers, which are often split (helical spring) type, use regular for Grade 2 bolts and high strength alloy steel for Grades 5 and 8, although there was a period when the SAE did not recommend using split lock washers with Grade 8 bolts. (Refer to our Tech Data section for additional information about nuts and washers.)
These concepts appear in Table 4.
Hex Bolt Nut Grades and Washer Types
Table 4. Compatible nut grades and flat and split lock washer types for the different SAE hex bolt grades. Click on link above to see table.
TIGHTENING TORQUE AND FASTENER REUSE
Hex bolts are often tightened to specific torque values, especially the higher grades. Be aware that thread lubricants substantially reduce friction—even most locking compounds act as a lubricant. Therefore, be sure to adjust torque values accordingly to prevent overtightening a lubricated bolt. The type of finish also affects friction, which affects torque. In addition, used hex bolts may produce a lower clamping force for the same torque value due to increased friction. Consequently, the connection will not be as "tight" as with a new bolt. Generally speaking, stressed bolts in critical applications should not be reused. And while bolts can fail from being overtightened, they can also fail by being undertightened. Always follow the manufacturer's recommendations regarding reuse, thread lubricants/locking compounds and torque values.
It is considered good practice to debur all holes to ensure a smooth, flat mating surface.
Do not align parts by "tapping" a bolt into position—serious damage to the bolt can occur. Instead, use a tapered dowel pin to first align the parts, then insert the bolt.
Never extend the threaded length of a bolt by cutting additional threads. "Jagged tears," created by both threading dies and lathes, are said to be stress raisers and a stress raiser can be a point of failure. A stress raiser is "A notch, hole, or other discontinuity in contour or structure which causes localized stress concentration."
It is advisable to match materials and finishes of bolts, washers and nuts. If, say, you are using a zinc plated hex bolt, use a zinc plated washer and zinc plated nut. Likewise for stainless steel: use a stainless steel bolt along with a stainless steel washer and nut.
Stainless steel hex bolts are susceptible to thread galling and seizing. It is believed that the self-generated chromium oxide film is destroyed during tightening, which exposes high points that shear or lock together. Increased adhesion of the thread surfaces from the clogging-shearing-locking action causes the galling phenomenon. If severe enough, seizing can occur preventing further rotation of the nut in either direction. While it may not be completely preventable, it can be substantially reduced. A thread lubricant is the most effective method. (Keep in mind that lubricants reduce friction, which can lead to overtightening.) Alternatively, stainless steel alloys having different hardnesses—like a 316 nut and a 304 hex bolt—have less tendency to gall.
Hex bolts are summarized in Table 5.
Hex Bolt Typical Characteristics
Table 5. Summary of typical hex bolt characteristics including sizes and lengths, head and drive styles, grades, materials and finishes. Click on link above to see table.
Printable hex bolt reference charts and information: