Hex Cap Screws

Hex cap screws are headed fasteners with external machine screw threads. They are used to fasten two or more parts together and allow for maintenance and repair disassembly.

Having either cut or rolled screw threads that comply with the Unified system—UNC (Unified National Coarse) and UNF (Unified National Fine)—right-hand threads are standard so turn clockwise to tighten. Short hex cap screws are fully threaded while longer lengths follow a standard formula: for screws 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 screw substantially more. In addition, some longer screws are fully threaded and are often called tap bolts. The point of hex cap screws is chamfered (beveled) to protect the first thread from damage and to facilitate entry into an internally threaded part such as a tapped hole or nut.

Ranging in size (diameter) from 1/4" to 1 1/2", 1/4" to 1 1/4" sizes are common. Lengths range from 3/8" to 24"; 1/2" to 10" is typical. Larger sizes are not manufactured in short lengths, just as smaller sizes are not available in long lengths. Size and length are specified in fractional inches; length is measured from under the head to the extreme point (threaded end) of the screw.

Hex cap screws have hex (short for "hexagon") heads along with an underhead circular boss bearing surface, called a washer face. The size of the head varies with the size of the screw.

Steel hex cap screws are available in different strength "grades" as designated by the SAE (Society of Automotive Engineers): Grade 2, 5 and 8. Another option is Grade 9, which is generally accepted to be about 20% stronger than SAE Grade 8. However, Grade 9 is not an SAE grade nor are its chemical and physical properties standardized.

Common materials are steel (unplated and plated), stainless steel, brass, silicon bronze, and nylon. 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.

Hot dip galvanized, stainless steel and silicon bronze are usually recommended if the hex cap screws 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 steel may show signs of severe pitting corrosion when submerged in salt water that lacks free oxygen, which makes silicon bronze a preferred material because stainless steel needs oxygen to create its self-healing, corrosion-resisting chromium oxide film.

Unlike other types of fasteners, which mainly come from offshore manufacturers, you have a choice with Grade 5 and 8 hex cap screws: imported (often from Asia), North America (Canada) and domestic (USA). Imported fasteners tend to be the least expensive, North America is priced a little higher, and domestic may be the most expensive (the number of dollar signs [$] indicate relative price).

When selecting a nut for use with a graded hex cap screw, it is very important that the nut be the same grade as the bolt; one grade higher is also acceptable as one source states. Grade 2 flat washers should only be used with Grade 2 cap screws—use hardened flat washers with Grade 5 and 8 hex cap screws. Also, use only SAE pattern flat washers with Grade 5 and 8 cap screws because, as it is explained, the oversized hole in USS washers is too large to properly support the bearing surface of the cap screw's head and one washer size smaller will be too tight and may cause the head to fail. As for lock washers, which are often split (helical spring) type, use regular for Grade 2 cap screws and high alloy for Grades 5 and 8.

Hex cap screws are also called:

cap screws, hex head cap screws, hex bolts, hex head bolts, machine bolts, hex machine bolts, hex head machine bolts and, if fully threaded, tap bolts, hex tap bolts and hex head tap bolts.

See the introduction to hex bolts for more information about hex cap screws including reference tables.

Hex Cap Screws
Brass
Coarse Thread	Brass
Steel, Grade 2
Coarse Thread, Hot Dip Galvanized Coarse Thread, Plain Finish Coarse Thread, Zinc Plated Coarse Thread, Fully Threaded, Zinc Plated	Grade 2, Galvanized Grade 2
Steel, Grade 5
Coarse Thread, Plain Finish, USA/Canada Coarse Thread, Zinc Plated, Global ($) Coarse Thread, Zinc Plated, USA/Canada ($$) Coarse Thread, Zinc Plated, USA ($$$) Coarse Thread, Fully Threaded, Zinc Plated Fine Thread, Zinc Plated, USA/Canada ($$) Fine Thread, Zinc Plated, USA ($$$)	Grade 5, USA/Canada Grade 5, Global Grade 5, USA
Steel, Grade 8
Coarse Thread, Zinc Yellow Plated, Global ($) Coarse Thread, Zinc Yellow Plated, USA/Canada ($$) Coarse Thread, Zinc Yellow Plated, USA ($$$) Fine Thread, Zinc Yellow Plated, USA/Canada ($) Fine Thread, Zinc Yellow Plated, USA ($$)	Grade 8, Global Grade 8, USA/Canada Grade 8, USA
Steel, Grade 9
Coarse Thread, Zinc Dichromate Plated, USA Fine Thread, Zinc Dichromate Plated, USA	Grade 9, USA
Nylon
Coarse Thread	Nylon
Silicon Bronze
Coarse Thread	Silicon Bronze
Stainless Steel
304 (18-8), Coarse Thread 304 (18-8), Fine Thread 316, Coarse Thread	Stainless Steel

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