Hex Bolts and Cap Screws Basics

Hex bolts and cap screws are fasteners that have a six-sided, hexagonal head. Typically, the terms hex bolt and hex cap screw are used interchangeably. The everyday user need not concern themselves with the differences between the two, as most fasteners are only available in either a bolt, or a cap screw. Rarely do you have a choice when selecting a fastener. For instance, all A325 hex fasteners are bolts. They are not commercially available as a cap screw.

 

Differences & Similarities - Hex Bolts vs Cap Screws

That is not to say there are not key differences between a bolt and a cap screw. The main difference is that a hex bolt is meant to have a hex nut tightened onto it, while a cap screw is meant to be threaded into a tapped hole and tightened by turning its head.

In addition, a hex bolt is normally used when the mechanical properties are more important than dimensional tolerances (usually in the construction industry).

A cap screw is often used in precise applications including, but not limited to, an OEM setting where tight tolerances are required. Cap screws have a washer face under the head, while hex bolts do not. Hex bolts are typically manufactured using a hot-forging process and have a flat end. Cap screws are commonly manufactured by a cold-heading process or a turned CNC operation and have a chamfered end.

Neither hex bolts nor cap screws are typically fully threaded (except for shorter fasteners) and are threaded based on the following formula: 2 X DIAMETER + 1/4" for fasteners under 6” in length.          

                      

Head Types of Hex Head Fasteners

Hex head bolts and cap screw fasteners are manufactured in two main head styles:

Finished Hex Bolts and Cap Screws

Standard head type found on most hex fasteners.

Heavy Hex Bolts and Cap Screws

Heavy hex bolts offer a heavy pattern head which utilizes a larger wrench size. The larger head increases the bearing surface and distributes the clamping load over a larger area. Heavy hex head bolts, cap screws, and heavy hex nuts are common in industrial flange bolting and structural bolting.

 

Hex Bolt Grades

Hex bolts are available in several different grades based on the mechanical and chemical properties of the material, and the intended application of the hex bolt.                                                              

ASTM A307 Hex Bolts

Common fasteners manufactured from low carbon steel. Grade A307 bolts have a finished head and are used for general purpose applications. A variation, ASTM A307 Grade B hex bolts have a heavy hex head and are commonly used for flanged joints in piping systems with cast iron flanges and structural applications. 

ASTM F3125 Grade A325 Type 1 Heavy Hex Bolts

High strength structural bolts manufactured from medium carbon steel, carbon boron or medium carbon alloy steel, quenched and tempered. Grade A325 Type 3 heavy hex bolts are manufactured from weathering steel and are also quenched and tempered. A325 Type 1 bolts can be galvanized to protect them from corrosion while Type 3 bolts are designed to weather, or rust, as a means of protection. Type 1 and Type 3 A325 bolts are intended for use in steel to steel structural connections.

ASTM A193 Grade B7 Heavy Hex Head Bolts

High strength fasteners manufactured from heat treated alloy steel AISI 4140/4142, quenched and tempered. Grade B7 heavy hex bolts are typically used in flange bolting applications with high temperature requirements or in high pressure service. A193 B7 hex bolts, as well as all thread stud bolts having diameters above 1" are most common 8UN (8 threads per inch) and are also available UNC (coarse thread).

 

Hex Cap Screw Grades

Hex cap screws also come in several different grades based on their strength, chemical and mechanical properties, and the intended application of the hex cap screw.

SAE J429 Grade 5 Hex Head Cap Screws

Common, economical fasteners manufactured from heat-treated medium carbon steel, quenched and tempered. Inch series fasteners meeting SAE J429 Grade 5 specifications are general use fasteners commonly used in automotive and construction related industries. Grade 5 hex cap screws are medium strength carbon steel fasteners that have a minimum tensile strength of 120,000 psi. When Grade 5 hex fasteners are offered in a fully threaded configuration, they are typically supplied as a bolt, rather than a cap screw.

SAE J429 Grade 8 Hex Head Cap Screws

High-strength fasteners manufactured from heat-treated medium carbon alloy steel and are also quenched and tempered. Inch series fasteners meeting SAE J429 Grade 8 specifications are general use fasteners commonly used in automotive and construction related industries. Grade 8 hex cap screws are high strength alloy steel fasteners that have a minimum tensile strength of 150,000 psi. When Grade 8 hex fasteners are offered in a fully threaded configuration, they are typically supplied as a bolt, rather than a cap screw.

304 Stainless Steel Hex Head Cap Screws

Common fasteners manufactured from austenitic 304 stainless steel. They have a chromium content of 17-19% and nickel content of 8-10%, yielding the common term 18-8 stainless steel. 304 stainless steel fasteners are known for their corrosion and heat resistance. Austenitic grade steels are high in chromium, with varying amounts of nickel, manganese, nitrogen and some carbon. The 10.5% minimum chromium content makes the metal approximately 200 times more resistant to corrosion than metals without chromium. 304 stainless steel cap screws are commonly used in applications that require general atmospheric corrosion resistance, such as chemical and food processing equipment, as well as marine and coastal facilities and other corrosive environments.

316 Stainless Steel Hex Head Cap Screws

Manufactured from 316 stainless steel, an austenitic alloy which differs from 304 stainless steel by its molybdenum content (2-3%) and a higher nickel content (10-14%) allowing for higher corrosion resistance. 316 stainless steel fasteners are known for their corrosion and heat resistance. Austenitic grade steels are high in chromium, with varying amounts of nickel, manganese, nitrogen and some carbon. The 10.5% minimum chromium content makes the metal approximately 200 times more resistant to corrosion than metals without chromium. 316 stainless steel cap screws are commonly used in applications that require general atmospheric corrosion resistance, such as chemical and food processing equipment, as well as marine and coastal facilities and other corrosive environments.

Silicon Bronze Hex Head Cap Screws

Fasteners manufactured from Alloy 651 or Alloy 655. They are commonly used in general construction, marine applications, refineries, and the power and energy industry. Non-magnetic and electrically conductive, these fasteners are best known for their corrosion resistance. Silicon Bronze cap screws are significantly stronger than brass.

Brass Hex Head Cap Screws

Manufactured from an alloy of copper and zinc. Brass fasteners are commonly used by electrical and communications industries, builders hardware and some marine applications. Brass hex head cap screws are electrically conductive, non-magnetic, and have good corrosion resistance.

 

Common Finishes of Hex Head Bolts and Cap Screws

Hex head bolts and cap screws are available in a variety of finishes and coatings, which play a role in the suitability of the fasteners in different applications, such as enhancing resistance to corrosion from environmental elements or chemicals.

Black or Plain Finish Hex Bolts and Cap Screws

Fasteners are unfinished or will have a minimally protective finish. Black fasteners offer near zero corrosion resistance, though black oxide coatings or oiled coatings do offer increased corrosion resistance over an unfinished plain black fastener.

• Black fasteners are commonly referred to as plain finish, black, black oxide, black phosphate, bare or light oil coated.

Galvanized Finished Hex Head Fasteners

Fasteners are coated with a sacrificial zinc coating that acts as an anode to prevent the fastener underneath from corroding.

• Hot Dipped Galvanized: Fasteners are dipped in a bath of molten zinc.
• Mechanically Galvanized: Fasteners are tumbled in a barrel with a mixture of water, zinc powder, other chemicals and glass beads.
• Electro Galvanized: Fasteners are immersed in a cold zinc bath while an electric current is run through it. This is also known as electroplating.

Zinc Plated Hex Head Bolts and Cap Screws

Fasteners are coated with a thin layer of zinc, typically via electroplating.

• Zinc plated fasteners will not corrode as quickly when covered with a zinc coating, even when a scratch or cut exposes the steel to air or moisture. The zinc coating will always tarnish and corrode first.

Teflon or PTFE Coated Hex Head Fasteners

Fasteners have a PTFE coating (commonly referred to as the Dupont tradename Teflon) and are formulated with superior characteristics for maximum fastener life.

• Technical components include a tensile strength of up to 4,000 psi and an operating pressure up to 100,000 psi. The absence of thread locking oxides enable easy and safe fastener removal. PTFE coatings have extreme chemical resistance and are ideal for applications including insulating piping, boiler rooms, water filled piping, water lines and more.