These terms are often incorrectly used interchangeably. The most basic difference between a cap screw and a bolt is the way in which these fasteners are installed. Technically, a bolt is installed by turning a nut to tighten the fastener, while a cap screw in installed by turning the head of the bolt to assemble and tighten.  Therefore, cap screws are often threaded into a tapped hole on a piece of equipment or machinery or installed in some other OEM application.

The variations between these fasteners are fairly significant both from a manufacturing and dimensional perspective as well as an application standpoint. Generally speaking, hex cap screws are used in precise applications like an OEM setting where tight tolerances are required. Hex bolts are often specified when the mechanical properties are more important than dimensional tolerances, like the construction industry. For example, SAE J429 Grade 2 is typically provided as a hex cap screw, whereas ASTM A307-A is a common hex bolt specification.

Below is an overview of some of the specifics these fasteners are required to meet.

Hex cap screws

• Flat washer facing under the head that meet specific tolerances as described under ASME B18.2.1-1996.
• They will usually be manufactured by a cold-heading process (large runs of standard sizes) or turned CNC operation.
• A radius under the head at the shank.
• Body diameter of plus nothing, minus .011″ for an 1-1/4″ diameter fastener.
• Some common fastener specifications unless otherwise specified: ASTM A449 and SAE J429 Grade 2, 5, and 8.

Hex bolts

• A die seam across the bearing surface is permissible.
• The typical bolt is manufactured by hot-forging process.
• Allows a reduced body diameter to be not less than the minimum pitch diameter of the thread.
• Some common fastener specifications unless otherwise specified: ASTM A307A, A354, and F1554.

This list is not inclusive, but represents some of the differences between these two designations. There is flexibility to use other materials, grades, and bolt standards as agreed by manufacturer and purchaser. So, be cautious when ordering bolts make sure you are clear about whether your projects requires cap screws or bolts.

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.

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.