Hex head cap screws and hex bolts. They look similar, and their names sound similar — but they’re different. How? Find out below.
What’s the Difference Between Hex Head Cap Screws and Hex Bolts? Definitions From ASME B18.2.1
It’s tricky to tell the difference between a hex head cap screw and a hex bolt. In fact, the topic falls squarely into the “depends on who you ask” category, right alongside the differences between generic bolts and screws.
However, there are a few manufacturers and industry organizations that have provided their expert opinion here. One explanation we like best is from The American Society of Mechanical Engineers (ASME) B18.2.1 standard. It’s just one of the many specifications that address this subject, but it’s pretty common, thorough, and simple to understand.
What Are Hex Head Cap Screws?
According to ASME B18.2.1, screws are externally threaded fasteners capable of being inserted into holes in assembled parts, mating with preformed internal threads, forming their own internal threads, and being tightened or released by torquing their heads.
Hex head cap screws are a particular type of screw with a hexagon-shaped head designed to be inserted into a pre-tapped hole. They typically have diameters between ¼ – 1 ½”, can be partially or fully threaded, and are available in a variety of grades.
Hex head cap screws are most commonly used in automotive and heavy machinery applications.
What Are Hex Bolts?
According to ASME B18.2.1, bolts are externally threaded fasteners designed for insertion through holes in assembled parts. They’re usually meant to be tightened or released by torquing a nut.
Hex bolts are a particular type of bolt with a hexagon-shaped cap on one end. They’re intended to be paired with a nut. They typically have diameters between ¼ – 1”, can be partially or fully threaded, and are available in a variety of grades.
Hex bolts are most commonly used in construction and heavy machinery applications.
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Physical Differences Between Hex Head Cap Screws and Hex Bolts, According to ASME B18.2.1
Now that we’ve proposed some basic definitions of hex head cap screws and hex bolts, let’s take a look at some of the more prominent differences in their physical features and performance.
Washer Face
Hex head cap screws have a washer face under their heads.
Hex bolts do not have a washer face.
Bearing Surface
In this spec, hex head cap screws have a maximum measurement of total bearing surface runout, which varies based on nominal size and diameter.
For hex bolts, this spec only dictates that a hex bolt’s diameter must be perpendicular to the axis of the body with a tolerance of 3° for 1” size and smaller, and 2° for greater than 1”.
Tolerance & Body Diameter
In general, hex head cap screws are a little more refined and precise than hex bolts. They have tighter tolerances across the board, no matter the size of the part. For example, the minimum body diameter for a ¼” nominal size hex head cap screw is 0.245” and the maximum is 0.250” — a 0.005” difference.
Hex bolts have “looser” tolerances and are a bit more lenient in this sense. For example, the minimum body diameter for a ¼” nominal size hex bolt is 0.237” and the maximum is 0.260” — a 0.023” difference.
Material & Grade
Unless otherwise specified, hex head cap screws made of steel should comply with ASTM A449, ASTM A354, or SAE J429. Hex head cap screws made of stainless steel should comply with ASTM F593.
Hex bolts made of steel should comply with ASTM A307 Grade A unless otherwise specified. For more information on different bolt grades and their applications, check out this blog.
Again, it’s important to remember that this is just one specification’s definition of the difference between hex head cap screws and hex bolts. Depending on what spec you look at, you may be presented with a different answer to this question.
Get Specialty Bolts, Screws & Studs at Wilson-Garner
Need hex head cap screws or hex bolts? At Wilson-Garner, we can manufacture both according to your specifications. Our team specializes in per-spec, limited-run fastener solutions for a range of industrial applications. Contact us online to learn more about how we can help you.
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