Ask the Expert

What are the differences between a hex cap screw and a hex bolt?

These terms are often incorrectly used interchangeably. The variations between these fasteners are fairly significant both from a manufacturing perspective and 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 A307-A, 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 that you make sure to be clear what type your project requires.

Posted on Friday, June 27th, 2008 by Greg Lindsay. Filed under Bolt Industry, Manufacturing. Comments may be followed using the RSS 2.0 feed.

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