hex machine bolts vs hex cap screws

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

These terms are often incorrectly used interchangeably. The most basic difference between a cap screw and a machine bolt is the way in which these fasteners are installed. Technically, a machine 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 machine 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 machine 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.