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.
Zinc Yellow-Chromate Plated Hex Head Screw
Grade 8 Steel, 5/8″-11
Thread Size, 2″ Long
Do get back to me with the Total quote and advice on the forms of payment that you accept. What’s the lead time
Thank you
@Bob- Apologies, but this isn’t the type of fastener that we typically handle, we are more of a specialty construction fastener manufacturer. A full line fastener distributor or hardware store might be the place to go.
Your first paragraphs were great, but the overview was… what the…!
“So, be cautious when ordering bolts make sure you are clear about whether your projects requires cap screws or bolts.”
I believe you know what your trying to describe. No wonder engineers can’t teach or write readable books. Way too technical for the average Joe. Thanks anyway bro.
Thank you, it was a big help!
If I were to order a 1″-8 UNC x 2″ LG Grade 5 Hex Head Bolt per ASTM B18.2.1, would you supply a Bolt or a Cap Screw?
@Doug- Most mass produced SAE grade 5 bolts are manufactured as cap screws.
Does Socket head cap screws break at head or at minor dia. ?
During tensile strength*
If yes, why?
Tq
@Deva- During axial tensile loading, the fracture point should be in the root (minor diameter) of the threads because that is the smallest point of the screw and therefore the weakest. If the screw is fully threaded, the fracture could occur up by the head, but should still be at the thread root.