Hex Head Cap Screw vs. Hex Bolt

Many in the fastener industry use the term “hex bolt” and “hex cap screw” interchangeably; technically there is a difference between the two. The industry standard that defines the dimensions for hex bolts and hex cap screws (ANSI/ASME B18.2.1) specifies a slightly wider tolerance for hex bolts as compared to hex cap screws on the following features:

• Body Diameter
• Head Thickness
• Width Across Flats (and Corners)
• Fillet Radius
• Overall Length
• Bearing Surface Flatness

The standard also permits several features to appear on hex bolts that are not permitted on hex cap screws:

• 1. Die seam on bearing face
• 2. Swell on body under head
• 3. Non-pointed (or chamfered) end

Both hex bolts and hex cap screws are made with the same basic thread length and body length requirements (basic thread length = 2 x dia + 1/4” for lengths up to 6” long).

The general rule of thumb for distinguishing a “hex bolt” from a “hex cap screw” is to look for a washer face under the head; if it does not have a washer face it is a hex bolt. Note that the features that are permitted to appear on a hex bolt are based on the assumption that a bolt is always assembled with a nut and that the nut
will be used to tighten the assembly down. A hex cap screw is designed so that it can be tightened by either the head or with a nut.

Some of our customers perceive that a hex bolt is a lower quality item than a hex cap screw. From a strength standpoint, there is no difference between the strength of a Grade 2, 5 or 8 hex bolt as compared to a hex cap screw. Both items have equivalent strength and load carrying capabilities. The tighter controls on the fillet radius and bearing face does make the hex cap screw a better choice if the applications are subjected to fatigue forces.

Also note that you can always provide a hex “cap screw” when a hex “bolt” is referenced on an order, but you cannot provide a hex bolt if a hex cap screw is ordered.
The same features discussed above also apply to “Heavy Hex Bolts” and “Heavy Hex Cap Screws”. The term “heavy” refers to the fact that these items have one size larger width across the flats than a “regular” hex head bolt or cap screw. For example, a 1/2” diameter “regular” hex head has a 3/4” width across the flats and a 1/2” “heavy” hex has a 7/8” width across the flats.