What are the differences between a hex cap screw and a hex bolt?
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.
Is ‘fillet’ meant by ‘radius’ in ‘A radius under the head at the shank.’? Thanks!
@David- we usually hear the word ‘fillet’ in relation to a weld. This is not a weld, but a forged head. The radius is there to maintain the structural integrity of the head.
I need to order Stainless Steel Hex Bolts. They have to be 316 Stainless Steel ASTM F593-02. When I try to order them online I see them called out as Hex Cap Screws and not as Hex Bolts. What is the correct call out? Is it Hex Bolt or Hex Cap Screw for this particular instance?
@Sean- Most off the shelf, mass produced F593s will be made as hex cap screws which are usually cold formed and have tighter dimensional tolerances. The product we manufacture will be hot formed and will be made as hex bolts. To the untrained eye, both will look the same and in many applications, both may be equally appropriate. If your application does not require one or the other, you could ask your supplier what they are stocking – if they are off the shelf, they are most likely cap screws.
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.
As stated above, Hex cap screws are usually to be manufactured by a cold-heading process or turned CNC operation and Hex bolts are typical manufactured by hot-forging process. Is the head of an A325 Structural Bolt hot-forged or cold-headed? I think it’s hot forged, which explains why by specification, A325 bolts are designed with sufficient ductility to pretension into the inelastic range.
@James- virtually all mass produced fasteners are cold headed, regardless of if they are a hex cap screw or a hex bolt. Mass produced A325s are also cold headed, and then heat treated to achieve mechanical properties and ductility. Short run specials like we can manufacture are typically hot forged.
why are using Stud bolt, if use instead of stud bolt hex head bolt there coming any problem pls. tell me……sir.
we are using Butterfly valve to flange connection insted of Stud bolt hex. Bolt.
@Muttu – We do not have any engineers of staff and are not able to make any recommendations regarding which type of fastener is better suited for a specific application.
I received a requirement for Bolts and the user specified the standar ASTM A193 and A194 for nuts. I got the material, but now he says that what he needs is Hex Bolts, so i think he is confused, because i know that ASTM A193 just cover Studs for intended use in oil and petrochemical industry. Could you confirm if i am right or no.
Thanks for you repply.
ASTM A193 covers material used in the manufacture of studs, hex bolts, bent bolts, etc; all for use in high temperature or high pressure applications (i.e. petrochemical). So there are a variety of fastener configurations that can all be certified to A193, studs and hex bolts included.
Are hex bolts typically supplied in material spec A193 B7? I have a valve drawing that calls for this spec and it is described as “TRIPLEX STL.”
Our normal encounter with this material spec is a threaded stud or rod.
THANK YOU!
@Jeff – Yes, A193 B7 hex bolts are typically available, although it may depend on the diameter and length you need. We’d be happy to look and quote you if you’d like to contact us with the details.
Wouod you be able to clairify the relationship between ASTM F593D and ASTM 193. I belive they are both standards for stainless steel hex bolts and 304 and 316 stainless steel are available under both standards, but F593D bolts are 20% stronger?? Also, I’ve been told 18-8 and 304 stainless steel are the same. If they are, why are there two grades?
Thank you
@Bob Hinzman – A193 was written as both a material and fastener standard to cover materials used in high temperature and high pressure applications. F593 was written as a general purpose, mass produced fastener standard. F593 fasteners are typically cold formed (A193 not necessarily so), which results in higher mechanical properties. AISI 304 stainless is a stainless alloy with a specific chemical analysis, whereas 18-8 refers to the family of stainlesses that 304 is a part of. AISI 302,303,304,305,309 and others all fall into the 18-8 family. Many times the terms 18-8 and 304 are used interchangeably, however that is not necessarily correct.