Frequently Asked Question

Answer

ASTM A325 bolts (now covered under the ASTM International F3125 specification) were originally designed as Heavy Hex Structural Bolts for steel-to-steel structural connections. Because of their strength, clamping force, and intended applications, the Heavy Hex head design remains the standard and most commonly used configuration.

However, A325, A490, and Grade 144 bolts can be manufactured in non-standard head styles if they still meet the mechanical and chemical requirements of the ASTM F3125 specification. Examples may include:

• Square head bolts
• Countersunk head bolts
• Carriage bolts
• Round head bolts
• Timber bolts
• Other specialty headed configurations

When a bolt uses a non-standard head style or non-standard thread length, it must be marked with an “S” designation, such as:

• A325S
• A490S
• 144S

The “S” marking identifies the bolt as a structural bolt that differs from the standard Heavy Hex Structural Bolt configuration.

Why Heavy Hex Heads Are Preferred

The ASTM F3125 specification was engineered around the performance characteristics of a Heavy Hex Head Structural Bolt. The larger bearing surface and head geometry help the bolt achieve the intended structural performance in high-strength steel connections.

Using alternate head styles — especially undersized heads like countersunk or carriage bolts — can affect:

• Clamp load performance
• Bearing surface strength
• Installation behavior
• Overall structural reliability

Even if a non-standard bolt passes tensile and yield testing, it may not perform identically to a standard Heavy Hex A325 bolt in real-world structural applications.

Important Considerations for Non-Standard A325 Bolts

Before specifying or using non-standard A325, A490, or Grade 144 head styles, engineers and contractors should consider:

• The connection design requirements
• Load distribution under the bolt head
• Installation method and torque performance
• Whether the application was designed for Heavy Hex hardware

There has also been ongoing discussion within ASTM committees about restricting certain undersized head styles due to performance concerns, although they are still permitted under the specification.

Summary

A325 bolts were designed primarily as Heavy Hex structural bolts, and that remains the industry standard. While other head styles can sometimes be certified to ASTM F3125 requirements, they must be marked with an “S” designation (A325S, A490S, or 144S), and their performance may differ from traditional Heavy Hex bolts depending on the application.