Heavy Hex Head Bolts

Product Description

Heavy Hex Head Bolts are specialized fasteners designed for large-scale structural and industrial applications. They differ from standard hex bolts (Finish Hex) by having a larger head width across flats, which provides a greater bearing surface to distribute the massive clamping loads required in heavy engineering.

1. Dimensional Differences

The "Heavy" designation refers specifically to the head dimensions. A Heavy Hex bolt head is consistently 1/8" wider across the flats than a standard hex bolt of the same thread diameter.

• Greater Bearing Area: The larger head distributes the load over a wider area, reducing the pressure on the clamped material and preventing the bolt head from "sinking" into the steel.

• Wrenching Surface: The increased size allows for larger, more robust tools to apply the high torque values these bolts require.

2. Primary Industrial Standards

Heavy Hex bolts are most commonly specified under ASTM standards for structural and high-temperature service:

• ASTM A325 / A490: The gold standard for structural steel connections (buildings, bridges). A490 is the higher-strength version, similar to a Grade 8.

• ASTM A193 Grade B7: A chromium-molybdenum alloy (AISI 4140) used extensively in high-pressure piping, valves, and flanges.

• ASTM A307 Grade B: A low-carbon steel bolt used for heavy-duty anchoring and piping joints where high strength is less critical than the heavy head profile.

3. Material and Metallurgy

Because these bolts are used in critical infrastructure, their chemistry and heat treatment are tightly controlled.

4. Technical Comparison: Heavy Hex vs. Finished Hex

5. Selection and Installation Notes

• Washer Requirements: Due to the high clamp loads, Heavy Hex bolts often require hardened circular washers (ASTM F436) to ensure the bolt doesn't gall the structural steel during tightening.

• Thermal Expansion: In piping applications (A193 B7), the bolt is often paired with Heavy Hex Nuts (ASTM A194 Grade 2H). Using a matching "Heavy" nut is critical because a standard nut may not be thick enough to handle the thread stripping loads generated by the bolt.

• Coatings: They are frequently found with Hot Dip Galvanized (HDG) or Zinc Flake coatings for atmospheric corrosion resistance. Note that HDG threads are over-tapped on the nut to account for the thickness of the zinc layer.