What is a Stud Bolt? Types, Selection, Materials, and Standards of Stud-Bolt

What is a Stud Bolt?

In the world of industrial piping and mechanical assembly, a Stud Bolt is a specialized fastener designed for high-integrity joints. Unlike a standard machine bolt with a fixed head, a Stud Bolt is a threaded rod that accepts a nut on both ends. This symmetrical design is critical for flange connections because it allows for uniform tensioning and easier removal during maintenance. In a typical flange assembly, the Stud Bolt passes through the bolt holes of two mating flanges, secured by heavy hex nuts to compress a gasket and create a leak-proof seal.

The engineering advantage of using a Stud Bolt over a headed bolt lies in its load distribution. Because the Stud Bolt is essentially a constant-diameter rod, it eliminates the stress concentration points typically found at the junction of a bolt head and its shank. For engineers following ASME B16.5 standards, the Stud Bolt is the preferred choice for all pressure-rated piping systems.

Primary Types of Stud Bolt Designs

Understanding the specific geometry of a Stud Bolt is essential for proper procurement and installation. There are three primary configurations used in 2026 industrial projects:

• 1. Continuous Thread Stud Bolt: This is the most common type used for flange bolting. It features threads running the entire length of the rod. It is versatile because it can be cut to custom lengths in the field while maintaining full functionality.
• 2. Tap-End Stud Bolt: Designed with a short threaded end intended to be permanently screwed into a tapped hole in a piece of equipment (like a pump casing), while the longer threaded end accepts a nut.
• 3. Double-End Stud Bolt: Features threads on both ends with a smooth, unthreaded shank in the center. These are often used in automotive or specific mechanical assemblies where the central shank acts as a dowel or guide.

Types of Stud Bolt Threads and Pitch Profiles

The thread profile of a Stud Bolt determines its resistance to vibration and its ease of assembly. Most industrial fasteners utilize the Unified Inch Screw Thread system. For a Stud Bolt with a diameter of 1 inch or smaller, the UNC (Coarse) thread series is standard, providing deep threads that are resistant to stripping.

However, for a Stud Bolt larger than 1 inch in diameter, the 8-UN (8-Thread Series) is the engineering requirement per ASME B1.1. This ensures that regardless of the bolt diameter, the thread pitch remains constant at 8 threads per inch. This consistency is vital for high-pressure applications where finer threads allow for higher tensioning and better torque-to-load conversion.

Critical Stud Bolt Standards (ASME & ASTM)

Standardization is the backbone of fastener safety. A Stud Bolt is governed by two primary organizations: ASME (for dimensions) and ASTM (for material properties).

Engineering Stud Bolt Materials for Critical Service

Selecting the correct metallurgical composition for a Stud Bolt is a balance between tensile strength, ductility, and corrosion resistance. The most prevalent specification in the oil, gas, and power industries is ASTM A193. This standard covers alloy and stainless steel bolting materials for high-temperature or high-pressure service. Within this standard, Grade B7—a chromium-molybdenum steel—is the workhorse for temperatures ranging from -20°F to 800°F.

For corrosive or high-temperature environments, engineers often specify ASTM A193 Grade B8 (SS304) or B8M (SS316). It is critical to note that "Class 2" Stud Bolt variants are strain-hardened to achieve significantly higher yield strengths than their "Class 1" counterparts. When working with sub-zero temperatures, such as in LNG processing, ASTM A320 Grade L7 is mandatory, requiring Charpy V-Notch impact testing to ensure the Stud Bolt does not undergo brittle fracture at low temperatures.

Technical Selection of Stud Bolt Materials

The selection of a Stud Bolt must be paired correctly with its nut. According to ASTM A194 standards, the nut should generally have a higher proof load than the bolt to ensure that if a failure occurs, the bolt threads strip rather than the nut fracturing, which is safer for pressure containment.

Stud Bolt Ends and Length Measurement

Proper length calculation is the difference between a secure joint and a safety violation. For a Stud Bolt used in flange connections, ASME B16.5 defines the length (L) as the distance from first thread to first thread, excluding the chamfered points. Common end finishes include Flat, Chamfered (most common), Round, or Conical. The chamfer facilitates easy nut starting, especially when using pneumatic torque wrenches in tight spaces.

Determining Stud Bolt Size and Dimension

The nominal diameter of a Stud Bolt is determined by the pressure class (150# to 2500#) and the pipe size. In 2026, many EPC firms utilize automated bolting charts to ensure compliance, but the manual formula remains:
Length = (2 × Nut Thickness) + (2 × Gasket Thickness) + (2 × Flange Thickness) + (Minimum 3 Threads).

Expert Insights: Lessons from 20 years in the field

References & Standards

For precise engineering calculations and compliance, refer to the following official authorities:

• ASME B16.5: Flange & Bolting Dimensions
• ASTM A193: High-Temp Bolting Specs
• ASTM A194: Heavy Hex Nut Standards
• ISO 898-1: Carbon Steel Fastener Properties

Frequently Asked Questions