Types of Stud Bolts: Full Thread vs. Half Thread vs. Double-End Studs

Introduction

Stud bolts are among the most critical fasteners in industrial applications, particularly in flanged connections, pressure vessels, and valve assemblies. Unlike hex bolts, stud bolts are threaded on both ends (or fully) and are used with nuts on each side. However, not all stud bolts are the same. The three primary types—full thread, half thread, and double-end studs—serve distinct purposes based on load requirements, assembly methods, and standards compliance. This guide provides an engineering-grade comparison to help procurement engineers and project managers select the correct type for their application.

Definitions and Standards

Full Thread Stud Bolts

As the name implies, full thread stud bolts have threads running the entire length of the shank. They are typically manufactured to standards such as ASTM A193 (for high-temperature or high-pressure service) or ASTM A320 (for low-temperature service). Full thread studs are commonly used in applications where the entire length needs to engage with nuts or tapped holes, providing maximum clamping force distribution.

Half Thread Stud Bolts

Half thread stud bolts have threads only on a portion of the shank, usually at both ends, leaving an unthreaded middle section (the grip length). The threaded portions are typically equal in length, and the unthreaded portion is often referred to as the 'body' or 'shank'. These are defined in standards like ASME B18.31.2 and are common in flange bolting where the unthreaded section passes through the flange bolt holes.

Double-End Studs

Double-end studs (also called 'tap-end studs') have threads of different lengths on each end. One end (the 'tap end') is longer and designed to be screwed into a tapped hole, while the other end (the 'nut end') is shorter and receives a nut. They are often used in applications where one side is inaccessible for a nut, such as in valve bonnets or engine blocks. Standards include ASTM A193 and DIN 939.

Application Comparison

Advantages and Disadvantages

Full Thread Stud Bolts

Advantages: Maximum thread engagement; can be used with nuts on both ends or one end in a tapped hole; versatile length adjustment. Disadvantages: Stress concentration at the thread runout; can cause galling in flange bolt holes; less resistant to bending fatigue.

Half Thread Stud Bolts

Advantages: Unthreaded shank provides better shear strength and fatigue resistance; reduces stress concentration in flange bolt holes; easier alignment during assembly. Disadvantages: Requires precise grip length; not suitable for tapped holes; limited adjustability.

Double-End Studs

Advantages: Ideal for blind holes; allows disassembly without removing the stud from the tapped side; provides a fixed stud for repeated assembly. Disadvantages: Requires accurate tapping depth; risk of cross-threading; more complex installation.

Selection Criteria Based on Load and Assembly

Load Type

For tensile loads, all three types can be designed to carry the same load if the thread root area is sufficient. However, for shear loads, half thread studs with an unthreaded shank are superior because the shank diameter is typically larger than the thread root diameter. For fatigue loads, half thread studs reduce stress concentrations at the thread runout, making them preferable in cyclic service.

Assembly Method

If both sides are accessible for nuts, half thread studs are generally recommended for flanged joints per ASME PCC-1 guidelines. If one side is a tapped hole (e.g., in valve bodies), double-end studs are the correct choice. Full thread studs are used when the stud must be threaded into a tapped hole on one side and also receive a nut on the other, or when the stud length needs to be cut to fit.

Standards Compliance

For PED 2014/68/EU applications, stud bolts must meet EN 10204 3.1 certification. NACE MR0175/ISO 15156 compliance is required for sour service. Always verify that the stud type is permitted by the applicable code (e.g., ASME VIII, EN 13445).

Common Mistakes and How to Avoid Them

• Using full thread studs in flange bolting: This can cause thread damage in bolt holes and reduce fatigue life. Always use half thread studs for flanges unless specified otherwise.
• Incorrect grip length for half thread studs: The unthreaded portion must be long enough to span the flange thickness without threads entering the bolt hole. Measure flange thickness and specify stud length accordingly.
• Over-tightening double-end studs: The tap end can strip if over-torqued. Follow manufacturer torque values and use thread lubricant.
• Ignoring material compatibility: For high-temperature or corrosive environments, select appropriate materials (e.g., ASTM A193 B7 for high temp, B8 for corrosion resistance).
• Neglecting certification: Always request EN 10204 3.1 or 3.2 certificates for critical applications.

LOKRON Solution

At LOKRON, we manufacture all three types of stud bolts in accordance with ASTM A193, ASTM A320, and DIN standards. Our products are certified to PED 2014/68/EU, ISO 9001, and IATF 16949. We offer full traceability with EN 10204 3.1 documentation. Whether you need full thread, half thread, or double-end studs for flanges, pressure vessels, or valves, LOKRON provides precision fasteners tailored to your specifications. Contact us for a quote.

FAQ

1. What is the difference between a full thread and a half thread stud bolt?

A full thread stud bolt has threads along the entire length, while a half thread stud bolt has an unthreaded middle section. Half thread studs are preferred for flange bolting because the unthreaded shank reduces stress concentration.

2. When should I use a double-end stud?

Use double-end studs when one side of the joint is a tapped hole (e.g., in valve bonnets or engine blocks). The longer tap end screws into the hole, and the shorter nut end receives a nut.

3. Are full thread stud bolts suitable for flanges?

Generally no. Full thread studs can cause thread damage in flange bolt holes and have lower fatigue resistance. Half thread studs are recommended per ASME PCC-1.

4. What standards apply to stud bolts for pressure vessels?

Common standards include ASTM A193 (for high-temperature), ASTM A320 (low-temperature), and ASME B18.31.2. For European applications, EN 1515 and EN 10269 are relevant.

5. How do I specify the correct stud bolt length?

For half thread studs, measure the total joint thickness and add allowance for nut height and washer. The unthreaded portion should be at least equal to the joint thickness. For double-end studs, the tap end length must match the tapped hole depth.

Summary

Selecting the right stud bolt type—full thread, half thread, or double-end—is crucial for joint integrity, safety, and longevity. Full thread studs offer versatility but are not ideal for flanges. Half thread studs provide superior fatigue and shear performance for flanged connections. Double-end studs are essential for tapped hole applications. Always adhere to relevant standards and consider load type, assembly method, and environmental conditions. LOKRON supplies certified stud bolts to meet your exact requirements.