Stud Bolt vs Hex Bolt: Key Differences

Stud bolts and hex bolts are both used to fasten flanged joints, but they differ in design, load distribution, and application. A stud bolt is a headless threaded rod secured by two hex nuts. A hex bolt has a forged hexagonal head on one end and a single nut on the other. ASME B16.5 mandates stud bolts for pressure piping flanges; hex bolts are limited to low-pressure, non-critical applications.

Feature	Stud Bolt	Hex Bolt
Head	None (headless)	Hexagonal forged head
Nuts	2 (one per end)	1 (opposite end from head)
Thread	Fully threaded (continuous or double-end)	Partially threaded shank + threaded end
Load distribution	Symmetric (equal from both ends	Asymmetric) head side vs nut side
Tensile area	Full rod cross-section under load	Reduced at thread root on nut end
Gasket compression	Uniform across flange face	Slightly uneven
Tightening	Both nuts torqued	Head held, nut torqued
Standard	ASME B16.5, ASME B31.3	ASME B18.2.1
Material spec	ASTM A193 (studs), A194 (nuts)	ASTM A307, A325, A490
Temperature range	Cryogenic to 550+ degC	Generally ambient to 200 degC
Cost	Higher (rod + 2 nuts)	Lower (single forging + 1 nut)

Why Stud Bolts Are Standard for Flanges

ASME B16.5 and ASME B31.3 require stud bolts for flanged connections in process piping. The reasons are structural:

1. Symmetric loading. Two nuts tightened from opposite ends produce balanced tension along the bolt axis. This is critical for compressing gaskets (SWG, RTJ, or soft gaskets) evenly.

2. Full engagement. Both ends thread into heavy hex nuts, giving maximum thread engagement and load-bearing capacity. A hex bolt head relies on the bearing surface under the head, which is smaller.

3. High-temperature performance. Stud bolt materials (A193 B7, B8, B16) are specifically designed for elevated-temperature service with controlled creep and relaxation properties. Standard hex bolt grades (A307, A325) are not rated for these conditions.

4. Controlled torque. Stud bolts allow precision torque application from both sides, which is required for achieving the target gasket stress per ASME PCC-1.

When Hex Bolts Are Acceptable

Hex bolts are used in low-pressure, low-temperature, non-critical services:

Application	Fastener	Reason
ASME B16.5 flanged joints (all classes)	Stud bolt	Required by code
Structural steel connections	Hex bolt (A325/A490)	Standard structural practice
ASME B16.1 cast iron flanges, Class 125/250	Hex bolt (A307 Grade B)	Low-pressure, no gasket stress concern
Equipment mounting, brackets, supports	Hex bolt	Non-pressure application
Pipeline pig launchers/receivers (body bolting)	Stud bolt	Pressure boundary

Common Mistake

Substituting hex bolts for stud bolts on ASME flanges. Even at Class 150, the code requires stud bolts with A194 heavy hex nuts. Hex bolts with standard nuts do not provide the symmetric load or material properties needed for a reliable gasket seal.

Identification

Stud bolts are marked with the ASTM grade on both the rod and the nuts. Hex bolts carry the grade marking on the head. In the field, a stud bolt is immediately recognizable by the absence of a head and the presence of a nut on each end. The hex nut dimensions for stud bolts are per ASME B18.2.2 (heavy hex series), which are larger than the standard hex nuts used with hex bolts.

Read the full guide to gasket selection