Double Ended Stud vs Full-Threaded Stud

Buyers who ask for a stud bolt do not always mean the same part: the term covers the double-end stud with a plain shank between two threaded ends and is often confused with the full-threaded stud that runs thread from end to end. The two differ in structure, function and ASME B18.31 designation, and the wrong choice undermines the clamping that keeps flange gaskets sealed.

Core Definitions and Structural Features

A stud bolt, also known as a double-end stud, is a cylindrical fastener machined with threads on both ends and a plain shank in the middle. The thread length at each end divides it into two structural forms.

• Equal thread lengths (Type A) — both ends carry exactly the same thread length over a plain or reduced shank. Both ends take nuts, for example when two flanges with through holes are drawn together or when a joint is tightened from both sides, so neither end acts as a tap end.
• Unequal thread lengths (Type B) — one end has a shorter thread (the tap end) and the other a longer thread (the nut end), with a plain shank between them. This is the common form: the tap end screws into a tapped hole in the equipment body while the nut end passes through the component and takes the nut.

The division of labor in the Type B design is deliberate.

• Tap end (shorter thread) — screws into the internal thread of the equipment base and seats the stud.
• Plain shank — locates and guides the joint and carries shear loads.
• Nut end (longer thread) — takes the nut that clamps the connected components.

Tap-end stud bolt vs double-end stud bolt

Stud Bolts vs Full-Threaded Studs

The two fasteners look similar but serve different duties under the same governing standards; the comparison below sets out the difference.

Type A and Type B stud bolts compared with full-threaded studs under ASME B18.31.1M / ASME B18.31.2.

Double-end stud vs full-threaded stud

Main Parameters in the ASME System

Under ASME/ANSI practice, stud bolt parameters are defined along three dimensions.

• Designation code — the Unified Inch Screw Thread series (UN/UNC/UNF). Example: 1/2-13 UNC-2A means 1/2 in nominal diameter, 13 threads per inch, coarse series.
• Material and performance grade — American fasteners are specified through ASTM material standards rather than numeric property classes. Common grades: ASTM A193 Grade B7 chromium-molybdenum alloy steel for high-temperature, high-pressure service such as petrochemical pipe flanges, the most widely used high-strength grade; ASTM A193 Grade B8/B8M in 304/316 stainless steel for chemical and food processing duty; and ASTM A320 Grade L7 alloy steel for low-temperature service.
• Tap end thread length — for the unequal Type B, ASME B18.31 sets the tap end thread (the end screwed into the equipment body) at about 1.5 times the nominal diameter.

Advantages and Typical Applications

Three advantages keep stud bolts irreplaceable in heavy industry.

• Protects equipment threads — in equipment opened often for maintenance, wear and thread stripping are confined to the replaceable studs and nuts, sparing the expensive internal threads of the body.
• Guidance and positioning — heavy end covers or valves can be hung on studs first screwed into the body as guide pins, which eases alignment and protects sealing faces from scratches.
• Adapts to extreme grip lengths — when the clamped stack is longer than a standard bolt allows, the stud anchors into the base material and the nut applies the load from outside.

Stud bolts cover a wide range of high-reliability duty.

• Petroleum and chemical — high-temperature, high-pressure pipe flanges, heat exchanger channel heads and valve bonnet connections, mostly in ASTM A193 B7.
• Power generation — sealing and fastening on thermal boilers, steam turbines and nuclear pressure vessels.
• Marine engineering — marine engine cylinder heads and deck machinery bases, with anticorrosion coatings or B8M stainless steel.
• Heavy machinery and construction — anchor elements for large gearbox bases and crusher liners in mining equipment.

Frequently Asked Questions

What is a stud bolt also known as?

A stud bolt is also called a double-end stud: a cylindrical fastener with threads on both ends and a plain shank in the middle.

When should I choose unequal thread lengths?

Use the Type B form when one end screws into a tapped hole in the equipment body and the other end takes a nut; ASME B18.31 sets the tap end thread at about 1.5 times the nominal diameter. Choose equal lengths when both ends take nuts.

Which material grade suits high-temperature flange service?

ASTM A193 Grade B7 chromium-molybdenum steel covers most high-temperature, high-pressure flanged joints; B8/B8M stainless suits corrosive duty, and A320 Grade L7 is for low-temperature service.

Can a full-threaded stud replace a stud bolt?

Only where the joint does not rely on a plain shank for guidance and shear. Full-threaded studs suit adjustment and load spreading; stud bolts protect equipment threads and position heavy covers.

What do UN, UNC and UNF mean in a designation?

They are Unified Inch Screw Thread series: UN is the basic series, UNC the coarse-pitch series and UNF the fine-pitch series. In 1/2-13 UNC-2A, 13 threads per inch is the coarse pitch for a 1/2 in nominal diameter.