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ASME B18.31.2-2014 — Continuous Thread Stud, Double-End Stud, and Flange Bolting Stud (Inch Series)
🔩 ASME B18.31.2-2014 is the complete American National Standard for inch-series studs, a revision of the 2008 edition issued on October 29, 2014. It covers three product families: continuous thread studs, double-end studs, and flange bolting studs (stud bolts). A stud is a headless rod threaded at both ends that joins components with nuts on either end or by threading one end into a tapped hole — and studs are the critical pressure-retaining fasteners of flange connections, pressure vessels, and chemical plant equipment. B18 Subcommittee 31 was formed in 1995 in response to shipbuilding industry demand, and its drafts were benchmarked against IFI and SAE stud standards.
💡 Unlike a bolt, a stud has no head — its full length is the rod. Flange service favors studs because nuts can be tightened from either end, and studs are rolled from high-strength bar stock for more uniform, more reliable mechanical properties.
1. Product Types and Configuration 📌
The Standard defines three stud types with explicit configuration and use:
- Continuous thread stud: threaded over its complete length; commonly supplied in coil and cut to length, suiting applications with many length variants.
- Double-end stud: threaded at both ends with a central unthreaded portion. Two subtypes — the clamping type (identical ends: same thread length and configuration each end, clamping two bodies with a nut on each end) and the tap-end type (installed into a tapped hole, usually with different threaded lengths per end; both regular Unified threads and interference-fit threads per ASME B1.12 are covered).
- Flange bolting stud (stud bolt): a threaded stud used primarily in flanged joints covered by ASME B16.5, made from ASTM A01 bolting materials.
⚠️ Double-end studs come in two body-diameter configurations: reduced-diameter body and full body. A reduced body gives more elastic elongation under preload and better resistance to thermal cycling and repeated tightening. Select by joint type and loading — the two are not freely interchangeable.
2. Dimensions and Length Series 🔧
The Standard tabulates complete dimensional data: thread size, body diameter, tap-end thread length, nut-end thread length, overall length, and tolerances for every type. Double-end studs cover diameters from 1/4 in. through 4 in. Threads conform to ASME B1.1 (Unified Inch Screw Threads, UN and UNR forms) and are gaged per ASME B1.3.
| Type | Configuration | Typical Service | Length Basis |
|---|---|---|---|
| Continuous thread stud | Full-length thread | Flange joints, general fixing | Overall length L |
| Double-end (clamping) | Identical threaded ends | Clamping two bodies with two nuts | L; equal thread each end |
| Double-end (tap-end 1.5D) | Tapped end + nut end | Engine blocks, bases, covers | L including tapped end |
| Flange bolting stud (stud bolt) | Full thread, B16.5 service | Flange-to-flange joints | L and thread length U per Fig. 1 |
The 2013 revision made pointed (chamfered) ends mandatory instead of optional, and unified the thread-length definition as the total thread length to the last scratch — minimum thread length plus five thread pitches. This simplifies thread gaging while preserving full suitability of the studs.
💡 Stud-bolt length is defined from the first full thread (see Fig. 1 for the L–U relationship): overall length L and effective end thread length U are explicitly related. Specify both clearly on purchase orders to avoid cutting errors that cause interference or insufficient thread engagement.
3. Materials, Mechanical Properties, and Finish ⚙️
The Standard requires stud materials and mechanical properties to conform to the referenced ASTM specifications; flange bolting studs are explicitly made from ASTM A01 bolting materials (e.g., A193) for high-temperature, high-pressure flange service. It also addresses finish (Sec. 13), workmanship (Sec. 14), straightness (Sec. 15, with further provision in Sec. 9), and inspection and quality assurance (Sec. 16).
🚨 In hot flanged joints, creep relaxation of the stud material is the principal cause of preload loss. When selecting high-temperature grades such as A193 B7 (Cr-Mo), B16, or B8M (stainless), verify allowable stress at design temperature per ASME B16.5/B31.3 and compensate for hot service in the tightening procedure.
4. Identification and Engineering Practice 🛠️
- 🔍 Selection workflow: define the joint (flange-to-flange or tapped/nut) → set thread size and class from B16.5 or other codes → choose clamping/tap-end or continuous thread → select body type (full or reduced) → compute overall length and thread lengths → pick material and property class.
- 🔧 Preload control: sealing and load capacity depend on preload. Use torque, angle, or tensioner methods; on multi-stud flanges (e.g., B16.5) tighten in a symmetric, incremental criss-cross sequence to avoid gasket crush and flange rotation.
- 📊 Thread acceptance: gage threads per the ASME B1.3 system; for interference-fit (B1.12) tap ends, control run-in torque and depth per specification.
- 📋 Length tolerances: Tables 5 and 6 give separate length tolerances for continuous/double-end studs and for flange bolting studs — treat them separately in procurement and inspection.
5. Frequently Asked Questions ❓
❓ Q: What is the difference between a stud bolt and a double-end stud?
A: A stud bolt is the full-thread flange stud made from ASTM A01 bolting material for ASME B16.5 flanged joints, with length set by flange design. A double-end stud is the general-purpose stud threaded at both ends, with equal (clamping) or unequal (tap-end) thread lengths. In flange engineering, the “stud bolt” is what is colloquially called the double-headed bolt.
A: A stud bolt is the full-thread flange stud made from ASTM A01 bolting material for ASME B16.5 flanged joints, with length set by flange design. A double-end stud is the general-purpose stud threaded at both ends, with equal (clamping) or unequal (tap-end) thread lengths. In flange engineering, the “stud bolt” is what is colloquially called the double-headed bolt.
❓ Q: How is the tapped-end thread length determined?
A: The Standard’s tap-end tables use approximately 1.5D engagement (1.5 times the thread diameter), the conventional practice for steel tapped materials. For brittle materials (e.g., cast iron) or interference-fit threads (B1.12), compute engagement length separately per the applicable rules.
A: The Standard’s tap-end tables use approximately 1.5D engagement (1.5 times the thread diameter), the conventional practice for steel tapped materials. For brittle materials (e.g., cast iron) or interference-fit threads (B1.12), compute engagement length separately per the applicable rules.
❓ Q: What is the advantage of a reduced-diameter body?
A: A reduced body gives the stud greater elastic elongation for the same preload — an “elastic reserve” that absorbs thermal expansion differences and vibration, slowing preload decay. The trade-off is a smaller net section, so strength must be checked on the reduced area.
A: A reduced body gives the stud greater elastic elongation for the same preload — an “elastic reserve” that absorbs thermal expansion differences and vibration, slowing preload decay. The trade-off is a smaller net section, so strength must be checked on the reduced area.
🔍 Q: Is there an ISO counterpart to B18.31.2?
A: The Standard states plainly that there is no comparable ISO standard. B18.31.2 is a uniquely U.S. stud product standard; the metric counterpart is ASME B18.31.1M, and inch studs should be specified to B18.31.2.
A: The Standard states plainly that there is no comparable ISO standard. B18.31.2 is a uniquely U.S. stud product standard; the metric counterpart is ASME B18.31.1M, and inch studs should be specified to B18.31.2.
