Walk through any refinery, power station, or process plant and the fastener you'll see most is not a hex bolt — it's the stud bolt: a fully threaded stud with a heavy hex nut on each end, clamping flange joints together. The dimensional standard behind this workhorse is ASME B18.31.2, and understanding what it covers (and what it doesn't) makes flange bolting RFQs far easier to specify and quote.
What ASME B18.31.2 Covers
ASME B18.31.2 — Continuous Thread Stud, Double-End Stud, and Flange Bolting Stud (Stud Bolt) (Inch Series) — provides the complete dimensional and general data for three product configurations:
| Configuration | Description | Typical Use |
|---|---|---|
| Continuous thread stud | Threaded over its complete length | General clamping, threaded rod applications |
| Double-end stud | Threaded both ends — clamping type (identical ends) or tap-end type (unequal thread lengths) | Nut-each-end joints; installation into tapped holes |
| Flange bolting stud (stud bolt) | Continuous thread stud used with a nut on each end | Pipe flange joints per ASME B16.5 |
The standard defines lengths and tolerances, thread requirements per ASME B1.1 (UN/UNR form), and end configurations — the current edition requires pointed (chamfered) ends on studs, which aids nut starting in the field.
Dimensions Only — Material Comes From ASTM
This is the point that trips up newcomers: B18.31.2 is a dimensional standard, not a material grade. A stud bolt callout is only complete when paired with an ASTM bolting specification. The common pairings in flange work:
- ASTM A193 B7 studs with A194 2H nuts — the default for general refinery and pipeline service
- A193 B8/B8M (Class 1 or 2) with A194 8/8M nuts — stainless for corrosive or high-temperature service
- A320 L7 with A194 grade 4/7 nuts — low-temperature service
- A453 Gr 660 and other specialty grades for demanding thermal cycling
So a proper RFQ line reads something like: stud bolt to ASME B18.31.2, 1"-8UN × 140 mm, ASTM A193 B7, with 2 heavy hex nuts A194 2H, PTFE-coated. Standard + size + material + nuts + finish.
Why Stud Bolts Instead of Hex Bolts?
Flange joints favour stud bolts for practical reasons: the continuous thread allows full nut engagement at any grip length, studs can be tensioned or torqued evenly from either side, corroded assemblies can be cut off and replaced without damaging the flange, and one stud diameter/length covers a flange set without worrying about grip-length increments.
Threads, Lengths, and the 8UN Convention
Flange bolting follows a long-standing convention: UNC (coarse) thread up to 1" diameter, then 8UN (8 threads per inch) for larger diameters. Stud lengths for standard flange classes are driven by ASME B16.5 joint stack-ups — flange thickness, gasket, and nut heights — so length selection usually starts from the flange rating table, not from scratch.
International Context
Unlike DIN hex bolt standards with direct ISO successors, B18.31.2 has no directly comparable ISO equivalent — inch-series flange stud bolting remains a distinctly ASME/ASTM ecosystem. Metric plant projects typically use equivalent-intent products specified through EN 13445/EN 1515 flange bolting practice, but cross-specifying requires engineering review, not a lookup table.
The Bottom Line
ASME B18.31.2 gives the stud bolt its shape; ASTM gives it its strength. Quote and specify the two together — dimensional standard, thread series, ASTM grade, matched nuts, and coating — and flange bolting becomes one of the most predictable product families in the fastener catalogue.