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ASME B18.5.2.3M-1990 — Round Head Square Neck Bolts With Large Head (Metric Series)
🔩 ASME B18.5.2.3M-1990 is the American National Standard for metric series round head square neck bolts with large head, approved by ANSI on October 25, 1990. Developed by Subcommittee 2 (square and hex bolts and nuts) of the ASME B18 Committee, its technical content is based on ISO 8677-1986, “Cup Head Square Neck Bolts with Large Head — Product Grade C,” modified in tolerances, thread, and material coverage. The bolt has a large rounded head and, immediately below the head, a square-section neck that seats into a square hole in the workpiece, preventing the bolt from rotating while the nut is tightened — making it ideal for soft wood and similar materials.
💡 The square neck is the anti-rotation core of this bolt: once it seats in a square hole in the wood or panel, the bolt cannot spin while you tighten the nut from the far side — one person, one side, reliable tightening. That is why square-neck bolts dominate timber structures, shelving, formwork, and temporary structures.
1. Scope and ISO Interchangeability 📌
Section 1 defines the scope: complete general and dimensional data for metric series round head square neck bolts with large head recognized as American National Standard. The Standard also notes that inclusion of dimensional data does not imply all sizes are stock items — consumers should consult suppliers about availability.
On interchangeability, the Standard states that bolts produced to it are functionally interchangeable with bolts conforming to ISO 8677-1986, “Cup Head Square Neck Bolts with Large Head — Product Grade C.” Differences from ISO 8677 include: minor variations in tolerances, use of thread tolerance class 6g for all property classes, recalculation (correction) of tabulated grip lengths, and inclusion of materials other than steel.
⚠️ Although functionally interchangeable with ISO 8677, the corrected grip lengths, the uniform 6g thread tolerance, and the broader material coverage mean you should always use this Standard’s tables for procurement and assembly — especially for grip-length checks. Do not substitute ISO table values directly.
2. Dimensioning and Geometric Tolerances 🔧
All dimensions are given in millimeters (mm) and apply before any coating unless otherwise specified. Symbols specifying geometric characteristics conform to ANSI Y14.5M-1982, Dimensioning and Tolerancing. The tables cover head diameter, head height, square-neck section dimensions and length, body diameter, thread length, overall length, and tolerances.
| Element | Standard Provision | Engineering Note |
|---|---|---|
| Head type | Large round (domed) head | Large bearing area for soft materials |
| Neck type | Square-section neck | Seats in square hole to prevent rotation |
| Thread tolerance | 6g for all property classes | More uniform than ISO 8677 |
| Units | Millimeters, pre-plate | Account for coating in dimensioning |
| Geometric tolerances | Per ANSI Y14.5M-1982 | Unified symbols |
| Materials | Steel and non-steel materials | Broader coverage than ISO |
💡 The square-neck dimensions and the hole must match precisely: a loose square hole lets the bolt spin; an undersized one stops the neck seating so the head cannot pull flat. The recommended clearance-hole series (normal series, Appendix II) is the engineering baseline for the mating hole.
3. Options, Terminology, and Clearance Holes 📏
Per para. 1.4, options are at the supplier’s discretion unless agreed otherwise between supplier and purchaser. Para. 1.5 refers terminology to ANSI B18.12, the Glossary of Terms for Mechanical Fasteners. Para. 1.6 specifies the recommended clearance holes for assemblies using these bolts — the normal series of Appendix II — for the mating workpieces.
Para. 1.7 defines the designation sequence: product name → ASME document number (followed by a dash) → thread size (bolt diameter) → thread pitch (preceded by X) → nominal length (preceded by X) → material and property class. A complete designation keeps the same size traceable and consistent across suppliers.
🚨 The round head transmits clamping and bearing load, and the square neck prevents rotation — but there is no socket or hex on the head for a wrench: preload is applied entirely from the nut side. If the neck does not fully seat in the square hole, tightening will spin the bolt, gouge the wood, and fail the joint. Always confirm the neck is seated before tightening.
4. Engineering Practice Essentials 🛠️
- 🔍 Selection workflow: identify the joined material (softwood, laminate, panels) and total thickness → choose bolt diameter and thread → verify grip length → confirm head and neck sizes → select material and finish.
- 🔧 Installation: machine a square hole (matching the neck) and clearance hole (normal series of Appendix II) in the workpiece, seat the neck, and tighten a washer-protected nut from the far side; use flat washers to spread pressure and prevent surface crush.
- 📊 Grip-length check: the tabulated grip lengths were recalculated in this Standard; verify the clamped thickness falls within the tabulated grip range so the threads engage fully.
- 📋 Finish: zinc plating or black oxide is common for wood service; outdoors and wet service call for corrosion-resistant coatings, and coated threads must still meet the 6g tolerance.
5. Frequently Asked Questions ❓
❓ Q: Why is the thread tolerance uniformly 6g?
A: It is one of the deliberate U.S. modifications of ISO 8677. A uniform 6g tolerance band keeps a consistent fit and interchangeability across property classes, simplifying inventory and assembly, without compromising functional interchange.
A: It is one of the deliberate U.S. modifications of ISO 8677. A uniform 6g tolerance band keeps a consistent fit and interchangeability across property classes, simplifying inventory and assembly, without compromising functional interchange.
❓ Q: Can these bolts directly replace ISO 8677 bolts?
A: They are functionally interchangeable, but tolerances, grip-length data, and material coverage differ. When substituting, recheck grip length and the mounting holes against this Standard’s tables before use.
A: They are functionally interchangeable, but tolerances, grip-length data, and material coverage differ. When substituting, recheck grip length and the mounting holes against this Standard’s tables before use.
❓ Q: What does the square neck do? What happens without it?
A: The neck seats in a square hole and prevents bolt rotation while the nut is tightened. If it is not seated or the hole is loose, the bolt spins during tightening, preload is lost, and the material may be damaged.
A: The neck seats in a square hole and prevents bolt rotation while the nut is tightened. If it is not seated or the hole is loose, the bolt spins during tightening, preload is lost, and the material may be damaged.
🔍 Q: What materials suit this bolt?
A: The Standard was prepared for soft wood and similar materials: the large round head spreads load so it will not sink into soft timber. It also suits laminated boards, particleboard, and plastic panels; for metal structures a conventional hex bolt is usually the better choice.
A: The Standard was prepared for soft wood and similar materials: the large round head spreads load so it will not sink into soft timber. It also suits laminated boards, particleboard, and plastic panels; for metal structures a conventional hex bolt is usually the better choice.
