Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
ASME B18.2.3.7M-1979 (2001) — Metric Heavy Hex Structural Bolts
🔩 ASME B18.2.3.7M-1979 (2001) is the dedicated American National Standard for metric heavy hex structural bolts, providing complete general and dimensional data for this product. Developed by the ANSI B18 Committee on the standardization of bolts, screws, nuts, rivets, and similar fasteners, the Standard was originally approved in 1979 and reaffirmed without change in 1995 and again in 2001. Unlike its inch-series counterparts, all dimensions in this Standard are expressed in millimeters (mm), making it the reference of choice for metric structural bolting in steel construction.
💡 Heavy hex structural bolts differ from ordinary heavy hex bolts in head proportions: their larger heads provide a wider bearing area that supports the high preload demanded by structural connections — the backbone of steel roofs, bridges, and heavy equipment foundations.
1. Scope and General Framework 📌
This Standard covers the complete general and dimensional data for metric heavy hex structural bolts recognized as “American National Standard.” Three points deserve emphasis:
- The inclusion of dimensional data is not intended to imply that all sizes, in conjunction with the various options described, are stock production items; purchasers should consult manufacturers regarding lists of stock production heavy hex structural bolts.
- Heavy hex structural bolts purchased for U.S. Government use must conform to this Standard and additionally to the requirements of Appendix III, which establishes standard items and a part-numbering system for government application.
- All dimensions are in millimeters unless otherwise stated; symbols specifying geometric characteristics follow ANSI Y14.5-1973, Dimensioning and Tolerancing.
⚠️ For government procurement, Appendix III and its Table 5 (item selection) must be used for ordering with the specified part-numbering system. Civil purchasers are not bound by Appendix III but are encouraged to follow the same standardized numbering for traceability.
2. Coordination With ISO Standards 🌐
The heavy hex structural bolts in this Standard have been coordinated, to the extent possible, with a draft ISO proposed standard. There are to be two ISO standards for heavy hex structural bolts, differing only in thread length; this ANSI Standard is essentially identical to the proposed ISO standard for bolts with the shorter thread lengths. Letter symbols designating dimensional characteristics follow ISO usage, except capitals are used for data-processing convenience instead of the lowercase letters used in ISO standards.
The dimensional differences between the ANSI standard and the ISO proposal are few, relatively minor, and none affect the functional interchangeability of bolts manufactured to the requirements of either — a practical benefit for international projects.
| Aspect | ASME B18.2.3.7M | ISO Proposal | Remark |
|---|---|---|---|
| Units | Millimeters (mm) | Millimeters (mm) | Both metric |
| Thread length | Shorter thread lengths | Two versions (long/short) | Equivalent to short version |
| Symbols | Capital letters | Lowercase letters | Data-processing convenience |
| Interchangeability | Fully interchangeable | Differences do not affect function | |
| Tolerancing basis | ANSI Y14.5-1973 | ISO tolerance system | Different symbol systems |
3. Head Configuration and Key Dimensions 🔧
The Standard specifies the bolt head as follows:
- Top of head: full form and chamfered or rounded; the diameter of the chamfer circle or the start of rounding shall equal the maximum width across flats within a tolerance of minus 15%.
- Head height: the distance measured parallel to the bolt axis from the top of the head to the plane of the bearing surface.
- Width across flats: the heavy hex head has a larger width across flats than a regular hex head, enabling higher tightening torques.
- Bearing surface: flat and perpendicular to the axis so that preload is distributed uniformly across the joint.
🚨 An undersized chamfer (below maximum across-flats minus 15%) prevents the wrench from seating properly; an oversized chamfer can weaken the head. Always verify the chamfer with standard gages rather than by visual inspection.
4. Threads and Tolerances 📏
Because the thread carries the structural load, the Standard requires:
- Thread lengths per the dimension tables, with complete, burr-free threads free of damage.
- Thread manufacturing and acceptance tolerances per the applicable metric screw-thread standards (e.g., ASME B1.13M).
- Sufficient unthreaded body length so that adequate grip is maintained when all threads extend through the joint.
5. Engineering Practice Essentials 🛠️
- 🔍 Selection workflow: establish design load and required preload → select nominal diameter and length from the dimension tables → match heavy hex nuts and washers → order per Appendix III numbering where applicable.
- 🔧 Preload control: structural bolts demand high, controlled preload; use the torque method, turn-of-nut method, or direct tension indicators on site, and avoid both undertightening and overtightening.
- 📊 Anti-loosening: reliance is primarily on adequate preload; add lock nuts or witness marks where vibration or thermal cycling is severe, and retorque periodically.
- 📋 Procurement and acceptance: government items must be selected per Appendix III; for general projects, request material certificates and dimensional inspection reports, and apply sampling inspection per the ASME B18.18 series.
6. Frequently Asked Questions ❓
❓ Q: What is the difference between heavy hex structural bolts and regular hex bolts?
A: Heavy hex structural bolts have larger across-flats dimensions and greater head height, so they withstand higher tightening torques and bearing pressures; they are used where high preload matters, whereas regular hex bolts suit general machinery assembly.
A: Heavy hex structural bolts have larger across-flats dimensions and greater head height, so they withstand higher tightening torques and bearing pressures; they are used where high preload matters, whereas regular hex bolts suit general machinery assembly.
❓ Q: Are bolts to this Standard interchangeable with ISO bolts?
A: Yes. The Standard states that dimensional differences from the ISO proposal are minor and do not affect functional interchangeability, but matching nuts and washers should also come from the same system.
A: Yes. The Standard states that dimensional differences from the ISO proposal are minor and do not affect functional interchangeability, but matching nuts and washers should also come from the same system.
❓ Q: Why are not all listed sizes available from stock?
A: The Standard lists dimensions for complete design reference, not as a promise of stock availability. Confirm the manufacturer’s stock list before ordering and arrange special production when necessary.
A: The Standard lists dimensions for complete design reference, not as a promise of stock availability. Confirm the manufacturer’s stock list before ordering and arrange special production when necessary.
🔍 Q: What extra requirements apply to government purchases?
A: Government-procured bolts must comply with Appendix III and be ordered through the part-numbering system of its Table 5, enabling standardized, traceable procurement.
A: Government-procured bolts must comply with Appendix III and be ordered through the part-numbering system of its Table 5, enabling standardized, traceable procurement.
