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ASME B18.2.3.6M-1979 (2001) — Metric Heavy Hex Bolts
🔩 ASME B18.2.3.6M-1979 (2001) is the American National Standard for metric heavy hex bolts, providing complete general and dimensional data for the product recognized as “American National Standard.” Approved by ANSI on April 26, 1979, it was reaffirmed in 1995 and again in 2001, and was adopted by the U.S. Department of Defense and Federal agencies for government procurement. Compared with regular hex bolts, heavy hex bolts use a larger width across flats and a larger bearing surface, allowing higher tightening torque and greater bearing pressure — the preferred choice for high-pressure flanges, heavy-duty structures, and critical connections.
💡 The design logic of heavy hex bolts is the same as for heavy hex screws: a larger bearing surface spreads the compressive stress during tightening, permitting higher preload and reducing the risk of crushing or distorting the clamped members.
1. Scope and ISO Harmonization 📌
The Standard covers metric heavy hex bolts recognized as “American National Standard.” As in companion documents, inclusion of dimensional data does not imply all sizes are stock production items — consumers should consult manufacturers about lists of stock production heavy hex bolts.
Harmonization with ISO standards:
- No ISO standard yet: ISO has not yet initiated development of a heavy hex bolt standard.
- Harmonized characteristics: nominal diameters and thread pitches, body diameters, widths across flats, head heights, thread lengths, thread dimensions, and nominal lengths agree with ISO standards for related hex screws and bolts.
- Symbol convention: letter symbols follow ISO usage, except capitals are used for data-processing convenience instead of the lowercase letters used in ISO standards.
- Units: unless stated otherwise, all dimensions are in millimeters; geometric characteristic symbols conform to ANSI Y14.5-1973.
⚠️ As with metric heavy hex screws (B18.2.3.3M), ISO has not yet developed a heavy hex bolt standard; however, all functional characteristics agree with related ISO hex products, so they can be used internationally. Note this Standard’s availability rules (next section) when purchasing.
2. Availability Rules and Principal Dimensions 🔧
This Standard features a distinctive availability clause:
- M12 through M24: standard only in lengths longer than 150 mm or 10 times the diameter, whichever is shorter; when shorter lengths of these sizes are ordered, heavy hex screws conforming to ANSI B18.2.3.3M are normally supplied.
- M30 and M36: standard in all lengths; however, at the manufacturer’s option, heavy hex screws may be substituted for any diameter-length combination.
Principal dimensional features (data in Table 1):
| Size | Width Across Flats (Nominal) | Head Height (Nominal) | Availability | Typical Applications |
|---|---|---|---|---|
| M12×1.75 | 21 mm | 7.5 mm | Long sizes only (>150 mm or 10D) | Long-shank heavy connections |
| M16×2 | 27 mm | 10 mm | Long sizes only | Large structures |
| M20×2.5 | 34 mm | 12.5 mm | Long sizes only | Heavy machinery |
| M24×3 | 41 mm | 15 mm | Long sizes only | Heavy-duty structures |
| M30×3.5 | 50 mm | 18.7 mm | All lengths | High-pressure flanges, large equipment |
| M36×4 | 60 mm | 22.5 mm | All lengths | Extra-heavy-duty service |
3. Manufacturing, Surface, and Marking Requirements 📊
The Standard sets manufacturing and marking requirements and includes government procurement provisions:
- Surface condition: bolts need not be finished on any surface except the threads.
- Top of head: the top of the head shall be full form and chamfered or rounded, with the top-of-head diameter per the applicable requirement.
- Grade marking: the property class and the manufacturer’s identification shall appear on top of the head.
- Material and finish codes: Appendix III defines the material and finish codes for government heavy hex bolts, with Table 4 used for item selection.
- Government procurement: heavy hex bolts purchased for Government use must conform to this Standard plus Appendix III.
🚨 Note the errata: the errata sheet for ANSI B18.2.4.6M-1979 (metric heavy hex nuts) shows the title-page government keywords changed to “Nuts, Heavy Hex – Metric” and the plating thickness for material and finish codes A and B changed from 0.005 mm to 5.1 µm. Such corrections remind us to check errata when referencing metric fastener standards, especially plating-thickness units.
4. Engineering Practice Essentials 🛠️
- 🔍 Short-shank demand → screws: short M12–M24 heavy hex bolts are normally supplied as heavy hex screws (B18.2.3.3M); plan procurement accordingly at the design stage.
- 🔧 Head matching: pair heavy hex bolts with heavy hex nuts (e.g., B18.2.4.6M) and heavy washers so bearing surfaces and torque transmission match.
- 📋 Government compliance: U.S. government and defense projects select items via the Appendix III material and finish codes; note the plating-thickness unit conversion (5.1 µm).
- 📐 Dimensional checks: check width across flats, head height, body diameter, and thread length per Table 1; threads are accepted per the applicable thread standards.
- 📝 Supply communication: state the required lengths and quantities clearly, confirming whether M12–M24 long-shank standard sizes apply or whether screw substitution will be used.
✅ Practice tip: when designing heavy bolted joints, check the availability clause of B18.2.3.6M first — use heavy hex screws for short-shank heavy loads and heavy hex bolts for long shanks; also match B18.2.8 clearance holes and heavy nuts.
5. Frequently Asked Questions ❓
❓ Q: What is the difference between heavy hex bolts and regular hex bolts?
A: Heavy hex bolts have a larger width across flats, head diameter, and bearing surface, enabling higher tightening torque and greater bearing pressure for high-pressure, heavy-duty, and critical joints; regular hex bolts (B18.2.3.5M) serve general assembly.
A: Heavy hex bolts have a larger width across flats, head diameter, and bearing surface, enabling higher tightening torque and greater bearing pressure for high-pressure, heavy-duty, and critical joints; regular hex bolts (B18.2.3.5M) serve general assembly.
❓ Q: Can short-length M12–M24 heavy hex bolts be purchased?
A: Per the availability clause, M12–M24 are standard only in lengths longer than 150 mm or 10 times the diameter (whichever is shorter); shorter lengths are normally supplied as heavy hex screws conforming to B18.2.3.3M.
A: Per the availability clause, M12–M24 are standard only in lengths longer than 150 mm or 10 times the diameter (whichever is shorter); shorter lengths are normally supplied as heavy hex screws conforming to B18.2.3.3M.
❓ Q: Does ISO have a heavy hex bolt standard?
A: As of this edition, ISO has not yet initiated one. However, this Standard’s functional characteristics agree with related ISO hex screws and bolts.
A: As of this edition, ISO has not yet initiated one. However, this Standard’s functional characteristics agree with related ISO hex screws and bolts.
❓ Q: What additional requirements apply to government heavy hex bolts?
A: They must conform to this Standard plus Appendix III, selecting standard items per Table 4; the material and finish codes (A/B, etc.) correspond to specific plating processes, thicknesses, and property classes.
A: They must conform to this Standard plus Appendix III, selecting standard items per Table 4; the material and finish codes (A/B, etc.) correspond to specific plating processes, thicknesses, and property classes.
