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ASME B18.2.3.5M-1979 (2016) — Metric Hex Bolts
🔩 ASME B18.2.3.5M-1979 (2016) is the American National Standard for metric hex bolts, providing complete general and dimensional data for the product recognized as “American National Standard.” Approved by ANSI on April 26, 1979, it has been reaffirmed several times, most recently by ASME in 2016, and was adopted by the U.S. Department of Defense and Federal agencies for government procurement (see the acceptance notice and Appendix III). It is the standalone “bolt” document of the B18.2.3 metric fastener series — bolts mate with nuts and usually have an unthreaded body — and is coordinated to the extent possible with ISO 4016.
💡 ISO 4016 harmonization is this Standard’s core value: in sizes M5 through M36, nominal diameters and thread pitches, body diameters, widths across flats, head heights, thread lengths, thread dimensions, and nominal lengths all agree with ISO 4016, guaranteeing international interchangeability.
1. Scope and ISO Harmonization 📌
The Standard covers metric hex bolts recognized as “American National Standard,” intended for general industrial assembly, structural connections, and maintenance. As elsewhere, inclusion of dimensional data does not imply all sizes are stock production items — consumers should consult manufacturers about lists of stock production hex bolts.
Harmonization with ISO 4016:
- Harmonized range: hex bolts in sizes M5 through M36 are coordinated with ISO 4016; dimensional differences are few, relatively minor, and none affect the functional interchangeability of bolts made to either standard.
- Harmonized characteristics: diameters and thread pitches, body diameters, widths across flats, head heights, thread lengths, thread dimensions, and nominal lengths.
- Width-across-flats series: at its May 1977 meeting in Varna, ISO/TC2 studied analyses of design considerations governing the best series of widths across flats for hex bolts, screws, and nuts, with the primary technical objective of a logical ratio between the underhead (nut) bearing surface area and the tensile stress area of the thread; Table 1 lists the ISO-selected sizes.
- M10 with 15 mm width across flats: M10 bolts with 15 mm across flats are produced and used in the U.S.A. and many other countries; although not an ISO size, B18.2.3.5M includes it.
⚠️ Two widths across flats coexist for M10 bolts: 16 mm is the ISO standard size, while 15 mm is the common U.S.-market size. Design and procurement must specify the width across flats explicitly, or the wrong wrench and nut will be matched.
2. Principal Dimensions and Tolerances 🔧
Table 1 gives the dimensional data for metric hex bolts from M5×0.8 through M100×6. Typical sizes are summarized below:
| Size | Thread Pitch | Width Across Flats (Nominal) | Head Height (Nominal) | Typical Applications |
|---|---|---|---|---|
| M5×0.8 | 0.8 mm | 8 mm | 3.5 mm | Precision small-size assembly |
| M6×1 | 1.0 mm | 10 mm | 4 mm | General machine assembly |
| M8×1.25 | 1.25 mm | 13 mm | 5.3 mm | General-purpose mainstay |
| M10×1.5 | 1.5 mm | 15 mm or 16 mm | 6.4 mm | Automotive and general assembly |
| M12×1.75 | 1.75 mm | 18 mm | 7.5 mm | Medium-heavy assembly |
| M16×2 | 2.0 mm | 24 mm | 10 mm | Structural connections |
| M20×2.5 | 2.5 mm | 30 mm | 12.5 mm | Heavy-duty equipment |
| M24×3 | 3.0 mm | 36 mm | 15 mm | Heavy structures |
| M36×4 | 4.0 mm | 55 mm | 22.5 mm | Large equipment connections |
3. Manufacturing, Marking, and Government Procurement 📊
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; the top of the head shall be full form and chamfered or rounded.
- Grade marking: the property class and the manufacturer’s identification shall appear on top of the head.
- Material and finish codes: Appendix III defines material and finish codes (e.g., A — steel with cadmium plating per QQ-P-416, Type II, Class 3, 5.1 µm, Property Class 10.9; B — steel with zinc plating per QQ-Z-325, Type II, Class 3, 5.1 µm, Property Class 10.9) for government item selection.
- Government procurement: hex bolts purchased for Government use must conform to this Standard plus Appendix III, using Table 4 of Appendix III for item selection under the part numbering system.
🚨 Note the errata: ANSI B18.2.3.5M-1979 was corrected by an errata sheet — Note 18 on page 3 was changed from “B1.13” to “B1.13M,” and the plating thickness for material and finish codes A and B on page 10 was changed from 0.005 mm to 5.1 µm. Always reference the corrected text.
4. Engineering Practice Essentials 🛠️
- 🔍 Screw vs. bolt: metric hex bolts (B18.2.3.5M) usually have an unthreaded body and mate with nuts to carry clamping load; metric hex cap screws (B18.2.3.1M) may be fully threaded. Choose by the clamped-grip situation.
- 🔧 Confirm width across flats: for M10 bolts, confirm 15 mm or 16 mm across flats and match the socket and nut specifications accordingly.
- 📋 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 head height, width across flats and corners, and body diameter per Table 1; length tolerances per the corresponding table.
- 📝 Stock confirmation: very large sizes (M72–M100) are usually made to order; confirm lead times and quantities in advance.
✅ Practice tip: use B18.2.3.5M hex bolts with metric hex nuts (B18.2.4 series) and washers for general assembly; upgrade to heavy hex bolts (B18.2.3.6M) for structural or high-pressure service. Purchase orders should state the edition, width across flats, and property class.
5. Frequently Asked Questions ❓
❓ Q: What is the difference between B18.2.3.5M and B18.2.3.1M?
A: B18.2.3.5M covers metric hex bolts (usually with an unthreaded body, mated with nuts), coordinated with ISO 4016; B18.2.3.1M covers metric hex cap screws (which may be fully threaded and thread into the workpiece), coordinated with ISO 4014/4017.
A: B18.2.3.5M covers metric hex bolts (usually with an unthreaded body, mated with nuts), coordinated with ISO 4016; B18.2.3.1M covers metric hex cap screws (which may be fully threaded and thread into the workpiece), coordinated with ISO 4014/4017.
❓ Q: Why do M10 bolts come in two widths across flats?
A: 16 mm is the ISO standard size; 15 mm is widely produced and used in North America and many other countries. Though not an ISO size, B18.2.3.5M includes it to serve U.S.-market needs.
A: 16 mm is the ISO standard size; 15 mm is widely produced and used in North America and many other countries. Though not an ISO size, B18.2.3.5M includes it to serve U.S.-market needs.
❓ Q: What additional requirements apply to government purchases?
A: Government hex bolts must also conform to 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: Government hex bolts must also conform to 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.
❓ Q: Has this Standard been corrected?
A: Yes. An errata to ANSI B18.2.3.5M-1979 changed Note 18 from B1.13 to B1.13M and changed the plating thickness for codes A and B from 0.005 mm to 5.1 µm. Use the corrected text when referencing the standard.
A: Yes. An errata to ANSI B18.2.3.5M-1979 changed Note 18 from B1.13 to B1.13M and changed the plating thickness for codes A and B from 0.005 mm to 5.1 µm. Use the corrected text when referencing the standard.
