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ASME B18.2.3.1M-1999 — Metric Hex Cap Screws
🔩 ASME B18.2.3.1M-1999 is the American National Standard for metric series hex cap screws, providing complete general and dimensional data for the product. It is the revised edition of the 1979 original (ANSI B18.2.3.1M-1979, reaffirmed 1989 and 1995); the 1998 edition was issued in December 1999 with an automatic addenda subscription service, with the next edition scheduled for 2004. As the opening document of the B18.2.3 metric fastener family, it is the foundation for procurement and acceptance of metric hex cap screws in North America, harmonized to the maximum extent possible with ISO 4014 / ISO 4017.
💡 ISO harmonization is the biggest strength of this Standard: for sizes M5 through M14, functional characteristics — nominal diameter and pitch, body diameter, widths across flats and corners, nominal head height, nominal length and thread length, and thread dimensions — agree with ISO 4014/4017, enabling “American standard, internationally interchangeable.”
1. Scope and ISO Harmonization 📌
The Standard covers metric hex cap screws recognized as American National Standard, intended for general machine assembly and maintenance. Inclusion of dimensional data does not imply that all sizes are stock production items — consumers should consult suppliers about lists of stock production hex cap screws.
Harmonization with ISO standards:
- Harmonized sizes: M5 through M14 are harmonized with ISO 4014 or, for short screws threaded full length, ISO 4017; sizes M16 through M24 (2D through 150 mm) and M30 through M64 (2D and longer) are also in agreement.
- Harmonized characteristics: nominal diameters and thread pitches, body diameters, widths across flats and corners, nominal head heights, nominal lengths and thread lengths, and thread dimensions.
- Differences: dimensional differences from ISO are few, relatively minor, and none affect functional interchangeability of screws made to either standard.
- Exclusive sizes: this Standard omits ISO 4014/4017 sizes M1.6–M4, M18, M22, M33, M39, M45, M52, and M60; it includes M10×1.5 with 15 mm width across flats, M72×6, M80×6, M90×6, and M100×6, which are not in ISO 4014/4017.
⚠️ Sizes not listed in B18.2.3.1M (e.g., M18, M22) still exist — they are defined in ISO 4014/4017. When engineering or procuring, choose the size according to the standard system in use (ASME or ISO); do not conclude that a missing table entry means the product does not exist.
2. Principal Dimensions and Tolerances 🔧
The tables give key dimensions for each size:
| Size | Thread Pitch | Width Across Flats (Nominal) | Nominal Head Height | Notes |
|---|---|---|---|---|
| M5×0.8 | 0.8 mm | 8 mm | 3.5 mm | Common small size |
| M6×1 | 1.0 mm | 10 mm | 4 mm | General assembly |
| M8×1.25 | 1.25 mm | 13 mm | 5.3 mm | Workhorse machine size |
| M10×1.5 | 1.5 mm | 15 mm (or 16 mm) | 6.4 mm | 15 mm across flats is a U.S. special |
| M12×1.75 | 1.75 mm | 18 mm | 7.5 mm | Heavy-duty assembly |
| M16×2 | 2.0 mm | 24 mm | 10 mm | Large standard size |
| M20×2.5 | 2.5 mm | 30 mm | 12.5 mm | Large equipment connections |
| M24×3 | 3.0 mm | 36 mm | 15 mm | Heavy structures |
3. Key Revisions and Quality Assurance 📊
The 1998 revision introduced a set of technically significant changes (summarized in the Foreword):
- Coating convention: it is now clear that all dimensions apply before any coating has been applied (para. 1.4.1).
- Washer face diameter: to be inspected 0.1 mm above the bearing plane rather than at midthickness as previously.
- Length of point redefined: para. 2.5 and Table 5 define a new method for checking an increased maximum point length.
- Screw straightness: formerly checked with a sleeve gage, now checked with a rail gage (para. 2.6).
- Body-to-thread position: the sleeve-gage check now applies only to cut threads (para. 2.7.6), with new tolerance-zone values in Table 1.
- Grip and body length: revised to correspond to ISO, making a series of standard diameter/length combinations fully threaded instead of partially threaded (e.g., M8×35, M10×40, M12×45, M16×55, M20×65, M24×80, and many others).
- Full-thread grip length: increased from about 2.5 to about 3 times the pitch (see Table 6).
- Root form: the former controlled root radius in the runout section for Property Class 10.9 and higher was replaced by a rounded root contour requirement (para. 2.7.5).
- Quality assurance: quality assurance and dimensional conformance are specified in paras. 2.13 and 2.14, respectively; Appendix IV gives a detailed comparison with ISO.
🚨 The revision changed a number of “partially threaded” combinations to “fully threaded” (e.g., M8×35, M10×40, M12×45). When reusing old drawings, recheck these diameter/length combinations one by one — purchasing to the old edition will deliver the wrong product.
4. Engineering Practice Essentials 🛠️
- 🔍 Standard-system selection: use B18.2.3.1M for North-American-bound or ASME-designed products; for international or OEM work, note the agreement with ISO 4014/4017, but flag diverging sizes such as M10×1.5 with 15 mm width across flats.
- 🔧 Pre-coating dimensions: all dimensions apply before coating; plating such as zinc changes finished dimensions, so agree post-plating tolerances with the supplier.
- 📋 Grade marking: property class markings (e.g., 8.8, 10.9, 12.9) appear on top of the head; verify the marking against the specified material per the applicable standard.
- 📐 Straightness and root checks: use the rail gage for straightness per the new edition; for Property Class 10.9 and higher, observe the rounded root contour requirement.
- 📝 Addenda subscription: the 1998 edition ships with an automatic addenda service; procurement and quality files should track the latest addenda.
✅ Practice tip: use B18.2.3.1M together with B18.2.8 (clearance holes) and the B18.2.4 series (metric nuts); when selecting for international projects, check functional characteristics item by item against ISO 4014/4017 to ensure interchangeability.
5. Frequently Asked Questions ❓
❓ Q: What differs between B18.2.3.1M and ISO 4014?
A: This Standard omits ISO 4014/4017 sizes M1.6–M4, M18, M22, M33, M39, M45, M52, and M60, and includes M10×1.5 (15 mm across flats) and M72×6 through M100×6, which are not in the ISO documents; all other principal functional characteristics agree.
A: This Standard omits ISO 4014/4017 sizes M1.6–M4, M18, M22, M33, M39, M45, M52, and M60, and includes M10×1.5 (15 mm across flats) and M72×6 through M100×6, which are not in the ISO documents; all other principal functional characteristics agree.
❓ Q: Do dimensions apply before or after coating?
A: Per para. 1.4.1, all dimensions apply before any coating has been applied. Finished dimensions of coated product are agreed between supplier and purchaser.
A: Per para. 1.4.1, all dimensions apply before any coating has been applied. Finished dimensions of coated product are agreed between supplier and purchaser.
❓ Q: Why did the washer face inspection position change?
A: The washer face diameter is now inspected 0.1 mm above the bearing plane rather than at midthickness, better reflecting the effective bearing surface and reducing measurement-position-related misjudgments.
A: The washer face diameter is now inspected 0.1 mm above the bearing plane rather than at midthickness, better reflecting the effective bearing surface and reducing measurement-position-related misjudgments.
❓ Q: Why do M10 screws come with a 15 mm width across flats?
A: M10 bolts and screws commonly use a 15 mm width across flats outside Europe (including North America); it is not an ISO standard size but a common U.S.-market dimension, so B18.2.3.1M includes it.
A: M10 bolts and screws commonly use a 15 mm width across flats outside Europe (including North America); it is not an ISO standard size but a common U.S.-market dimension, so B18.2.3.1M includes it.
