What You Will Find in This Guide
This is not a summary. This is an exhaustive, specification-level reference covering every major category of metric fastener standardized under the ANSI/ASME B18 series, coordinated with ISO international standards.
You will find:
- Complete dimensional tables for every major metric screw and bolt type
- Mechanical property classes with strength grade decoding
- Thread series and tolerance specifications
- Clearance hole sizing for close, normal, and loose fits
- Drill and counterbore dimensions for socket head cap screws
- Diameter-length combination matrices for proper procurement
- Identification symbol systems for head marking verification
- Designation conventions for unambiguous ordering
- Practical decision frameworks for fastener selection
Every dimension is in millimeters. Every specification is traceable to its governing standard.
ANSI-ISO Coordination: Why Your Metric Bolts Are (Mostly) Interchangeable
Before diving into specific fastener types, you need to understand the single most important fact about metric fasteners used in North America:
American National Standards for metric bolts, screws, and nuts have been coordinated to the maximum extent possible with comparable ISO Standards.
The dimensional differences between ANSI and ISO standards are few, relatively minor, and—critically—none will affect the functional interchangeability of bolts, screws, and nuts manufactured to the requirements of either system.
Here is what is in agreement between the ANSI and ISO standards:
- Diameters and thread pitches
- Body diameters
- Widths across flats (with one notable exception discussed below)
- Bearing surface diameters (except for metric hex bolts)
- Flange diameters (for metric hex flange screws)
- Head heights
- Thread lengths
- Thread dimensions
- Nominal lengths
The M10 Exception You Must Know
There is one persistent deviation between North American production and ISO standards that catches people regularly:
The M10 size for hex cap screws, formed hex screws, hex bolts, hex nuts (Styles 1 and 2), slotted hex nuts, hex jam nuts, and prevailing-torque hex nuts is currently produced in the United States with a width across flats of 15 mm. This width is not an ISO Standard. Unless M10 fasteners with 15 mm width across flats are specifically ordered, the M10 size with 16 mm width across flats will be furnished.
This matters for wrench clearance, assembly tooling, and interchangeability with imported components. If you are working with mixed-source M10 fasteners, verify the width across flats before assuming wrench compatibility.
The Complete ANSI Standards Map
Every metric fastener discussed in this guide traces to one of these governing standards:
| ANSI Standard | Fastener Type |
|---|---|
| ANSI B18.2.3.1M-1979 (R1995) | Metric Hex Cap Screws |
| ANSI B18.2.3.2M-1979 (R1995) | Metric Formed Hex Screws |
| ANSI B18.2.3.3M-1979 (R1995) | Metric Heavy Hex Screws |
| ANSI B18.2.3.4M-1984 | Metric Hex Flange Screws |
| ANSI B18.2.3.5M-1979 (R1995) | Metric Hex Bolts |
| ANSI B18.2.3.6M-1979 (R1989) | Metric Heavy Hex Bolts |
| ANSI B18.2.3.7M-1979 (R1989) | Metric Heavy Hex Structural Bolts |
| ANSI B18.2.3.8M-1981 (R1991) | Metric Hex Lag Screws |
| ANSI B18.2.3.9M-1984 | Metric Heavy Hex Flange Screws |
| ANSI B18.5.2.2M-1982 | Metric Round Head Square Neck Bolts |
| ANSI B18.3.1M-1986 | Socket Head Cap Screws (Metric Series) |
| ANSI B18.2.4.1M-1979 (R1995) | Metric Hex Nuts, Style 1 |
| ANSI B18.2.4.2M-1979 (R1995) | Metric Hex Nuts, Style 2 |
| ANSI B18.2.4.3M-1979 (R1989) | Metric Slotted Hex Nuts |
| ANSI B18.2.4.4M-1982 | Metric Hex Flange Nuts |
| ANSI B18.2.4.5M-1979 | Metric Hex Jam Nuts |
| ANSI B18.2.4.6M-1979 (R1990) | Metric Heavy Hex Nuts |
| ANSI B18.16.3M-1998 | Prevailing-Torque Metric Hex Nuts |
| ANSI B18.16.3M-1998 | Prevailing-Torque Metric Hex Flange Nuts |
| ANSI B18.22M-1981 (R1990) | Metric Plain Washers |
Pro Tip: ANSI letter symbols designating dimensional characteristics are in accord with those used in ISO Standards, except that ANSI uses capital letters for data processing convenience instead of the lower case letters used in ISO Standards.
Metric Hex Cap Screws — ANSI/ASME B18.2.3.1M
The metric hex cap screw is the workhorse of precision metric fastening. It features a machined or cold-formed hex head with a washer face on the bearing surface, tighter dimensional tolerances than hex bolts, and is available in sizes from M5 through M100.
Dimensional Reference Table
All dimensions in millimeters. Basic thread lengths (B) are the same as those given for Metric Hex Bolts.
| Nominal Size & Thread Pitch | Body Dia. Ds (Max) | Body Dia. Ds (Min) | Width Across Flats S (Max) | Width Across Flats S (Min) | Width Across Corners E (Max) | Width Across Corners E (Min) | Head Height K (Max) | Head Height K (Min) | Wrenching Height K1 (Min) | Washer Face Thick. C (Max) | Washer Face Thick. C (Min) | Washer Face Dia. Dw (Min) |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| M5 × 0.8 | 5.00 | 4.82 | 8.00 | 7.78 | 9.24 | 8.79 | 3.65 | 3.35 | 2.4 | 0.5 | 0.2 | 7.0 |
| M6 × 1 | 6.00 | 5.82 | 10.00 | 9.78 | 11.55 | 11.05 | 4.15 | 3.85 | 2.8 | 0.5 | 0.2 | 8.9 |
| M8 × 1.25 | 8.00 | 7.78 | 13.00 | 12.73 | 15.01 | 14.38 | 5.50 | 5.10 | 3.7 | 0.6 | 0.3 | 11.6 |
| M10 × 1.5 | 10.00 | 9.78 | 16.00 | 15.73 | 18.48 | 17.77 | 6.63 | 6.17 | 4.5 | 0.6 | 0.3 | 14.6 |
| M12 × 1.75 | 12.00 | 11.73 | 18.00 | 17.73 | 20.78 | 20.03 | 7.76 | 7.24 | 5.2 | 0.6 | 0.3 | 16.6 |
| M14 × 2 | 14.00 | 13.73 | 21.00 | 20.67 | 24.25 | 23.35 | 9.09 | 8.51 | 6.2 | 0.6 | 0.3 | 19.6 |
| M16 × 2 | 16.00 | 15.73 | 24.00 | 23.67 | 27.71 | 26.75 | 10.32 | 9.68 | 7.0 | 0.8 | 0.4 | 22.5 |
| M20 × 2.5 | 20.00 | 19.67 | 30.00 | 29.16 | 34.64 | 32.95 | 12.88 | 12.12 | 8.8 | 0.8 | 0.4 | 27.7 |
| M24 × 3 | 24.00 | 23.67 | 36.00 | 35.00 | 41.57 | 39.55 | 15.44 | 14.56 | 10.5 | 0.8 | 0.4 | 33.2 |
| M30 × 3.5 | 30.00 | 29.67 | 46.00 | 45.00 | 53.12 | 50.85 | 19.48 | 17.92 | 13.1 | 0.8 | 0.4 | 42.7 |
| M36 × 4 | 36.00 | 35.61 | 55.00 | 53.80 | 63.51 | 60.79 | 23.38 | 21.62 | 15.8 | 0.8 | 0.4 | 51.1 |
| M42 × 4.5 | 42.00 | 41.38 | 65.00 | 62.90 | 75.06 | 71.71 | 26.97 | 25.03 | 18.2 | 1.0 | 0.5 | 59.8 |
| M48 × 5 | 48.00 | 47.38 | 75.00 | 72.60 | 86.60 | 82.76 | 31.07 | 28.93 | 21.0 | 1.0 | 0.5 | 69.0 |
| M56 × 5.5 | 56.00 | 55.26 | 85.00 | 82.20 | 98.15 | 93.71 | 36.20 | 33.80 | 24.5 | 1.0 | 0.5 | 78.1 |
| M64 × 6 | 64.00 | 63.26 | 95.00 | 91.80 | 109.70 | 104.65 | 41.32 | 38.68 | 28.0 | 1.0 | 0.5 | 87.2 |
| M72 × 6 | 72.00 | 71.26 | 105.00 | 101.40 | 121.24 | 115.60 | 46.45 | 43.55 | 31.5 | 1.2 | 0.6 | 96.3 |
| M80 × 6 | 80.00 | 79.26 | 115.00 | 111.00 | 132.72 | 126.54 | 51.58 | 48.42 | 35.0 | 1.2 | 0.6 | 105.4 |
| M90 × 6 | 90.00 | 89.13 | 130.00 | 125.50 | 150.11 | 143.07 | 57.74 | 54.26 | 39.2 | 1.2 | 0.6 | 119.2 |
| M100 × 6 | 100.00 | 99.13 | 145.00 | 140.00 | 167.43 | 159.60 | 63.90 | 60.10 | 43.4 | 1.2 | 0.6 | 133.0 |
Note: The M10 × 1.5 size with width across flats of 15 mm is not standard. Unless specifically ordered, M10 hex cap screws with 16 mm width across flats will be furnished.
Metric Formed Hex Screws — ANSI/ASME B18.2.3.2M
Formed hex screws differ from hex cap screws in their manufacturing method—they are produced by cold forming (heading) rather than machining, resulting in slightly less precise head geometry but faster, more economical production. The body diameter matches the nominal size exactly (not oversized like hex bolts).
Dimensional Reference Table
All dimensions in millimeters.
| Nominal Size & Thread Pitch | Body Dia. Ds (Max) | Body Dia. Ds (Min) | Width Across Flats S (Max) | Width Across Flats S (Min) | Width Across Corners E (Max) | Width Across Corners E (Min) | Head Height K (Max) | Head Height K (Min) | Wrenching Height K1 (Min) | Washer Face Thick. C (Max) | Washer Face Thick. C (Min) | Washer Face Dia. Dw (Max) |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| M5 × 0.8 | 5.00 | 4.82 | 8.00 | 7.64 | 9.24 | 8.56 | 3.65 | 3.35 | 2.4 | 0.5 | 0.2 | 6.9 |
| M6 × 1 | 6.00 | 5.82 | 10.00 | 9.64 | 11.55 | 10.80 | 4.15 | 3.85 | 2.0 | 0.5 | 0.2 | 8.9 |
| M8 × 1.25 | 8.00 | 7.78 | 13.00 | 12.57 | 15.01 | 14.08 | 5.50 | 5.10 | 3.7 | 0.6 | 0.3 | 11.6 |
| M10 × 1.5 | 10.00 | 9.78 | 16.00 | 15.57 | 18.48 | 17.43 | 6.63 | 6.17 | 4.5 | 0.6 | 0.3 | 14.6 |
| M12 × 1.75 | 12.00 | 11.73 | 18.00 | 17.57 | 20.78 | 19.68 | 7.76 | 7.24 | 5.2 | 0.6 | 0.3 | 16.6 |
| M14 × 2 | 14.00 | 13.73 | 21.00 | 20.16 | 24.25 | 22.58 | 9.09 | 8.51 | 6.2 | 0.6 | 0.3 | 19.6 |
| M16 × 2 | 16.00 | 15.73 | 24.00 | 23.16 | 27.71 | 25.94 | 10.32 | 9.68 | 7.0 | 0.8 | 0.4 | 22.5 |
| M20 × 2.5 | 20.00 | 19.67 | 30.00 | 29.16 | 34.64 | 32.66 | 12.88 | 12.12 | 8.8 | 0.8 | 0.4 | 27.7 |
| M24 × 3 | 24.00 | 23.67 | 36.00 | 35.00 | 41.57 | 39.20 | 15.44 | 14.56 | 10.5 | 0.8 | 0.4 | 33.2 |
Key Difference: Hex Cap Screws vs. Formed Hex Screws
| Feature | Hex Cap Screws | Formed Hex Screws |
|---|---|---|
| Manufacturing | Machined or precision cold-formed | Cold-formed (headed) |
| Head tolerance | Tighter (width across flats tolerance narrower) | Wider tolerance on width across flats |
| Body diameter | Equal to nominal size | Equal to nominal size |
| Washer face | Standard | Standard |
| Cost | Higher | Lower |
| Best application | Precision assemblies, critical joints | General-purpose fastening |
Metric Heavy Hex Screws — ANSI B18.2.3.3M
Heavy hex screws feature a larger head than standard hex cap screws—wider across flats, taller, and with a larger bearing surface. They are used in applications requiring greater wrenching capability and increased bearing area. Available from M12 through M36.
Dimensional Reference Table
All dimensions in millimeters.
| Nominal Size & Thread Pitch | Body Dia. Ds (Max) | Body Dia. Ds (Min) | Width Across Flats S (Max) | Width Across Flats S (Min) | Width Across Corners E (Max) | Width Across Corners E (Min) | Head Height K (Max) | Head Height K (Min) | Wrenching Height K1 (Min) | Washer Face Thick. C (Max) | Washer Face Thick. C (Min) | Washer Face Dia. Dw (Min) |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| M12 × 1.75 | 12.00 | 11.73 | 21.00 | 20.67 | 24.25 | 23.35 | 7.76 | 7.24 | 5.2 | 0.6 | 0.3 | 19.6 |
| M14 × 2 | 14.00 | 13.73 | 24.00 | 23.67 | 27.71 | 26.75 | 9.09 | 8.51 | 6.2 | 0.6 | 0.3 | 22.5 |
| M16 × 2 | 16.00 | 15.73 | 27.00 | 26.67 | 31.18 | 30.14 | 10.32 | 9.68 | 7.0 | 0.8 | 0.4 | 25.3 |
| M20 × 2.5 | 20.00 | 19.67 | 34.00 | 33.00 | 39.26 | 37.29 | 12.88 | 12.12 | 8.8 | 0.8 | 0.4 | 31.4 |
| M24 × 3 | 24.00 | 23.67 | 41.00 | 40.00 | 47.34 | 45.20 | 15.44 | 14.56 | 10.5 | 0.8 | 0.4 | 38.0 |
| M30 × 3.5 | 30.00 | 29.67 | 50.00 | 49.00 | 57.74 | 55.37 | 19.48 | 17.92 | 13.1 | 0.8 | 0.4 | 46.6 |
| M36 × 4 | 36.00 | 35.61 | 60.00 | 58.80 | 69.28 | 66.44 | 23.38 | 21.72 | 15.8 | 0.8 | 0.4 | 55.9 |
Design note: When hex bolts in sizes M12 through M24 are ordered in lengths shorter than 150 mm or 10D (whichever is shorter), heavy hex screws are normally supplied in place of heavy hex bolts, at the manufacturer's option.
Metric Screw and Bolt Diameters: Full Body vs. Reduced Body
This is a critical specification detail that many engineers overlook:
Metric screws and bolts are furnished with full diameter body within the limits shown in the respective dimensional tables, or are threaded to the head, unless the purchaser specifies "reduced body diameter."
The following fastener types may be obtained with reduced body diameter if specified:
- Formed hex screws
- Hex flange screws
- Heavy hex flange screws
- Hex bolts
- Heavy hex bolts
- Hex lag screws
- Round head square neck bolts
Important restrictions on reduced body diameter:
- Formed hex screws, hex flange screws, heavy hex flange screws, hex bolts, or heavy hex bolts with nominal lengths shorter than 4D (where D is the nominal diameter) are not recommended for reduced body
- Formed hex screws, hex flange screws, heavy hex flange screws, and hex lag screws with reduced body diameter will be furnished with a shoulder under the head
- For hex bolts and heavy hex bolts, the shoulder is optional with the manufacturer
Allowable Body Swell Adjacent to Head
For bolts and lag screws, there may be a reasonable swell, fin, or die seam on the body adjacent to the head not exceeding the nominal bolt diameter by:
| Nominal Diameter Range | Maximum Swell Beyond Nominal |
|---|---|
| M5 | 0.50 mm |
| M6 | 0.65 mm |
| M8 through M14 | 0.75 mm |
| M16 | 1.25 mm |
| M20 through M30 | 1.50 mm |
| M36 through M48 | 2.30 mm |
| M56 through M72 | 3.00 mm |
| M80 through M100 | 4.80 mm |
Reduced Body Diameter Reference
All dimensions in millimeters.
Metric Formed Hex Screws (Reduced Body):
| Size | Shoulder Dia. Ds (Max/Min) | Body Dia. Dsi (Max/Min) | Shoulder Length Lsh (Max/Min) |
|---|---|---|---|
| M5 × 0.8 | 5.00 / 4.82 | 4.46 / 4.36 | 3.5 / 2.5 |
| M6 × 1 | 6.00 / 5.82 | 5.39 / 5.21 | 4.0 / 3.0 |
| M8 × 1.25 | 8.00 / 7.78 | 7.26 / 7.04 | 5.0 / 4.0 |
| M10 × 1.5 | 10.00 / 9.78 | 9.08 / 8.86 | 6.0 / 5.0 |
| M12 × 1.75 | 12.00 / 11.73 | 10.95 / 10.68 | 7.0 / 6.0 |
| M14 × 2 | 14.00 / 13.73 | 12.77 / 12.50 | 8.0 / 7.0 |
| M16 × 2 | 16.00 / 15.73 | 14.77 / 14.50 | 9.0 / 8.0 |
| M20 × 2.5 | 20.00 / 19.67 | 18.49 / 18.16 | 11.0 / 10.0 |
| M24 × 3 | 24.00 / 23.67 | 22.13 / 21.80 | 13.0 / 12.0 |
Metric Hex Bolts — ANSI/ASME B18.2.3.5M
The metric hex bolt is distinguishable from the hex cap screw by its wider body diameter tolerance (the body can be significantly oversize or undersize relative to nominal), the absence of a mandatory washer face, and a generally lower level of dimensional precision. Hex bolts are the standard choice for structural and general-purpose connections.
Dimensional Reference Table
All dimensions in millimeters.
| Nominal Dia. D & Thread Pitch | Body Dia. Ds (Max) | Body Dia. Ds (Min) | Width Across Flats S (Max) | Width Across Flats S (Min) | Width Across Corners E (Max) | Width Across Corners E (Min) | Head Height K (Max) | Head Height K (Min) | Wrenching Ht. K1 (Min) |
|---|---|---|---|---|---|---|---|---|---|
| M5 × 0.8 | 5.48 | 4.52 | 8.00 | 7.64 | 9.24 | 8.63 | 3.58 | 3.35 | 2.4 |
| M6 × 1 | 6.19 | 5.52 | 10.00 | 9.64 | 11.55 | 10.89 | 4.38 | 3.55 | 2.8 |
| M8 × 1.25 | 8.58 | 7.42 | 13.00 | 12.57 | 15.01 | 14.20 | 5.68 | 5.10 | 3.7 |
| M10 × 1.5 | 10.58 | 9.42 | 16.00 | 15.57 | 18.48 | 17.59 | 6.85 | 6.17 | 4.5 |
| M12 × 1.75 | 12.70 | 11.30 | 18.00 | 17.57 | 20.78 | 19.85 | 7.95 | 7.24 | 5.2 |
| M14 × 2 | 14.70 | 13.30 | 21.00 | 20.16 | 24.25 | 22.78 | 9.25 | 8.51 | 6.2 |
| M16 × 2 | 16.70 | 15.30 | 24.00 | 23.16 | 27.71 | 26.17 | 10.75 | 9.68 | 7.0 |
| M20 × 2.5 | 20.84 | 19.16 | 30.00 | 29.16 | 34.64 | 32.95 | 13.40 | 12.12 | 8.8 |
| M24 × 3 | 24.84 | 23.16 | 36.00 | 35.00 | 41.57 | 39.55 | 15.90 | 14.56 | 10.5 |
| M30 × 3.5 | 30.84 | 29.16 | 46.00 | 45.00 | 53.12 | 50.55 | 19.75 | 17.92 | 13.1 |
| M36 × 4 | 37.00 | 35.00 | 55.00 | 53.80 | 63.51 | 60.79 | 23.55 | 21.72 | 15.8 |
| M42 × 4.5 | 43.00 | 41.00 | 65.00 | 62.90 | 75.06 | 71.71 | 27.05 | 25.03 | 18.2 |
| M48 × 5 | 49.00 | 47.00 | 75.00 | 72.60 | 86.60 | 82.76 | 31.07 | 28.93 | 21.0 |
| M56 × 5.5 | 57.20 | 54.80 | 85.00 | 82.20 | 98.15 | 93.71 | 36.20 | 33.80 | 24.5 |
| M64 × 6 | 65.52 | 62.80 | 95.00 | 91.80 | 109.70 | 104.65 | 41.32 | 38.68 | 28.0 |
| M72 × 6 | 73.84 | 70.80 | 105.00 | 101.40 | 121.24 | 115.60 | 46.45 | 43.55 | 31.5 |
| M80 × 6 | 82.16 | 78.80 | 115.00 | 111.00 | 132.79 | 126.54 | 51.58 | 48.42 | 35.0 |
| M90 × 6 | 92.48 | 88.60 | 130.00 | 125.50 | 150.11 | 143.07 | 57.74 | 54.26 | 39.2 |
| M100 × 6 | 102.80 | 98.60 | 145.00 | 140.00 | 167.43 | 159.60 | 63.90 | 60.10 | 43.4 |
Basic Thread Lengths for Metric Hex Bolts
Thread length varies by bolt length and nominal diameter:
| Nominal Dia. & Thread Pitch | Bolt Length < 125 mm | Bolt Length 125–200 mm | Bolt Length > 200 mm |
|---|---|---|---|
| M5 × 0.8 | 16 | 22 | 35 |
| M6 × 1 | 18 | 24 | 37 |
| M8 × 1.25 | 22 | 28 | 41 |
| M10 × 1.5 | 26 | 32 | 45 |
| M12 × 1.75 | 30 | 36 | 49 |
| M14 × 2 | 34 | 40 | 53 |
| M16 × 2 | 38 | 44 | 57 |
| M20 × 2.5 | 46 | 52 | 65 |
| M24 × 3 | 54 | 60 | 73 |
| M30 × 3.5 | 66 | 72 | 85 |
| M36 × 4 | 78 | 84 | 97 |
| M42 × 4.5 | 90 | 96 | 109 |
| M48 × 5 | 102 | 108 | 121 |
| M56 × 5.5 | — | 124 | 137 |
| M64 × 6 | — | 140 | 153 |
| M72 × 6 | — | 156 | 169 |
| M80 × 6 | — | 172 | 185 |
| M90 × 6 | — | 192 | 205 |
| M100 × 6 | — | 212 | 225 |
Critical note: Basic thread length B is a reference dimension. The actual thread length is controlled by the grip gaging length, which equals the nominal screw length minus the basic thread length.
Metric Hex Lag Screws — ANSI B18.2.3.8M
Hex lag screws are designed for wood-to-wood or wood-to-metal connections. They have a special gimlet-point thread (not a standard machine thread) and are available from M5 through M24.
Thread length formula: The minimum thread length for hex lag screws equals one-half the nominal screw length plus 12 mm, or 150 mm, whichever is shorter. Screws too short for this formula are threaded as close to the head as practicable.
Socket Head Cap Screw Thread Lengths (for reference comparison)
| Nominal Size | Length of Complete Thread (LT) |
|---|---|
| M1.6 | 15.2 mm |
| M2 | 16.0 mm |
| M2.5 | 17.0 mm |
| M3 | 18.0 mm |
| M4 | 20.0 mm |
| M5 | 22.0 mm |
| M6 | 24.0 mm |
| M8 | 28.0 mm |
| M10 | 32.0 mm |
| M12 | 36.0 mm |
| M14 | 40.0 mm |
| M16 | 44.0 mm |
| M20 | 52.0 mm |
| M24 | 60.0 mm |
| M30 | 72.0 mm |
| M36 | 84.0 mm |
| M42 | 96.0 mm |
| M48 | 108.0 mm |
Heavy Hex Flange Screws & Hex Flange Screws
Metric Hex Flange Screws — ANSI/ASME B18.2.3.4M
Hex flange screws incorporate an integral washer-like flange beneath the hex head, eliminating the need for a separate washer and providing a larger bearing surface area. Available from M5 through M16.
All dimensions in millimeters.
| Nominal Size & Thread Pitch | Body Dia. Ds (Max/Min) | Width Across Flats S (Max/Min) | Width Across Corners E (Max/Min) | Flange Dia. Dc (Max) | Bearing Circle Dia. Dw (Min) | Flange Edge Thick. C (Min) | Head Height K (Max) | Wrenching Ht. K1 (Min) | Fillet Radius R (Max) |
|---|---|---|---|---|---|---|---|---|---|
| M5 × 0.8 | 5.00 / 4.82 | 7.00 / 6.64 | 8.08 / 7.44 | 11.4 | 9.4 | 1.0 | 5.6 | 2.30 | 0.3 |
| M6 × 1 | 6.00 / 5.82 | 8.00 / 7.64 | 9.24 / 8.56 | 13.6 | 11.6 | 1.1 | 6.8 | 2.90 | 0.4 |
| M8 × 1.25 | 8.00 / 7.78 | 10.00 / 9.64 | 11.55 / 10.80 | 17.0 | 14.9 | 1.2 | 8.5 | 3.80 | 0.5 |
| M10 × 1.5 | 10.00 / 9.78 | 13.00 / 12.57 | 15.01 / 14.08 | 20.8 | 18.7 | 1.5 | 9.7 | 4.30 | 0.6 |
| M12 × 1.75 | 12.00 / 11.73 | 15.00 / 14.57 | 17.32 / 16.32 | 24.7 | 22.0 | 1.8 | 11.9 | 5.40 | 0.7 |
| M14 × 2 | 14.00 / 13.73 | 18.00 / 17.57 | 20.78 / 19.68 | 28.6 | 25.9 | 2.1 | 12.9 | 5.60 | 0.8 |
| M16 × 2 | 16.00 / 15.73 | 21.00 / 20.48 | 24.25 / 22.94 | 32.8 | 30.1 | 2.4 | 15.1 | 6.70 | 1.0 |
Heavy Hex Bolts & Heavy Hex Structural Bolts
Heavy Hex Structural Bolts — ANSI B18.2.3.7M
These are the high-strength structural fasteners used in steel-to-steel connections in buildings, bridges, and heavy equipment. They conform to ASTM A325M or ASTM A490M and are available from M16 through M36.
All dimensions in millimeters.
| Nominal Dia. D & Thread Pitch | Body Dia. Ds (Max/Min) | Width Across Flats S (Max/Min) | Width Across Corners E (Max/Min) | Head Height K (Max/Min) | Wrenching Ht. K1 (Min) | Washer Face Dia. Dw (Min) | Washer Face Thick. C (Max/Min) | Thread Length B ≤ 100 mm | Thread Length B > 100 mm | Transition Thread X (Max) |
|---|---|---|---|---|---|---|---|---|---|---|
| M16 × 2 | 16.70 / 15.30 | 27.00 / 26.16 | 31.18 / 29.56 | 10.75 / 9.25 | 6.5 | 24.9 | 0.8 / 0.4 | 31 | 38 | 6.0 |
| M20 × 2.5 | 20.84 / 19.16 | 34.00 / 33.00 | 39.26 / 37.29 | 13.40 / 11.60 | 8.1 | 31.4 | 0.8 / 0.4 | 36 | 43 | 7.5 |
| M22 × 2.5 | 22.84 / 21.16 | 36.00 / 35.00 | 41.57 / 39.55 | 14.90 / 13.10 | 9.2 | 33.3 | 0.8 / 0.4 | 38 | 45 | 7.5 |
| M24 × 3 | 24.84 / 23.16 | 41.00 / 40.00 | 47.34 / 45.20 | 15.90 / 14.10 | 9.9 | 38.0 | 0.8 / 0.4 | 41 | 48 | 9.0 |
| M27 × 3 | 27.84 / 26.16 | 46.00 / 45.00 | 53.12 / 50.85 | 17.90 / 16.10 | 11.3 | 42.8 | 0.8 / 0.4 | 44 | 51 | 9.0 |
| M30 × 3.5 | 30.84 / 29.16 | 50.00 / 49.00 | 57.74 / 55.37 | 19.75 / 17.65 | 12.4 | 46.5 | 0.8 / 0.4 | 49 | 56 | 10.5 |
| M36 × 4 | 37.00 / 35.00 | 60.00 / 58.80 | 69.28 / 66.44 | 23.55 / 21.45 | 15.0 | 55.9 | 0.8 / 0.4 | 56 | 63 | 12.0 |
Structural bolts conform to ASTM A325M or ASTM A490M. These are not interchangeable with general-purpose hex bolts—they have mandatory property class requirements, marking requirements, and specific torque/tension installation procedures. The bridge failure in our opening story occurred precisely because this distinction was ignored.
Round Head Square Neck Bolts — ANSI B18.5.2.2M
Also known as carriage bolts, these feature a domed head and a square section beneath it that prevents rotation when the nut is tightened. Available from M5 through M24.
Reduced Body Diameters for Round Head Square Neck Bolts
| Nominal Dia. & Thread Pitch | Reduced Body Dia. Dr (Max/Min) |
|---|---|
| M5 × 0.8 | 5.00 / 4.36 |
| M6 × 1 | 6.00 / 5.21 |
| M8 × 1.25 | 8.00 / 7.04 |
| M10 × 1.5 | 10.00 / 8.86 |
| M12 × 1.75 | 12.00 / 10.68 |
| M14 × 2 | 14.00 / 12.50 |
| M16 × 2 | 16.00 / 14.50 |
| M20 × 2.5 | 20.00 / 18.16 |
| M24 × 3 | 24.00 / 21.80 |
Clearance Holes for Round Head Square Neck Bolts
| Nominal Dia. & Thread Pitch | Close Clearance | Normal Clearance | Loose Clearance | Corner Radius Rh |
|---|---|---|---|---|
| M5 × 0.8 | 5.5 | — | 5.8 | 0.2 |
| M6 × 1 | 6.6 | — | 7.0 | 0.3 |
| M8 × 1.25 | — | 9.0 | 10.0 | 0.4 |
| M10 × 1.5 | — | 11.0 | 12.0 | 0.4 |
| M12 × 1.75 | 13.0 | 13.5 | 14.5 | 0.6 |
| M14 × 2 | 15.0 | 15.5 | 16.5 | 0.6 |
| M16 × 2 | 17.0 | 17.5 | 18.5 | 0.6 |
| M20 × 2.5 | 21.0 | 22.0 | 24.0 | 0.8 |
| M24 × 3 | 25.0 | 26.0 | 28.0 | 1.0 |
Mechanical Properties: The Property Class System Decoded
This is where the practitioner's bridge went wrong. Understanding property classes is not optional—it is the difference between a joint that holds and one that kills.
How the Property Class Number Works
The metric property class designation consists of two numbers separated by a decimal point. Here is how to decode it:
- First number: Equals approximately 1/100 of the minimum tensile strength in megapascals (MPa)
- Second number (after decimal): Approximates the ratio of the minimum yield stress to the minimum tensile stress
Example: Property Class 8.8
- First digit "8" → Minimum tensile strength ≈ 800 MPa
- Second digit "8" → Yield/Tensile ratio ≈ 0.8, so minimum yield ≈ 640 MPa
Example: Property Class 12.9
- "12" → Minimum tensile strength ≈ 1200 MPa
- ".9" → Yield/Tensile ratio ≈ 0.9, so minimum yield ≈ 1080 MPa
Strength Grade Designation Table (ISO/BS System)
| Strength Grade | Min. Tensile Strength (kgf/mm²) | Min. Yield Stress Re (kgf/mm²) | Min. Stress at Permanent Set R0.2 (kgf/mm²) |
|---|---|---|---|
| 4.6 | 40 | 24 | — |
| 4.8 | 40 | 32 | — |
| 5.6 | 50 | 30 | — |
| 5.8 | 50 | 40 | — |
| 6.6 | 60 | 36 | — |
| 6.8 | 60 | 48 | — |
| 8.8 | 80 | — | 64 |
| 10.9 | 100 | — | 90 |
| 12.9 | 120 | — | 108 |
| 14.9 | 140 | — | 126 |
Note: For classes 8.8 and above, the "yield" measurement is the 0.2% offset proof stress (R0.2) rather than the traditional yield stress (Re), because these higher-strength fasteners are quenched and tempered steels that do not exhibit a well-defined yield point.
Materials and Standards Governing Metric Fasteners
| Fastener Type | Governing Standard |
|---|---|
| Steel metric screws and bolts (general) | SAE J1199 or ASTM F568 |
| Steel heavy hex structural bolts | ASTM A325M or ASTM A490M |
| Alloy steel socket head cap screws | ASTM 574M, property class 12.9 |