Heavy hex bolts, heavy hex screws and A325 / A490 heavy hex structural bolts from ½″ to 3″ and M12 to M36, hot forged and thread rolled in alloy steel, weathering steel, stainless, duplex and nickel alloys, supplied as matched bolt-nut-washer assemblies with EN 10204 3.1 / 3.2 certificates.
A heavy hex bolt is a hex bolt with a bigger head. The width across flats is 1½ times the diameter plus ⅛″ instead of 1½ times the diameter, so the head bears on more of the joint and the spanner has more metal to grip. It is the standard bolt for flanged joints, pressure equipment and structural steel.
Exotic Alloy manufactures heavy hex bolts and heavy hex screws to ASME B18.2.1 and B18.2.3.6M, and heavy hex structural bolts to ASME B18.2.6 in ASTM F3125 Grades A325, A490, A325M and A490M, at our works in Mumbai, India. Every size is hot forged, thread rolled and supplied with the matching heavy hex nut and hardened washer. We supply India and export to Saudi Arabia, the UAE, Qatar, Oman, Kuwait, Europe and the Americas.
A193 B7, B8, B8M and A307 Grade B heavy hex bolts ship from stock. A325 and A490 structural assemblies, weathering Type 3, A320 L7 low-temperature bolts, and heavy hex bolts in duplex, Inconel, Monel and Hastelloy are forged to order in 7 to 15 business days, in any length, to drawing, at factory price.
The envelope we manufacture and stock. Anything outside it is made to drawing.
Three inch patterns share the same heavy hex head but differ under the head, in head height and in thread length, and they are specified by different ASTM grades. Metric heavy hex bolts follow ASME or the European HR / HV systems.
Plain underhead, the taller heavy hex bolt head and the 2D + ¼″ thread length. The machine bolt for ASME B16.5 flanges, valves, pumps and pressure equipment where the joint is tightened against a gasket. Normally supplied with A194 2H, 8 / 8M or A563 B heavy hex nuts.
Washer face under the head, the cap-screw head height and closer head and body tolerances than the bolt. Chosen for machined assemblies, equipment bases and wherever the bolt turns against a finished surface. Same across-flats and thread length as the heavy hex bolt.
½″ to 1½″ with a short thread (1.00″ to 2.25″) so that threads stay out of the shear plane, and supplied as a bolt, A563 DH nut and F436 washer assembly. Installed snug-tight or pretensioned to 70 % of tensile under the RCSC Specification; Type 3 is weathering steel for unpainted bridges and towers.
M12 to M36 with the ASME heavy series across flats (M20 = 34 mm, M24 = 41 mm). A325M and A490M structural bolts are the metric equivalents of A325 and A490, marked 8S and 10S, and are pretensioned to the same 70 % rule in kN.
The European preloaded assemblies with large widths across flats (M20 = 32 mm, M24 = 41 mm, M36 = 60 mm), CE-marked as bolt, nut and washer sets with a declared k-class for torque or combined tightening. HV sets use a thinner 10 nut and a shorter thread; HR sets a full-height nut.
The same heavy hex pattern forged in 304 / 316, duplex 2205 / 2507, Inconel 625 / 718, Monel 400 / K-500 and Hastelloy C-276 for seawater, sour gas and chemical flanges, with heavy hex nuts in the same alloy and anti-seize to prevent galling.
| Feature | Heavy hex bolt | Heavy hex screw | Heavy hex structural bolt |
|---|---|---|---|
| Standard | ASME B18.2.1 | ASME B18.2.1 | ASME B18.2.6 |
| Width across flats | 1½D + ⅛″ for all three | ||
| Head height, ¾″ example | ½″ | 15⁄32″ | 15⁄32″ |
| Underhead | Plain | Washer face | Plain, with fillet |
| Thread length, ¾″ example | 1¾″ (2D + ¼) | 1¾″ (2D + ¼) | 1.38″ (reference) |
| Size range | ½″ to 3″ | ½″ to 3″ | ½″ to 1½″ |
| Usual grades | A307 B, A193 B7 / B8M, A320 L7 | A449, A193 B7, SAE Gr. 5 / 8 | F3125 A325 / A490 Types 1 and 3 |
| Nut | A563 A / B, A194 2H / 8M | A563 B / DH | A563 DH (A194 2H), washer F436 |
| Typical joint | Flanges, valves, gasketed pressure joints | Machinery, bases, finished surfaces | Steel frames, bridges, towers, cranes |
| Item | ASME / ASTM (inch) | ASME / ASTM (metric) | EN / DIN | IS (India) | JIS |
|---|---|---|---|---|---|
| Heavy hex bolt, dimensions | ASME B18.2.1 | ASME B18.2.3.6M | — | — | — |
| Heavy hex screw, dimensions | ASME B18.2.1 | ASME B18.2.3.3M | — | — | — |
| Heavy hex structural bolt, dimensions | ASME B18.2.6 | ASME B18.2.3.7M | EN 14399-3 (HR), EN 14399-4 (HV), DIN 6914 | IS 3757 | JIS B 1186 |
| Structural bolt, material | ASTM F3125 A325 / A490 | ASTM F3125 A325M / A490M | EN 14399-1, ISO 898-1 8.8 / 10.9 | IS 3757 (8.8, 10.9) | JIS B 1186 (F8T, F10T) |
| Heavy hex nut | ASME B18.2.2; ASTM A563 / A194 | ASME B18.2.4.6M; A563M / A194 | EN 14399-3 / -4 (HR / HV nuts) | IS 6623 | JIS B 1186 |
| Hardened washer | ASTM F436 | ASTM F436M | EN 14399-5 / -6 | IS 6649 | JIS B 1186 |
| Installation | RCSC Specification for Structural Joints; AISC 360 | EN 1090-2 | IS 4000 | — | |
| Flange bolting | ASME B16.5 / B16.47; ASME PCC-1 | EN 1092-1; EN 1591-4 | — | — | |
Basic (nominal) dimensions. The ASME tables carry maximum and minimum limits on every figure; the metric table shows them. Thread lengths for bolts and screws are 2D + ¼″ up to 6″ long and 2D + ½″ above; structural bolts use the shorter B18.2.6 reference length.
| Size | UNC tpi | F across flats in (1½D + ⅛) | H heavy hex bolt in | H heavy hex screw / structural in | Thread length, L ≤ 6″ in | Thread length, L > 6″ in | Structural (B18.2.6) thread in, ref. |
|---|---|---|---|---|---|---|---|
| 1/2″ | 13 | 7/8 | 11/32 | 5/16 | 1.25 | 1.50 | 1.00 |
| 5/8″ | 11 | 1-1/16 | 27/64 | 25/64 | 1.50 | 1.75 | 1.25 |
| 3/4″ | 10 | 1-1/4 | 1/2 | 15/32 | 1.75 | 2.00 | 1.38 |
| 7/8″ | 9 | 1-7/16 | 37/64 | 35/64 | 2.00 | 2.25 | 1.50 |
| 1″ | 8 | 1-5/8 | 43/64 | 39/64 | 2.25 | 2.50 | 1.75 |
| 1-1/8″ | 7 | 1-13/16 | 3/4 | 11/16 | 2.50 | 2.75 | 2.00 |
| 1-1/4″ | 7 | 2 | 27/32 | 25/32 | 2.75 | 3.00 | 2.00 |
| 1-3/8″ | 6 | 2-3/16 | 29/32 | 27/32 | 3.00 | 3.25 | 2.25 |
| 1-1/2″ | 6 | 2-3/8 | 1 | 15/16 | 3.25 | 3.50 | 2.25 |
| 1-3/4″ | 5 | 2-3/4 | 1-5/32 | 1-3/32 | 3.75 | 4.00 | — |
| 2″ | 4-1/2 | 3-1/8 | 1-11/32 | 1-7/32 | 4.25 | 4.50 | — |
| 2-1/4″ | 4-1/2 | 3-1/2 | 1-1/2 | 1-3/8 | 4.75 | 5.00 | — |
| 2-1/2″ | 4 | 3-7/8 | 1-21/32 | 1-17/32 | 5.25 | 5.50 | — |
| 2-3/4″ | 4 | 4-1/4 | 1-13/16 | 1-11/16 | 5.75 | 6.00 | — |
| 3″ | 4 | 4-5/8 | 2 | 1-7/8 | 6.25 | 6.50 | — |
Heavy hex bolts carry the taller H; heavy hex screws and ASME B18.2.6 structural bolts share the lower H. Width across corners is about 1.155 × F. Heavy hex nuts (ASME B18.2.2) have the same F and a thickness of about one diameter.
| Size | Pitch mm | s across flats max / min mm | k head height max / min mm | k1 wrenching height min mm | As mm² |
|---|---|---|---|---|---|
| M12 | 1.75 | 21 / 20.16 | 7.95 / 7.24 | 5.2 | 84.3 |
| M14 | 2 | 24 / 23.16 | 9.25 / 8.51 | 6.2 | 115 |
| M16 | 2 | 27 / 26.16 | 10.75 / 9.68 | 7 | 157 |
| M20 | 2.5 | 34 / 33 | 13.4 / 12.12 | 8.8 | 245 |
| M24 | 3 | 41 / 40 | 15.9 / 14.56 | 10.5 | 353 |
| M30 | 3.5 | 50 / 49 | 19.75 / 17.92 | 13.1 | 561 |
| M36 | 4 | 60 / 58.8 | 23.55 / 21.72 | 15.8 | 817 |
ASME B18.2.3.6M heavy hex bolts, M12 to M36 from stock tooling. Note that these across-flats values are larger than ISO 4014 (M20 = 30 mm, M24 = 36 mm) and differ again from EN 14399 (M20 = 32 mm): the three metric systems are not interchangeable at the spanner. Larger metric sizes are forged to drawing.
| Size | Pitch mm | s across flats mm | k head height mm | Property class | As mm² | Min. preload Fp,C = 0.7 fub As kN (EN 1090-2) |
|---|---|---|---|---|---|---|
| M12 | 1.75 | 22 | 8 | 10.9 | 84.3 | 59 |
| M16 | 2 | 27 | 10 | 10.9 | 157 | 110 |
| M20 | 2.5 | 32 | 13 | 10.9 | 245 | 172 |
| M22 | 2.5 | 36 | 14 | 10.9 | 303 | 212 |
| M24 | 3 | 41 | 15 | 10.9 | 353 | 247 |
| M27 | 3 | 46 | 17 | 10.9 | 459 | 321 |
| M30 | 3.5 | 50 | 19 | 10.9 | 561 | 393 |
| M36 | 4 | 60 | 23 | 10.9 | 817 | 572 |
System HV, class 10.9 bolt with class 10 nut, to EN 14399-4. The minimum preload Fp,C is the EN 1090-2 value, 70 % of the 1000 MPa nominal tensile strength on the stress area. HR assemblies (EN 14399-3) are available in 8.8 and 10.9 with a full-height nut.
The heavy hex head is a shape; the grade is the material specification stamped on it. Structural joints use ASTM F3125, pressure joints use ASTM A193 / A320 or A307 Grade B.
| Grade | Type | Head / marking | Diameter | Tensile ksi (MPa) | Proof load ksi (MPa) | Hardness | Steel | Galvanising |
|---|---|---|---|---|---|---|---|---|
| F3125 A325 | Type 1 | Heavy hex, “A325” | ½ to 1½″ | 120 (827) min | 85 (586) | 25–34 HRC | Medium carbon, carbon boron or alloy, Q&T | Hot-dip (F2329) or mechanical (B695 Cl. 55) permitted |
| F3125 A325 | Type 3 | Heavy hex, “A325” underlined | ½ to 1½″ | 120 (827) min | 85 (586) | 25–34 HRC | Weathering steel (A588 type), Q&T | Not galvanised (weathering) |
| F3125 A490 | Type 1 | Heavy hex, “A490” | ½ to 1½″ | 150 (1034) to 173 (1193) | 120 (827) | 33–38 HRC | Alloy steel, Q&T | Not permitted (hydrogen embrittlement) |
| F3125 A490 | Type 3 | Heavy hex, “A490” underlined | ½ to 1½″ | 150 (1034) to 173 (1193) | 120 (827) | 33–38 HRC | Weathering alloy steel, Q&T | Not permitted |
| F3125 A325M | Type 1 / 3 | Heavy hex, “A325M” or “8S” | M12 to M36 | 830 MPa min | 600 MPa | 25–34 HRC | As A325 | As A325 |
| F3125 A490M | Type 1 / 3 | Heavy hex, “A490M” or “10S” | M12 to M36 | 1040–1210 MPa | 830 MPa | 33–38 HRC | As A490 | Not permitted |
| F3125 F1852 | Type 1 / 3 | Round head with spline (twist-off), A325 strength | ½ to 1¼″ | 120 (827) min | 85 (586) | 25–34 HRC | As A325 | Mechanical only |
| F3125 F2280 | Type 1 / 3 | Round head with spline (twist-off), A490 strength | ½ to 1¼″ | 150 (1034) to 173 (1193) | 120 (827) | 33–38 HRC | As A490 | Not permitted |
F3125 (2015 onward) consolidated A325, A490, A325M, A490M, F1852 and F2280 into one specification and set A325 at 120 ksi for every diameter, dropping the earlier 105 ksi tier above 1″. Hardness ranges apply to the finished bolt. A490 has a maximum tensile strength because higher hardness raises the risk of hydrogen embrittlement, which is also why it must not be galvanised or reused.
| Grade / spec | Head marking | Diameter | Tensile min ksi (MPa) | Yield min ksi (MPa) | Use in the heavy hex pattern |
|---|---|---|---|---|---|
| ASTM A307 Gr. A | 307A | ¼ to 4″ | 60 (414) | — | General-purpose heavy hex bolts and A563 Gr. A nuts |
| ASTM A307 Gr. B | 307B | ¼ to 4″ | 60 (414) to 100 (689) | — | Heavy hex bolts for ASME B16.5 Class 150 / 300 flanged joints, limited tensile so the gasket is not crushed |
| ASTM A193 B7 | B7 | ≤ 2½″ | 125 (862) | 105 (724) | Heavy hex machine bolts for flanges, valves and pressure equipment to 400 °C |
| ASTM A193 B8 / B8M Cl. 1 | B8 / B8M | all | 75 (517) | 30 (207) | Stainless heavy hex bolts for corrosive and high temperature flanged joints |
| ASTM A320 L7 | L7 | ≤ 2½″ | 125 (862) | 105 (724) | Low temperature flanged joints, Charpy at −101 °C |
| ASTM A449 | 3 radial lines | ¼ to 3″ | 120 (827) (≤ 1″) | 92 (634) (≤ 1″) | Heavy hex pattern where A325 strength is wanted outside the F3125 size range or head style |
| ASTM A354 Gr. BD | BD + 6 lines | ¼ to 4″ | 150 (1034) (≤ 2½″) | 130 (896) (≤ 2½″) | A490 strength level in heavy hex bolts larger than 1½″ or in non-structural joints |
| ISO 898-1 8.8 / 10.9 | 8.8 / 10.9 | M12 to M100 | 800 / 1040 MPa | 640 / 940 MPa | Metric heavy hex bolts to ASME B18.2.3.6M and EN 14399 HR / HV systems |
| Grade | Material | Diameter | Tensile min ksi (MPa) | Yield min ksi (MPa) | Elong. min % | RA min % | Hardness max | Impact test |
|---|---|---|---|---|---|---|---|---|
| A193 B7 | AISI 4140-type Cr-Mo, Q&T | ≤ 2½″ | 125 (862) | 105 (724) | 16 | 50 | 35 HRC | — |
| A193 B7M | Cr-Mo, Q&T, controlled hardness | ≤ 4″ | 100 (689) | 80 (552) | 18 | 50 | 235 HBW / 22 HRC | — |
| A193 B16 | Cr-Mo-V, Q&T | ≤ 2½″ | 125 (862) | 105 (724) | 18 | 50 | 35 HRC | — |
| A193 B8 Cl.1 | AISI 304, solution annealed | all | 75 (517) | 30 (207) | 30 | 50 | 223 HBW / 96 HRB | — |
| A193 B8M Cl.1 | AISI 316, solution annealed | all | 75 (517) | 30 (207) | 30 | 50 | 223 HBW / 96 HRB | — |
| A193 B8 Cl.2 | AISI 304, strain hardened | ≤ ¾″ | 125 (862) | 100 (689) | 12 | 35 | 321 HBW / 35 HRC | — |
| A193 B8 Cl.2 | AISI 304, strain hardened | > ¾ to 1″ | 115 (793) | 80 (552) | 15 | 35 | 321 HBW / 35 HRC | — |
| A193 B8M Cl.2 | AISI 316, strain hardened | ≤ ¾″ | 110 (758) | 95 (655) | 15 | 45 | 321 HBW / 35 HRC | — |
| A193 B8M Cl.2 | AISI 316, strain hardened | > ¾ to 1″ | 100 (689) | 80 (552) | 20 | 45 | 321 HBW / 35 HRC | — |
| A320 L7 | Cr-Mo (4140-type), Q&T | ≤ 2½″ | 125 (862) | 105 (724) | 16 | 50 | 35 HRC | 27 J (20 ft·lbf) at −101 °C |
| A320 L7M | Cr-Mo, Q&T, controlled hardness | ≤ 2½″ | 100 (689) | 80 (552) | 18 | 50 | 235 HBW | 27 J (20 ft·lbf) at −73 °C |
| A320 L43 | AISI 4340-type Ni-Cr-Mo, Q&T | ≤ 4″ | 125 (862) | 105 (724) | 16 | 50 | 35 HRC | 27 J (20 ft·lbf) at −101 °C |
| A320 B8 / B8M Cl.1 | AISI 304 / 316, solution annealed | all | 75 (517) | 30 (207) | 30 | 50 | 223 HBW | 27 J at −196 °C |
A193 covers high-temperature and high-pressure service; A320 adds a Charpy impact requirement for low temperature. B7M and L7M are the controlled-hardness versions for sour service to NACE MR0175 / ISO 15156. Heavy hex bolts in these grades are paired with A194 2H, 2HM, 8, 8M or 7 heavy hex nuts.
Every grade links to its full datasheet on this site. The heavy hex pattern is forged in all of them from certified bar.
1035 to 1045 and boron-treated grades, quenched and tempered for A325, A449 and 8.8; as-rolled for A307. The bulk of structural and flange bolting.
4140 / 4142 for A490, B7, L7 and 10.9; 4340 for L43 and large diameters; Cr-Mo-V for B16; A588-type weathering steel for Type 3 bolts on unpainted bridges and transmission towers.
904L and Alloy 20 for sulphuric acid plant flanges; 254 SMO for seawater; Nitronic 60 for galling resistance; A-286 for 650 °C turbine casing bolting.
450 MPa yield with chloride SCC resistance for offshore topside flanges, FPSO piping and flue-gas desulphurisation.
PREN above 40 and 550 MPa yield for seawater, desalination and subsea flanges; NACE MR0175 compliant.
| Alloy | UNS | Condition supplied | Tensile min MPa | Yield 0.2 % min MPa | Elongation % | Why it is chosen for hex bolts |
|---|---|---|---|---|---|---|
| Inconel 625 | N06625 | Solution annealed (Grade 1) | 827 | 414 | 30 | Seawater, sour gas, cryogenic to 980 °C oxidation; subsea flange bolting |
| Inconel 718 | N07718 | Solution treated + aged (AMS 5662) | 1240 | 1035 | 12 | Highest strength to 650 °C; turbine and wellhead bolting |
| Monel 400 | N04400 | Hot-finished annealed | 517 | 207 | 35 | Seawater, hydrofluoric acid, no chloride SCC; marine and valve bolting |
| Monel K-500 | N05500 | Age hardened (AMS 4676) | 960 | 690 | 18 | Monel 400 corrosion resistance with three times the yield; pump shafts, subsea |
| Hastelloy C-276 | N10276 | Solution annealed (ASTM B574) | 690 | 283 | 40 | Wet chlorine, hot acids, mixed oxidising / reducing media; chemical reactors |
| Incoloy 825 | N08825 | Annealed (ASTM B425) | 586 | 241 | 30 | Sulphuric and phosphoric acid, sour gas (NACE MR0175); pickling plant |
| Duplex 2205 | S32205 | Solution annealed | 655 | 450 | 25 | Twice the yield of 316 with chloride SCC resistance; offshore topsides |
| Super Duplex 2507 | S32750 | Solution annealed | 750 | 550 | 15 | PREN > 40; seawater and desalination bolting |
| Titanium Grade 5 | R56400 | Annealed (ASTM B348) | 895 | 828 | 10 | Half the weight of steel, immune to seawater; aerospace and chemical |
| A-286 | S66286 | Solution treated + aged | — | — | — | Iron-base superalloy for 650 °C bolting; on request with certified values |
Values are the minimums published on the linked datasheets for the supply condition shown; they are material properties, not a structural bolt grade. Alloy heavy hex bolts are not substitutes for A325 / A490 in RCSC joints unless the project engineer specifies them.
The heavy head needs more metal gathered under the die than a standard hex, so from ¾″ upward the head is hot forged. Structural grades are heat treated as full bolts and tested by the lot.
Bar received by heat against mill certificates; each production lot is one heat, one diameter, one heat-treatment batch, as F3125 requires. PMI on stainless and nickel alloys.
Induction-heated bar is upset in a two-blow forging press that gathers the extra volume of the 1½D + ⅛″ head and forms the underhead fillet without folding, keeping grain flow continuous into the head.
Hexagon trimmed to the across-flats limits, washer face turned on heavy hex screws, point chamfered. Sizes above 1½″ and short runs are CNC machined from hexagon or round bar.
Quench and temper to the hardness window of the grade: 25 to 34 HRC for A325, 33 to 38 HRC for A490, 35 HRC max for B7, with a minimum tempering temperature of 427 °C (800 °F) for A325 and A490 and 593 °C (1100 °F) for B7. Solution annealing for stainless; ageing for K-500 and 718.
Threads rolled after heat treatment on B7, L7 and 10.9 for the best fatigue life, and before heat treatment on structural grades where the standard allows. UNC class 2A or metric 6g, checked with go / no-go gauges.
Hot-dip or mechanical galvanising, zinc flake, PTFE or passivation; galvanised nuts overtapped and lubricated; bolt, nut and washer assembled by lot and rotational-capacity tested.
Hardness, full-size wedge tensile, proof load and dimensional checks on every lot; Charpy, PMI and NDT where specified; EN 10204 3.1 or 3.2 certificates with lot numbers that match the head markings.
F3125 and A193 require the tensile test on the full-size bolt with a wedge under the head. The wedge loads the head off-axis, so a bolt with a sharp underhead fillet, a cold shut in the forging, decarburised threads or an over-hard head fails at the head instead of in the thread. A heavy hex head forged with a proper fillet and tempered to the grade's hardness window passes; a machined head with no fillet often does not. We wedge-test every F3125 and B7 lot and keep the fractured specimens with the lot record.
Galvanised A325 assemblies additionally pass the rotational-capacity test: bolt, nut and washer are turned to twice the installation rotation and must reach the required tension without stripping, seizing or shearing.
With heavy hex bolts the coating choice is governed by the grade as much as by the environment: the high-strength grades restrict what may be done to them. See also our coated fasteners page.
| Finish | Specification | Typical thickness | Permitted on | Not for | Notes for heavy hex bolts |
|---|---|---|---|---|---|
| Plain / black, light oil | As manufactured | — | All grades | — | Standard supply for A490, A354 BD, B7 and L7; the only finish for A490 apart from zinc flake. Store dry; oil film affects turn-of-nut little but affects torque a lot |
| Hot-dip galvanised | ASTM F2329 / ISO 10684 / IS 1367 Pt 13 | ≥ 43 μm (F2329 on ≥ ⅜″) | A307, A325, A449, A193 B7, 8.8 | A490, A354 BD, A490M, 12.9 | Nuts overtapped and lubricated (waxed) and supplied with the bolts as an assembly; rotational-capacity test on every A325 lot; 10.9 only with controlled pickling and baking |
| Mechanically galvanised | ASTM B695 Class 55 | ≥ 53 μm (Class 55) | A325, F1852, A307, B7 | A490, F2280 | No heat, no acid pickling of the finished bolt, so no hydrogen risk and better thread fit; the usual choice for galvanised tension-control bolts |
| Zinc flake (Geomet® / Dacromet® type) | ISO 10683 / ASTM F1136 | 8 – 12 μm | All grades including A490 and 10.9 / 12.9 where the project allows | — | Non-electrolytic, no hydrogen embrittlement; 600 to 1000 h salt spray; integral lubricant gives a repeatable friction coefficient |
| Zinc electroplate | ASTM B633 / F1941 / ISO 4042 | 5 – 12 μm | A307, B7, 8.8, SAE Gr. 5 | A490, 12.9; not recommended for A325 structural joints | Bake within 4 h of plating above 320 HV; thin coating, indoor or sheltered service only |
| PTFE / Xylan® fluoropolymer | Over phosphate or zinc | 20 – 30 μm | B7, B7M, L7, B8M flange bolting | Pretensioned structural joints (friction too low for turn-of-nut tables) | Friction coefficient 0.08 to 0.12, easy break-out after years offshore; blue, green, black; standard on subsea and refinery flange bolting |
| Weathering (uncoated Type 3) | F3125 Type 3, A588-type steel | — | A325 / A490 Type 3 | — | Forms its own protective oxide; used only with weathering steel members, never galvanised or painted |
| Pickled & passivated | ASTM A967 / A380 | — | B8, B8M, A4, duplex, nickel alloys | — | Supplied with nickel or molybdenum-disulphide anti-seize to stop galling in the heavy hex nut |
Structural heavy hex bolts are specified by tension, not torque. The RCSC Specification sets the minimum pretension at 70 % of the bolt's minimum tensile strength and accepts four ways to get there: turn-of-nut, calibrated wrench, twist-off tension-control bolt and direct tension indicator. Torque figures are a by-product that depends on lubrication; use them for planning the tooling, not as the acceptance criterion.
Minimum pretension = 0.70 × minimum tensile strength × stress area (RCSC / EN 1090-2). Turn-of-nut from the snug-tight condition for both faces normal to the bolt axis: ⅓ turn for L ≤ 4D, ½ turn for 4D < L ≤ 8D, ⅔ turn for 8D < L ≤ 12D. Indicative torque T = K · F · d.
| Size | As in² | A325 min. pretension kips (kN) | A490 min. pretension kips (kN) | Turn-of-nut rotation, L ≤ 4D | L > 4D to 8D | L > 8D to 12D |
|---|---|---|---|---|---|---|
| 1/2″ | 0.142 | 12 (53) | 15 (66) | ⅓ turn | ½ turn | ⅔ turn |
| 5/8″ | 0.226 | 19 (84) | 24 (106) | ⅓ turn | ½ turn | ⅔ turn |
| 3/4″ | 0.334 | 28 (125) | 35 (156) | ⅓ turn | ½ turn | ⅔ turn |
| 7/8″ | 0.462 | 39 (173) | 49 (216) | ⅓ turn | ½ turn | ⅔ turn |
| 1″ | 0.606 | 51 (226) | 64 (283) | ⅓ turn | ½ turn | ⅔ turn |
| 1-1/8″ | 0.763 | 64 (285) | 80 (356) | ⅓ turn | ½ turn | ⅔ turn |
| 1-1/4″ | 0.969 | 81 (362) | 102 (453) | ⅓ turn | ½ turn | ⅔ turn |
| 1-3/8″ | 1.155 | 97 (432) | 121 (539) | ⅓ turn | ½ turn | ⅔ turn |
| 1-1/2″ | 1.405 | 118 (525) | 148 (656) | ⅓ turn | ½ turn | ⅔ turn |
Pretension values are 0.70 × minimum tensile strength × UNC stress area, as tabulated in the RCSC Specification and AISC 360 Table J3.1. Rotation is from snug-tight with both outer faces normal to the bolt axis; a sloped face without a bevel washer adds ⅞ turn per face. The RCSC edition in force sets the rotation tolerance.
| Size | As mm² | A325M (830 MPa) min. pretension kN | A490M (1040 MPa) min. pretension kN | EN 14399 HV 10.9 Fp,C kN |
|---|---|---|---|---|
| M16 | 157 | 91 | 114 | 110 |
| M20 | 245 | 142 | 178 | 172 |
| M22 | 303 | 176 | 221 | 212 |
| M24 | 353 | 205 | 257 | 247 |
| M27 | 459 | 267 | 334 | 321 |
| M30 | 561 | 326 | 408 | 393 |
| M36 | 817 | 475 | 595 | 572 |
A325M and A490M to AISC 360 Table J3.1M; HV 10.9 to EN 1090-2 (Fp,C = 0.7 fub As). For heavy hex flange bolts in B7, B8M or alloy grades the target is set by the gasket and the flange, not by these tables: follow ASME PCC-1 and the joint calculation.
A325 and A490 assemblies for beam-column connections, splices and bracing; A325M / A490M and EN 14399 HV sets for metric-designed frames; snug-tight or pretensioned to RCSC.
Type 3 weathering bolts for unpainted structures, galvanised A325 assemblies with DH nuts for painted steel, slip-critical joints with Class A / B faying surfaces.
A193 B7 / B8M and A307 B heavy hex machine bolts with A194 2H / 8M nuts for ASME B16.5 Class 150 and 300 flanges, valve bonnets, pump casings and heat exchanger covers.
PTFE-coated B7 / L7 flange bolting, B7M / L7M for NACE sour service, Inconel 625 and super duplex heavy hex bolts for seawater and subsea flanges.
10.9 HV and A490 bolting for tower flanges and foundations, B16 and A-286 heavy hex bolts for turbine casings and steam flanges to 540 °C and beyond.
A490, A354 BD and 10.9 heavy hex screws for crane slew rings, excavator frames, mining equipment and rail bogies where the larger head resists bearing damage under cyclic load.
| Test | Method | Applied to |
|---|---|---|
| Chemical analysis / PMI | OES on bar; XRF PMI on finished bolts | Every heat; 100 % of stainless and nickel alloy lots |
| Full-size wedge tensile | ASTM F606, wedge 4° to 10° by size | Every F3125 and A193 / A320 lot; fracture must be in the thread or shank |
| Proof load | ASTM F606 length-measurement or yield-strength method | A325, A490, A449, A354, 8.8 and above |
| Hardness | Rockwell C on the end or a cross-section; surface and core | Every lot; decarburisation and carburisation check on A490 and 10.9 |
| Rotational capacity | RCSC / ASTM F3125 Annex, twice the installation rotation | Every galvanised A325 assembly lot |
| Charpy V-notch impact | ASTM A370, −46 to −196 °C | A320 L7 / L7M / L43 / B8; duplex and nickel alloys on request |
| Thread & dimensions | Go / no-go gauges to ASME B1.2; across flats, head height, length, fillet radius | Every lot, sampled to ASME B18.18 / F3125 Annex |
| Coating thickness | Magnetic gauge; ASTM A123 / F2329 / B695 sampling | All galvanised lots; thread fit checked with overtapped nuts |
| Non-destructive testing | Magnetic particle or ultrasonic | Critical-service and large-diameter bolts on request |
Tell us the pattern, grade, size, length, finish and quantity, and whether you need nuts and washers as an assembly. Our technical sales engineers respond within 2 to 4 business hours.