Hexagon socket head cap screws to ISO 4762 / DIN 912 from M1.6 to M64 and ASME B18.3 from No. 0 to 1½″. We make them in 12.9 and ASTM A574 alloy steel, in A2-70, A4-80 and F837 stainless, and in duplex, Inconel, Monel and titanium. Every socket is gauged, every lot is hardness tested, and each shipment carries an EN 10204 3.1 / 3.2 certificate.
A socket head bolt, properly a hexagon socket head cap screw, has a round head with a hexagon socket and is driven from above with a hex key. The head is about 1½ times the thread diameter across and as high as the diameter. It sinks into a counterbore and needs no room for a spanner, which is why it is the standard screw of machine tools, dies and moulds.
Exotic Alloy manufactures socket head cap screws in Mumbai, India, to ISO 4762 (the successor to DIN 912) and ASME B18.3. Alloy steel screws are cold headed with the socket formed in the die, then quenched and tempered to 12.9 or ASTM A574. Stainless and exotic alloy screws are headed or machined from certified bar. We stock and supply across India and export to Bahrain, the UAE, Saudi Arabia, Qatar, Kuwait, Oman, Europe and the Americas.
Standard metric and inch lengths in 12.9 / A574 black oxide and A2-70 / A4-70 stainless ship from stock. A4-80, F837 CW1, duplex, Inconel, Monel, Hastelloy and titanium socket cap screws, fine threads and special lengths are made to order in 7 to 15 business days.
The range we manufacture and stock.
This page covers the socket head cap screw itself. Related socket-drive products have their own pages, linked below.
The world's most common precision screw, with partial thread to the ISO 4762 b length or full thread on shorter screws. Made in 12.9 alloy steel with black oxide, in A2 / A4 stainless, and in exotic alloys.
UNRC and UNRF threads, class 3A, with head height equal to D and head diameter 1½D from ¼″ up. ASTM A574 alloy steel at 180 ksi up to ½″, or F837 stainless.
Fine pitches (M8×1, M10×1.25, M12×1.5 and more) for thin walls, fine adjustment and better resistance to loosening. Left-hand threads are made for rotating assemblies.
For food, pharmaceutical, marine and clean-room equipment. A4 (316) is used where chlorides are present, and A4-80 where more strength is wanted. Supplied passivated.
ISO 4762 geometry in super duplex, Inconel 625 / 718, Monel 400 / K-500, Hastelloy C-276 and titanium Grade 5, for subsea equipment, chemical process pumps, valves and aerospace.
Hollow and vented socket screws, countersunk socket screws, socket shoulder screws and Allen bolts are covered on their own pages.
| M12 example | Socket head cap screw (ISO 4762) | Hex bolt (ISO 4014) |
|---|---|---|
| Head | Round, 18 mm across, 12 mm high | Hexagon, 18 mm across flats, 7.5 mm high |
| Drive | 10 mm hex key, from above | 18 mm spanner or socket, from the side |
| Room needed | Above the head only; counterbore possible | Around the head for the spanner |
| Usual grade | 12.9 (1220 MPa) | 8.8 (800 MPa) |
| Typical use | Machine tools, dies, moulds, fixtures, compact assemblies | Steel structures, flanges, equipment, general engineering |
Metric dimensions from DIN EN ISO 4762:2004 and inch dimensions from ASME B18.3. The counterbore tab gives the minimum counterbore needed for each metric size, calculated from the standard's head limits.
| Size | Pitch P | dk max plain / knurled | k max head height | s hex key | t min key engagement | w min wall under socket | b thread length, ref. |
|---|---|---|---|---|---|---|---|
| M1.6 | 0.35 | 3 / 3.14 | 1.6 | 1.5 | 0.7 | 0.55 | 15 |
| M2 | 0.4 | 3.8 / 3.98 | 2 | 1.5 | 1 | 0.55 | 16 |
| M2.5 | 0.45 | 4.5 / 4.68 | 2.5 | 2 | 1.1 | 0.85 | 17 |
| M3 | 0.5 | 5.5 / 5.68 | 3 | 2.5 | 1.3 | 1.15 | 18 |
| M4 | 0.7 | 7 / 7.22 | 4 | 3 | 2 | 1.4 | 20 |
| M5 | 0.8 | 8.5 / 8.72 | 5 | 4 | 2.5 | 1.9 | 22 |
| M6 | 1 | 10 / 10.22 | 6 | 5 | 3 | 2.3 | 24 |
| M8 | 1.25 | 13 / 13.27 | 8 | 6 | 4 | 3.3 | 28 |
| M10 | 1.5 | 16 / 16.27 | 10 | 8 | 5 | 4 | 32 |
| M12 | 1.75 | 18 / 18.27 | 12 | 10 | 6 | 4.8 | 36 |
| M14 | 2 | 21 / 21.33 | 14 | 12 | 7 | 5.8 | 40 |
| M16 | 2 | 24 / 24.33 | 16 | 14 | 8 | 6.8 | 44 |
| M20 | 2.5 | 30 / 30.33 | 20 | 17 | 10 | 8.6 | 52 |
| M24 | 3 | 36 / 36.39 | 24 | 19 | 12 | 10.4 | 60 |
| M30 | 3.5 | 45 / 45.39 | 30 | 22 | 15.5 | 13.1 | 72 |
| M36 | 4 | 54 / 54.46 | 36 | 27 | 19 | 15.3 | 84 |
| M42 | 4.5 | 63 / 63.46 | 42 | 32 | 24 | 16.3 | 96 |
| M48 | 5 | 72 / 72.46 | 48 | 36 | 28 | 17.5 | 108 |
| M56 | 5.5 | 84 / 84.54 | 56 | 41 | 34 | 19 | 124 |
| M64 | 6 | 96 / 96.54 | 64 | 46 | 38 | 22 | 140 |
Dimensions in mm, product grade A. dk max is shown for plain and knurled heads. s is the hex key, t the minimum key engagement, w the minimum wall under the socket, and b the reference thread length of partially threaded screws. Shorter screws are threaded to the head.
| Size | Threads / in UNC / UNF | A head dia. max / min, in | H head height max / min, in | J hex key in | T key engagement min, in | G wall min, in |
|---|---|---|---|---|---|---|
| No. 0 | — / 80 | 0.096 / 0.091 | 0.060 / 0.057 | 0.050 | 0.025 | 0.020 |
| No. 1 | 64 / 72 | 0.118 / 0.112 | 0.073 / 0.070 | 1/16 | 0.031 | 0.025 |
| No. 2 | 56 / 64 | 0.140 / 0.134 | 0.086 / 0.083 | 5/64 | 0.038 | 0.029 |
| No. 3 | 48 / 56 | 0.161 / 0.154 | 0.099 / 0.095 | 5/64 | 0.044 | 0.034 |
| No. 4 | 40 / 48 | 0.183 / 0.176 | 0.112 / 0.108 | 3/32 | 0.051 | 0.038 |
| No. 5 | 40 / 44 | 0.205 / 0.198 | 0.125 / 0.121 | 3/32 | 0.057 | 0.043 |
| No. 6 | 32 / 40 | 0.226 / 0.218 | 0.138 / 0.134 | 7/64 | 0.064 | 0.047 |
| No. 8 | 32 / 36 | 0.270 / 0.262 | 0.164 / 0.159 | 9/64 | 0.077 | 0.056 |
| No. 10 | 24 / 32 | 0.312 / 0.303 | 0.190 / 0.185 | 5/32 | 0.090 | 0.065 |
| 1/4″ | 20 / 28 | 0.375 / 0.365 | 0.250 / 0.244 | 3/16 | 0.120 | 0.095 |
| 5/16″ | 18 / 24 | 0.469 / 0.457 | 0.312 / 0.306 | 1/4 | 0.151 | 0.119 |
| 3/8″ | 16 / 24 | 0.562 / 0.550 | 0.375 / 0.368 | 5/16 | 0.182 | 0.143 |
| 7/16″ | 14 / 20 | 0.656 / 0.642 | 0.438 / 0.430 | 3/8 | 0.213 | 0.166 |
| 1/2″ | 13 / 20 | 0.750 / 0.735 | 0.500 / 0.492 | 3/8 | 0.245 | 0.190 |
| 5/8″ | 11 / 18 | 0.938 / 0.921 | 0.625 / 0.616 | 1/2 | 0.307 | 0.238 |
| 3/4″ | 10 / 16 | 1.125 / 1.107 | 0.750 / 0.740 | 5/8 | 0.370 | 0.285 |
| 7/8″ | 9 / 14 | 1.312 / 1.293 | 0.875 / 0.864 | 3/4 | 0.432 | 0.333 |
| 1″ | 8 / 12 | 1.500 / 1.479 | 1.000 / 0.988 | 3/4 | 0.495 | 0.380 |
| 1-1/4″ | 7 / 12 | 1.875 / 1.852 | 1.250 / 1.236 | 7/8 | 0.620 | 0.475 |
| 1-1/2″ | 6 / 12 | 2.250 / 2.224 | 1.500 / 1.485 | 1 | 0.745 | 0.570 |
ASME B18.3 socket head cap screws (1960 series), dimensions in inches. Head height H equals D for every size; head diameter A is 1½D from ¼″ up and slightly larger on the numbered sizes. J is the hex key size, T the minimum key engagement and G the minimum wall thickness.
| Size | Clearance hole ISO 273 medium, mm | Head dk max knurled, mm | Min. counterbore dia. dk max + 0.5, mm | Head height k max mm | Depth for flush head k max, mm | Depth, head 0.5 mm below mm |
|---|---|---|---|---|---|---|
| M3 | 3.4 | 5.68 | 6.5 | 3 | 3 | 3.5 |
| M4 | 4.5 | 7.22 | 8 | 4 | 4 | 4.5 |
| M5 | 5.5 | 8.72 | 9.5 | 5 | 5 | 5.5 |
| M6 | 6.6 | 10.22 | 11 | 6 | 6 | 6.5 |
| M8 | 9 | 13.27 | 14 | 8 | 8 | 8.5 |
| M10 | 11 | 16.27 | 17 | 10 | 10 | 10.5 |
| M12 | 13.5 | 18.27 | 19 | 12 | 12 | 12.5 |
| M14 | 15.5 | 21.33 | 22 | 14 | 14 | 14.5 |
| M16 | 17.5 | 24.33 | 25 | 16 | 16 | 16.5 |
| M20 | 22 | 30.33 | 31 | 20 | 20 | 20.5 |
| M24 | 26 | 36.39 | 37 | 24 | 24 | 24.5 |
| M30 | 33 | 45.39 | 46 | 30 | 30 | 30.5 |
| M36 | 39 | 54.46 | 55 | 36 | 36 | 36.5 |
Our calculation, not a standard table. The counterbore must clear the maximum head diameter, including knurled heads; we add 0.5 mm and round up to 0.5 mm. For a flush head the depth must be at least k max. Clearance holes are ISO 273 medium series. Where your drawing standard gives preferred counterbore sizes, such as DIN 974-1, use those.
Socket cap screws are normally the strongest standard fasteners in a machine. Choose alloy steel for strength, and stainless or an exotic alloy where corrosion matters.
| Grade / class | Standard | Tensile Rm min, MPa | Yield Rp0.2 min, MPa | Hardness | Head marking | Typical use |
|---|---|---|---|---|---|---|
| 8.8 | ISO 898-1 | 800 (≤ M16) / 830 | 640 / 660 | 22–34 HRC | 8.8 + maker | General machinery where 12.9 is not needed |
| 10.9 | ISO 898-1 | 1040 | 940 | 32–39 HRC | 10.9 + maker | Machine tools, fixtures, automotive |
| 12.9 | ISO 898-1 | 1220 | 1100 | 39–44 HRC | 12.9 + maker | The standard socket cap screw: dies, moulds, high-load machinery |
| A574 (≤ ½″) | ASTM A574 | 1241 (180 ksi) | 1117 (162 ksi) | C39 min | Maker's mark | Inch alloy steel socket cap screws |
| A574 (≥ ⅝″) | ASTM A574 | 1172 (170 ksi) | 1055 (153 ksi) | C37 min | Maker's mark | Larger inch alloy steel socket cap screws |
| A2-70 / A4-70 | ISO 3506-1 | 700 | 450 | — | A2-70 / A4-70 | Stainless, up to M24 per ISO 4762 |
| A4-80 | ISO 3506-1 | 800 | 600 | — | A4-80 | Higher-strength 316 stainless |
| A2-50 / A4-50 | ISO 3506-1 | 500 | 210 | — | A2-50 / A4-50 | Stainless above M24 to M39 per ISO 4762 |
| Size | Stress area UNC, in² | Tensile strength ksi | Min. tensile load lbf (kN) | Min. yield load lbf (kN) | Hardness |
|---|---|---|---|---|---|
| 1/4″ | 0.0318 | 180 | 5,730 (25.5) | 5,160 (22.9) | C39 min |
| 5/16″ | 0.0524 | 180 | 9,440 (42.0) | 8,490 (37.8) | C39 min |
| 3/8″ | 0.0775 | 180 | 13,950 (62.0) | 12,550 (55.8) | C39 min |
| 7/16″ | 0.1063 | 180 | 19,140 (85.1) | 17,220 (76.6) | C39 min |
| 1/2″ | 0.1419 | 180 | 25,540 (113.6) | 22,990 (102.3) | C39 min |
| 5/8″ | 0.2260 | 170 | 38,420 (170.9) | 34,580 (153.8) | C37 min |
| 3/4″ | 0.3345 | 170 | 56,860 (252.9) | 51,170 (227.6) | C37 min |
| 7/8″ | 0.4617 | 170 | 78,490 (349.2) | 70,650 (314.2) | C37 min |
| 1″ | 0.6057 | 170 | 102,980 (458.1) | 92,680 (412.3) | C37 min |
| 1-1/4″ | 0.9691 | 170 | 164,750 (732.8) | 148,270 (659.6) | C37 min |
| 1-1/2″ | 1.4053 | 170 | 238,890 (1062.6) | 215,000 (956.4) | C37 min |
Our calculation: UNC tensile stress area (ASME B1.1) times the A574 minimum tensile (180 ksi to ½″, 170 ksi from ⅝″) and yield (162 / 153 ksi). The results agree with published A574 load tables within 0.2 %.
| Group / condition | Designation | Diameter in | Full-size tensile ksi | Yield ksi | Elongation % min | Hardness |
|---|---|---|---|---|---|---|
| Group 1 or 2, AF (annealed) | F837A (Gr 1) / F837D (Gr 2) | 0.060–1.500 | 85 max | 55 max | 40 | 95 HRB max |
| Group 1 or 2, CW (cold worked) | F837B (Gr 1) / F837E (Gr 2) | 0.060–0.750 | 80 min | 40 min | 25 | 50 HRA min |
| Group 1 or 2, CW1 (cold worked) | F837C (Gr 1) / F837F (Gr 2) | 0.060–0.750 | 102 min | 65 min | 20 | 59 HRA min |
| Group 5, HT (martensitic) | F837G | 0.060–0.190 | 180 min | — | — | 36–43 HRC |
| Group 5, HT (martensitic) | F837H | 0.250–1.500 | 180 min | 120 min | 12 | 36–43 HRC |
Group 1 covers the 300-series austenitic grades such as 304, Group 2 the 316 family and Group 5 a martensitic grade. CW and CW1 apply up to ¾″; above that, properties and marking are agreed between buyer and manufacturer.
Every grade links to its datasheet on this site.
4137 / 4140 type steels with enough hardenability to reach 39 to 44 HRC right through the head and the thread. The workhorse of socket cap screws.
904L for acids, Nitronic 60 against galling, A-286 for strength at temperature.
About twice the yield strength of 316, with resistance to chloride stress corrosion cracking.
| 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 |
Bar minimums from the linked datasheets for the supply condition shown. Exotic alloy screws are certified to the material specification, not to an ISO 898-1 property class.
A socket cap screw is a high-strength part with a hole in its head. Most of the manufacturing effort goes into keeping the head, the socket and the fillet sound.
Cr-Mo wire rod annealed so that it can be cold headed. It is received by heat with mill certificates and checked for surface seams, which would open under the heading punch.
A multi-station header upsets the head and backward-extrudes the hex socket in the same blows. This keeps grain flow continuous around the socket and the underhead fillet. Exotic alloys and large sizes are hot forged or machined.
Head knurled on request, top edge chamfered and the point formed. Lengths are cut and the blank is checked for head height and concentricity.
Threads rolled to 6g (5g6g / 4g6g on request) or UNRC / UNRF class 3A with the rounded root that the inch standards require. They are rolled before or after heat treatment as the specification demands.
Hardened and tempered in protective-atmosphere furnaces to 39 to 44 HRC for 12.9, 32 to 39 HRC for 10.9, and ASTM A574 minimums. Carbon potential is controlled to avoid decarburization at the thread and socket.
Black oxide and oil for alloy screws, zinc flake or baked plating on request, and passivation for stainless.
Hardness, wedge or axial tensile and proof load per lot. Socket gauged go / no-go to ISO 23429, thread gauged, and decarburization checked on 12.9.
A cap screw that fails in service usually fails at the first engaged thread or at the underhead fillet. Cold heading pushes the metal into the head and around the socket, so the grain lines follow the outline of the part. A head turned from large bar cuts through the grain right at the fillet and leaves the corner weaker in fatigue.
That is why standard socket cap screws are cold headed. Sizes or alloys that cannot be headed are hot forged, and we machine from bar only where neither is possible. The method is stated on the certificate.
The finish on a 12.9 screw is chosen as much for its effect on the steel as for corrosion. See also our coated fasteners page.
| Finish | Specification | Corrosion | Suitable for | Notes |
|---|---|---|---|---|
| Black oxide & oil | Thermal or chemical black, oiled | Indoor, oiled storage | 8.8 to 12.9, A574 | The traditional socket cap screw finish; no dimensional change, no hydrogen risk from the thermal process |
| Zinc flake | ISO 10683 / ASTM F1136 | 600 to 1000 h salt spray | All classes including 12.9 | Non-electrolytic, no hydrogen embrittlement; the recommended corrosion finish for 12.9 |
| Zinc-nickel | ISO 19598 / ASTM B841 | High | 8.8, 10.9; 12.9 with care | Electroplated, so baked after plating to relieve hydrogen |
| Zinc electroplate | ISO 4042 / ASTM F1941 | Moderate | 8.8; 10.9 / 12.9 only with baking | Thin plating can affect socket and thread fit on small sizes |
| Passivated | ASTM A967 / A380 | Full stainless resistance | A2, A4, F837, duplex | Restores the passive film after heading and rolling |
| PTFE / fluoropolymer | To drawing | High with a zinc base | Offshore and chemical plant | Low and consistent friction; reduce tightening torque accordingly |
Guide values by the VDI 2230 method for ISO 4762 screws, coarse thread, at 90 % of minimum yield. The head bearing face is taken as the head diameter dk over an ISO 273 medium clearance hole. The table uses a friction coefficient of 0.12, typical of oiled black oxide; use the calculator for other finishes.
FM = 0.9 · Rp0.2 · A0 / √(1 + 3[2 (d2/d0)(P/πd2 + 1.155 μ)]²); MA = FM(0.16 P + 0.58 d2 μ + μ DKm/2), DKm = (dk + dh)/2. Guide values only; confirm for the joint.
| Size | FM 8.8 kN | MA 8.8 N·m | FM 10.9 kN | MA 10.9 N·m | FM 12.9 kN | MA 12.9 N·m |
|---|---|---|---|---|---|---|
| M3 | 2.3 | 1.3 | 3.4 | 1.8 | 4.0 | 2.1 |
| M4 | 4.1 | 2.9 | 6.0 | 4.2 | 7.0 | 4.9 |
| M5 | 6.6 | 5.7 | 9.8 | 8.4 | 11.4 | 9.8 |
| M6 | 9.4 | 9.7 | 13.8 | 14 | 16.1 | 17 |
| M8 | 17.2 | 23 | 25.3 | 34 | 29.6 | 40 |
| M10 | 27.4 | 46 | 40.3 | 68 | 47.1 | 79 |
| M12 | 40.0 | 79 | 58.7 | 116 | 68.7 | 136 |
| M14 | 54.9 | 126 | 80.6 | 185 | 94.3 | 217 |
| M16 | 75.5 | 195 | 110.8 | 287 | 129.7 | 336 |
| M20 | 121.6 | 394 | 173.2 | 561 | 202.6 | 656 |
| M24 | 175.1 | 678 | 249.4 | 966 | 291.8 | 1131 |
| M30 | 280.1 | 1353 | 399.0 | 1927 | 466.9 | 2255 |
| M36 | 409.7 | 2358 | 583.5 | 3358 | 682.8 | 3930 |
FM is the assembly preload and MA the torque that produces it. Into tapped holes in aluminium or cast iron, check the thread engagement before using 12.9 values: the internal thread, not the screw, may set the limit.
12.9 and A574 cap screws in counterbored die sets, injection moulds, press tools and jigs, where strength and flush heads are essential.
Spindles, slides, ball-screw mounts, robot arms and linear guides, fastened in close patterns that a spanner could not reach.
Cylinder heads, gearboxes, pumps and suspension parts in 10.9 and 12.9, often with zinc flake coatings.
Manifold blocks, valve bonnets and cylinder end caps, where the socket head sits in a counterbore and carries high pressure loads.
A2 and A4 stainless socket cap screws in hygienic equipment, packaging machines and clean rooms.
A4-80, super duplex, Inconel 625 and Monel K-500 socket cap screws for seawater, subsea controls and chemical pumps.
| Test | Method | Applied to |
|---|---|---|
| Chemical analysis / PMI | Mill certificate; XRF PMI | Every heat; 100 % of stainless and exotic alloy lots |
| Tensile & wedge tensile | ISO 898-1 / ASTM F606 / A574 | Every heat-treatment lot |
| Proof load | ISO 898-1 / ASTM F606 | 8.8 to 12.9 and A574 |
| Hardness | Rockwell C, surface and core | Every lot |
| Decarburization | ISO 898-1 hardness traverse or micro | 12.9 and A574 lots |
| Socket | Go / no-go hexagon gauges to ISO 23429; key engagement t | Every lot |
| Thread & dimensions | Thread gauges; head diameter, height, length, fillet | Every lot to ISO 4762 or ASME B18.3 |
| Head soundness | Visual and magnified inspection for heading cracks and seams | Every lot; magnetic particle on request |
Tell us the standard, size, length, grade, finish and quantity. Our engineers respond within 2 to 4 business hours with pricing and lead time.