12.9 Black Allen Cap Screws
ISO 4762 / DIN 912 · M3 – M36The workhorse of tooling, dies, moulds and machinery, with full 12.9 strength and black oxide finish.
12.9 High Tensile Allen Cap Screws & the Allen Family: Manufacturer & Supplier
An Allen bolt is a screw driven by a hex key in a socket in its head. The black, cylindrical-head version in the photo is the hexagon socket head cap screw to ISO 4762 / DIN 912 in class 12.9, one of the strongest standard bolts made.
Exotic Alloy manufactures Allen bolts in Mumbai, India, and supplies tool rooms, machine builders, mould makers and maintenance teams across India. We also export to Malaysia, the Gulf, Europe and Africa. Heads and sockets are cold formed in one piece. 12.9 bolts are made from Cr-Mo or boron alloy steel, quenched and tempered, and black oxide finished.
Why choose an Allen bolt?
The head is only about 1.5 to 1.8 × d across and is turned from above. Bolts can sit close together, close to walls, or below the surface in a counterbore, and they reach about 50 % more tensile load than an 8.8 hex bolt of the same size.
DIN 912 / ISO 4762
Class 12.9, 10.9, 8.8
Black oxide & zinc flake
A2 / A4 stainless
Inch ASME B18.3 / A574
Allen bolt at a glance
- Head
- Cylindrical, dk 1.5–1.8 d, height k = d
- Drive
- Hex socket; M10 takes an 8 mm key
- Class
- 12.9: 1220 MPa tensile, 39–44 HRC
- Finish
- Black oxide (standard), zinc flake
- Standards
- ISO 4762, DIN 912, ASME B18.3
- Related
- Socket head bolts, hollow Allen, shoulder bolts
Allen Bolt Specification
The range we manufacture and supply.
StandardsISO 4762 / DIN 912 cap screws; DIN 7984 low head; ISO 7380-1 / -2 button head; ISO 10642 countersunk; ISO 4026–4029 set screws; ASME B18.3 inch
Size rangeM3 to M36 (larger to order); No. 4 to ¾″ and above in inch; lengths per standard, special lengths to order
ThreadMetric coarse 6g (5g6g for 12.9 cap screws per ISO 4762), fine pitch to order; UNC / UNF 3A; part or full thread by length
Property class12.9, 10.9, 8.8 (ISO 898-1); ASTM A574 inch; A2-70, A4-70, A4-80 (ISO 3506-1)
MaterialsCr-Mo alloy steel (4135, 4137, 4140), boron steel, stainless 304 / 316; titanium and nickel alloys to order
FinishBlack oxide and oil (standard for 12.9); zinc flake (ISO 10683); plain; passivated stainless
MarkingProperty class and maker’s mark on the head (top or side) for M5 and above; A2-70 / A4-70 on stainless
Head optionsPlain or knurled cap head; low head; button; flanged button; countersunk; hollow / vented (see hollow Allen)
CertificationMill certificate; EN 10204 3.1 with hardness, tensile and dimensional results
PackingBoxes of 100 / 200 / 500, bulk cartons, or labelled kits with matching Allen keys
Types of Allen Bolts We Manufacture
The same hex drive, six heads. Each one sits differently above or below the part surface.
| Allen type (M8) | Standard | Head dia. mm | Head height mm | Hex key mm | Socket depth t min, mm | Load rating |
|---|---|---|---|---|---|---|
| Cap screw | ISO 4762 / DIN 912 | 13 | 8 | 6 | 4 | Full class load (8.8, 10.9, 12.9) |
| Low head cap screw | DIN 7984 | 13 | 5 | 5 | 3.8 | Reduced head: lower load |
| Button head | ISO 7380-1 | 14 | 4.4 | 5 | 2.6 | Reduced loadability |
| Flanged button head | ISO 7380-2 | 13.2 (flange 17.8) | 4.4 | 5 | 2.6 | Reduced loadability |
| Countersunk head | ISO 10642 | 17.92 | 4.96 | 5 | 2.9 | Reduced loadability |
| Set screw, flat point | ISO 4026 | — | — | 4 | 3 | Not for tensile load |
Dimensions in mm, maximum head sizes. ISO 7380-1, ISO 7380-2 and ISO 10642 are titled “with reduced loadability”: their heads cannot carry the full ISO 898-1 loads of the class marked on them. Use an ISO 4762 cap screw wherever the bolt must carry high tensile load.
Low Head Allen Bolts
DIN 7984 · M3 – M24A head about 60 % as tall as ISO 4762, for shallow counterbores and tight spaces.
Button & Flanged Button Head
ISO 7380-1 / -2 · M3 – M16Smooth, low dome for guards, covers and panels. The flanged version spreads the load without a washer.
Countersunk Allen & Set Screws
ISO 10642 · ISO 4026–4029Flush heads for slides and fixtures, and headless set (grub) screws for collars and pulleys. See countersunk bolts.
Allen Key Size Chart: Metric & Inch
The key depends on the head type as well as the thread size. An M8 bolt can take a 6, 5 or 4 mm key.
| Thread | Cap screw ISO 4762 / DIN 912 | Low head DIN 7984 | Button head ISO 7380-1 / -2 | Countersunk ISO 10642 | Set screw ISO 4026–4029 |
|---|---|---|---|---|---|
| M3 | 2.5 mm | 2 mm | 2 mm | 2 mm | 1.5 mm |
| M4 | 3 mm | 2.5 mm | 2.5 mm | 2.5 mm | 2 mm |
| M5 | 4 mm | 3 mm | 3 mm | 3 mm | 2.5 mm |
| M6 | 5 mm | 4 mm | 4 mm | 4 mm | 3 mm |
| M8 | 6 mm | 5 mm | 5 mm | 5 mm | 4 mm |
| M10 | 8 mm | 7 mm | 6 mm | 6 mm | 5 mm |
| M12 | 10 mm | 8 mm | 8 mm | 8 mm | 6 mm |
| M14 | 12 mm | 10 mm | — | 10 mm | — |
| M16 | 14 mm | 12 mm | 10 mm | 10 mm | 8 mm |
| M20 | 17 mm | 14 mm | — | 12 mm | 10 mm |
| M24 | 19 mm | 17 mm | — | — | 12 mm |
| M30 | 22 mm | — | — | — | — |
| M36 | 27 mm | — | — | — | — |
Nominal hex socket size s (= Allen key across flats) from ISO 4762, DIN 7984:2022, ISO 7380-1 / -2, ISO 10642:2020 and ISO 4026. Cone, dog and cup point set screws (ISO 4027 to 4029) use the same keys as ISO 4026. — = size not in that standard.
| Size | Cap screw B18.3, A574 | Button head | Flat (countersunk) head | Set screw |
|---|---|---|---|---|
| No. 4 | 3/32″ | 1/16″ | 1/16″ | 0.050″ |
| No. 6 | 7/64″ | 5/64″ | 5/64″ | 1/16″ |
| No. 8 | 9/64″ | 3/32″ | 3/32″ | 5/64″ |
| No. 10 | 5/32″ | 1/8″ | 1/8″ | 3/32″ |
| 1/4″ | 3/16″ | 5/32″ | 5/32″ | 1/8″ |
| 5/16″ | 1/4″ | 3/16″ | 3/16″ | 5/32″ |
| 3/8″ | 5/16″ | 7/32″ | 7/32″ | 3/16″ |
| 1/2″ | 3/8″ | 5/16″ | 5/16″ | 1/4″ |
| 5/8″ | 1/2″ | 3/8″ | 3/8″ | 5/16″ |
| 3/4″ | 5/8″ | — | 1/2″ | 3/8″ |
Nominal hex key sizes per ASME B18.3 for socket head cap, button head, flat countersunk head and set screws, in inches. Inch button heads are not standardized above ⅝″.
| Metric socket | Largest inch key that goes in | Key across flats | Slack | Result |
|---|---|---|---|---|
| 2 mm | 5/64″ | 1.98 mm | 0.02 mm | Close fit, but use the metric key |
| 2.5 mm | 3/32″ | 2.38 mm | 0.12 mm | Loose: the key twists and rounds the socket corners |
| 3 mm | 7/64″ | 2.78 mm | 0.22 mm | Loose: the key twists and rounds the socket corners |
| 4 mm | 5/32″ | 3.97 mm | 0.03 mm | Close fit, but use the metric key |
| 5 mm | 3/16″ | 4.76 mm | 0.24 mm | Loose: the key twists and rounds the socket corners |
| 8 mm | 5/16″ | 7.94 mm | 0.06 mm | Close fit, but use the metric key |
Our calculation from nominal key sizes. Several inch keys slide into metric sockets and feel right, but a loose key twists under load and bears on the socket corners. A rounded socket is the most common reason a good Allen bolt cannot be removed.
Tightening Allen Bolts Without Rounding the Socket
Do
- Use a metric key in metric screws, and replace worn keys
- Clean the socket and push the key to the bottom
- Do the final tightening with a straight hex bit on a torque wrench
- Seat the head square in a counterbore with a little clearance
Don't
- Apply final torque through the ball end of a key
- Extend the key with a pipe: it overloads key and socket
- Reuse a 12.9 bolt with a rounded socket or damaged thread
- Swap a 12.9 cap screw for a button or countersunk screw in a loaded joint
Allen Bolt Grades & 12.9 Load Table
| Grade | Standard | Tensile Rm min, MPa | Yield Rp0.2 min, MPa | Proof stress MPa | Hardness | Use |
|---|---|---|---|---|---|---|
| 8.8 | ISO 898-1 | 800 (≤ M16) / 830 | 640 / 660 | 580 / 600 | 22–32 / 23–34 HRC | General machinery |
| 10.9 | ISO 898-1 | 1040 | 940 | 830 | 32–39 HRC | Automotive, fixtures |
| 12.9 | ISO 898-1 | 1220 | 1100 | 970 | 39–44 HRC | The standard black Allen bolt: dies, moulds, machine tools |
| A574 (≤ ½″) | ASTM A574 | 1241 (180 ksi) | 1117 (162 ksi) | — | C39 min | Inch alloy steel socket cap screws |
| A574 (≥ ⅝″) | ASTM A574 | 1172 (170 ksi) | 1055 (153 ksi) | — | C37 min | Larger inch sizes |
| A2-70 / A4-70 | ISO 3506-1 | 700 | 450 | — | — | Stainless Allen bolts to M24 |
| A4-80 | ISO 3506-1 | 800 | 600 | — | — | Higher-strength 316 stainless |
8.8 values: first figure up to M16, second above M16. ISO 4762 lists stainless classes 70 up to M24 and 50 above.
Minimum loads by size
| Size | Stress area As mm² | 8.8 min. tensile kN | 10.9 min. tensile kN | 12.9 proof load kN | 12.9 min. tensile kN | 12.9 hex key mm |
|---|---|---|---|---|---|---|
| M3 | 5.03 | 4.0 | 5.2 | 4.9 | 6.1 | 2.5 |
| M4 | 8.78 | 7.0 | 9.1 | 8.5 | 10.7 | 3 |
| M5 | 14.2 | 11.4 | 14.8 | 13.8 | 17.3 | 4 |
| M6 | 20.1 | 16.1 | 20.9 | 19.5 | 24.5 | 5 |
| M8 | 36.6 | 29.3 | 38.1 | 35.5 | 44.7 | 6 |
| M10 | 58 | 46.4 | 60.3 | 56.3 | 70.8 | 8 |
| M12 | 84.3 | 67.4 | 87.7 | 81.8 | 102.8 | 10 |
| M14 | 115 | 92.0 | 119.6 | 111.5 | 140.3 | 12 |
| M16 | 157 | 125.6 | 163.3 | 152.3 | 191.5 | 14 |
| M20 | 245 | 203.3 | 254.8 | 237.7 | 298.9 | 17 |
| M24 | 353 | 293.0 | 367.1 | 342.4 | 430.7 | 19 |
| M30 | 561 | 465.6 | 583.4 | 544.2 | 684.4 | 22 |
| M36 | 817 | 678.1 | 849.7 | 792.5 | 996.7 | 27 |
Calculated as As × stress (8.8: 800 / 830 MPa; 10.9: 1040; 12.9: proof 970, tensile 1220 MPa). These match the ISO 898-1 load tables to within their rounding, and apply to ISO 4762 cap screws only.
Allen Key & Tightening Torque Finder
Choose the screw type and size to get the Allen key and socket depth. For ISO 4762 cap screws you also get the preload and torque at 90 % of yield (VDI 2230 method).
Your screw
μ 0.12 suits black oxide and oiled 12.9 bolts; use 0.10 for zinc flake with lubricant and 0.14 to 0.16 for dry or plain parts. The torque values are guidance; confirm them by test for critical joints.
Result
8 mm
Allen key
79
N·m torque
Socket depth t min:5 mm
Preload FM:47.1 kN
Proof load (class):56.3 kN
Allen Bolt Materials & Finishes
Every grade links to its datasheet on this site.
| Finish | Specification | Corrosion protection | Notes for 12.9 |
|---|---|---|---|
| Black oxide + oil | Chemical / thermal blackening | Light: indoor, oiled machinery | Standard finish; no thickness, no hydrogen risk from coating |
| Zinc flake | ISO 10683 (e.g. silver top coat) | High: outdoor and automotive | Non-electrolytic, so the preferred coating for 12.9 |
| Zinc / zinc-nickel plated | ISO 4042 | Medium | Hydrogen embrittlement risk at 39–44 HRC; baking required, only when specified |
| Passivated stainless | ASTM A967 / A380 | High | A2 / A4 Allen bolts; not hardened |
How Our Allen Bolts Are Made
1
Wire
Spheroidize-annealed Cr-Mo or boron wire, received by heat with mill certificates.
2
Heading
Multi-station cold heading forms the head and the hex socket in one piece, with no machining.
3
Heat treatment
Hardened and tempered in controlled atmosphere to 39–44 HRC without decarburizing.
4
Thread rolling
Threads rolled; rolling after heat treatment improves fatigue strength.
5
Finishing
Black oxide and oil, zinc flake, or passivation for stainless.
6
Inspection
Socket gauged, hardness and tensile tested, threads checked with ring gauges.
Where Our Allen Bolts Are Used
Press tools, dies & moulds
12.9 cap screws in counterbores clamp die sets and mould plates.
Machine tools & spindles
Slides, way covers, tool holders and fixtures in tight spaces.
Automation & robotics
Aluminium profiles, linear guides and grippers, with button heads on covers.
Hydraulics
Valve blocks, cylinder heads and manifolds bolted with 12.9.
Automotive & motorsport
Engine, gearbox and chassis parts; titanium Allen bolts where weight matters.
Maintenance stores
Boxed mixed kits of cap, button and set screws with matching keys.
Allen Bolt Inspection & Certification
| Check | Method | Applied to |
|---|---|---|
| Hex socket | Go / no-go socket gauges (ISO 23429) | Every lot |
| Head & length | Callipers against dk, k, l | Every lot |
| Thread | Ring gauges | Every lot |
| Hardness | Rockwell C, core and surface | Every heat-treatment lot |
| Tensile / proof load | ISO 898-1 | 12.9, 10.9 and 8.8 lots |
| Decarburization | Microsection, ISO 898-1 limits | 12.9 and 10.9 lots |
Every shipment includes
- Mill certificate with heat number
- Hardness and dimensional report
- Lot traceability on every box
On request
- EN 10204 3.1 certificate
- Hydrogen embrittlement test (ISO 15330) for coated 12.9
- Salt spray test for coated lots
Allen Bolt Questions, Answered
An Allen bolt is a screw with a hexagon socket in its head, turned with an Allen (hex) key instead of a spanner. The most common type is the cylindrical head socket cap screw to ISO 4762 or DIN 912, usually in class 12.9 alloy steel with a black oxide finish. The name comes from the Allen Manufacturing Company, an early US maker of hex socket screws and keys.
An M8 cap screw to ISO 4762 or DIN 912 takes a 6 mm Allen key. Other M8 Allen screws use smaller keys: 5 mm for DIN 7984 low head, ISO 7380 button head and ISO 10642 countersunk head screws, and 4 mm for ISO 4026 to 4029 set screws. The full chart for M3 to M36 is on this page.
12.9 is the ISO 898-1 property class. The first number times 100 is the nominal tensile strength (1200 MPa; the minimum is 1220 MPa) and the second number means the yield strength is 90 % of that, so at least 1100 MPa. A 12.9 bolt is quenched and tempered to 39 to 44 HRC and is the usual grade for black Allen cap screws.
Yes. Allen bolt is the everyday name for a hexagon socket head cap screw to ISO 4762, DIN 912 or ASME B18.3. Its small cylindrical head fits in a counterbore and is turned from above with a key, so it needs less space than a hex bolt and spanner. Button, countersunk and low head screws and set screws use the same Allen key drive.
Most 12.9 Allen bolts are black oxide finished: the heat-treated alloy steel is blackened and oiled. The finish adds no thickness, so the thread fit is unchanged, but it gives only light corrosion protection for oiled or indoor machinery. For outdoor or wet service use zinc flake coated or A2 / A4 stainless Allen bolts.
Electroplating a 12.9 bolt carries a real risk of hydrogen embrittlement because of its high hardness (39 to 44 HRC), which can make it crack without warning after tightening. If plating is unavoidable it must be followed by baking per ISO 4042. The safer choice is a non-electrolytic zinc flake coating to ISO 10683.
For an ISO 4762 M10 class 12.9 Allen bolt with a friction coefficient of 0.12, tightening to 90 % of yield gives about 79 N·m and a preload of about 47 kN (VDI 2230 method). The same size in 8.8 takes about 46 N·m. Oiled, coated or stainless parts need different values, so use the calculator on this page and confirm by test for critical joints.
We supply Allen cap screws from M3 to M36 in class 12.9, 10.9 and 8.8 and in A2 and A4 stainless, inch socket cap screws to ASME B18.3 and ASTM A574, and low head, button, flanged button, countersunk and set screws. Larger sizes, special lengths and other materials are made to order.