Hex Socket Head Cap SEMS Screw FW + LW – Alloy Steel 12.9 Black
FULLERKREG Class 12.9 Alloy Steel Socket Cap Head SEMS Screws combine an ISO 4762 hex socket cap head screw with two permanently captivated washers — a DIN 125 standard flat washer (FW) and a lock washer (LW) — assembled onto the shank by thread rolling. Manufactured from 40Cr alloy steel and finished with a black oxide (blackening) surface treatment, these screws deliver the highest strength class available in the SEMS range — minimum tensile strength of 1220 MPa — combined with the assembly efficiency of a pre-assembled captive washer system. Designed for high-load, vibration-intensive, and space-critical applications in automotive, machinery, tooling, and heavy industrial environments.
What are Socket Cap Head SEMS Screws?
SEMS (pre-assembled screw and washer) screws are integrated fasteners that permanently combine a socket cap head screw with one or more captive washers into a single handling unit. The washers are retained on the screw shank during manufacturing via a thread-rolling process: washers are loaded onto the screw blank before threads are rolled, and the resulting thread shoulder prevents washer removal while allowing free rotation.
The FW + LW (Flat Washer + Lock Washer) double washer configuration pairs a DIN 125 standard flat washer with a spring lock washer. The flat washer distributes the clamping load over a wider bearing area, while the lock washer provides mechanical resistance to loosening under vibration and dynamic loading. The hex socket (Allen key) drive head conforms to ISO 4762, delivering high torque capacity and compatibility with standard hex key tooling.
In the Class 12.9 alloy steel grade, the screw body is manufactured from 40Cr chromium alloy steel, heat-treated to achieve the exceptional mechanical properties of property class 12.9. The black oxide finish provides a uniform matte black appearance, mild corrosion protection, and reduced light reflection — making these screws a preferred choice in precision machinery, automotive, and tooling applications where both performance and aesthetics matter.
Benefits of Using SEMS Screws
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Exceptional tensile strength — Class 12.9 alloy steel delivers a minimum tensile strength of 1220 MPa and proof load stress of 970 MPa, making these the highest-strength SEMS screws in the FULLERKREG range, suitable for the most demanding structural and dynamic load applications.
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Superior vibration resistance — the integrated spring lock washer provides mechanical anti-loosening performance under high vibration, shock, and dynamic loading without additional locking compounds or inserts.
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Eliminates loose washers — the DIN 125 flat washer and lock washer are permanently captivated; they cannot be lost during kitting, handling, or installation, eliminating assembly errors and line stoppages.
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Faster assembly — one integrated component replaces three separate parts (screw + flat washer + lock washer), reducing cycle time and operator handling steps in high-volume production.
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Consistent washer positioning — the washer is always correctly seated and centred under the head, ensuring uniform clamping load and preventing misalignment or tilting.
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Black oxide finish — provides a uniform matte black appearance for aesthetic consistency in visible assemblies, reduces light reflection in optical and precision instrument applications, and offers mild corrosion protection in dry indoor environments.
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Compact high-strength fastening — the socket cap head profile provides a low-profile, high-strength fastening solution suitable for confined spaces and recessed hole applications where hex head bolts cannot be used.
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Reduced inventory complexity — one SKU replaces three separate part numbers, simplifying procurement, storage, and BOM management.
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Automation-friendly — the single-piece design is compatible with automated feeding and driving equipment, supporting high-volume production lines.
Limitations of Using SEMS Screws
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Limited corrosion resistance — alloy steel with black oxide finish provides only mild corrosion protection; not suitable for outdoor, marine, or high-humidity environments without additional protective coatings. Specify stainless steel grades for corrosive environments.
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Hydrogen embrittlement risk — Class 12.9 high-strength alloy steel is susceptible to hydrogen embrittlement during electroplating processes; black oxide finish is preferred over zinc plating for this reason. Avoid acid cleaning or electroplating after assembly.
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Fixed washer configuration — the DIN 125 washer type and size are determined at manufacture; they cannot be changed or substituted in the field.
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Higher unit cost than lower grades — Class 12.9 alloy steel carries a material and processing premium over Class 8.8 or 10.9; specify 12.9 only where the load requirements justify the cost.
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Requires controlled tightening — the high strength of Class 12.9 requires accurate torque control during installation to avoid over-tightening and thread stripping in lower-strength mating materials.
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Washer cannot be removed for inspection — the captive design means washer condition cannot be independently inspected or replaced without replacing the entire fastener.
Material / Finish
| Component |
Material |
Grade / Standard |
Surface Treatment |
| Screw Body |
40Cr Chromium Alloy Steel |
Class 12.9 (ISO 898-1) |
Black Oxide (Blackening) |
| Standard Flat Washer (DIN 125) |
Alloy Steel / Carbon Steel |
DIN 125 / ISO 7089 |
Black Oxide |
| Lock Washer |
Alloy Steel / Carbon Steel |
ISO Standard |
Black Oxide |
Class 12.9 Mechanical Properties (ISO 898-1)
| Property |
Class 8.8 |
Class 10.9 |
Class 12.9 |
| Min. Tensile Strength (MPa) |
800 |
1040 |
1220 |
| Min. Yield Strength (MPa) |
640 |
940 |
1100 |
| Proof Load Stress (MPa) |
580 |
830 |
970 |
| Min. Elongation (%) |
12 |
9 |
8 |
| Hardness (HRC) |
22–32 |
32–39 |
39–44 |
Black Oxide Finish Properties
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Appearance: Uniform matte black; no dimensional change (coating thickness <1 μm).
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Corrosion resistance: Mild — suitable for dry indoor environments; apply oil or wax for enhanced protection.
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Temperature resistance: Stable up to approximately 120°C continuous service.
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No hydrogen embrittlement risk: Black oxide is a chemical conversion coating, not an electroplating process, making it safe for Class 12.9 high-strength fasteners.
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RoHS compliant: Black oxide process does not use hexavalent chromium or other restricted substances.
Dimensions of Pre-Assembled Washers in SEMS Screws
The FW + LW configuration uses DIN 125 standard flat washers (equivalent to ISO 7089). DIN 125 specifies the most widely used standard washer outer diameter, providing a balanced bearing surface for general industrial fastening. The following table shows the inner diameter (d1), outer diameter (d2), and thickness (t) of the captive DIN 125 flat washers by thread size:
DIN 125 Standard Plain Washer (FW – used in this SEMS configuration)
| Thread Size |
M2 |
M2.5 |
M3 |
M4 |
M5 |
M6 |
M8 |
M10 |
M12 |
M14 |
M16 |
| d1 – Inner Dia. (mm) |
2.2 |
2.7 |
3.2 |
4.3 |
5.3 |
6.4 |
8.4 |
10.5 |
13.0 |
15.0 |
17.0 |
| d2 – Outer Dia. (mm) |
5.0 |
6.0 |
7.0 |
9.0 |
10.0 |
12.5 |
17.0 |
21.0 |
24.0 |
28.0 |
30.0 |
| t – Thickness (mm) |
0.3 |
0.5 |
0.5 |
0.8 |
1.0 |
1.6 |
1.6 |
2.0 |
2.5 |
2.5 |
3.0 |
All dimensions per DIN 125 / ISO 7089 standard. All dimensions in mm.
Dimensional Data – Metric Series (ISO 4762 Head, Class 12.9)
All dimensions in mm. Head dimensions conform to ISO 4762. Thread tolerance Class 6g. Washer diameter column (D) reflects the DIN 125 standard washer outer diameter.
| Thread |
Pitch (mm) |
A – Head Diam (mm) |
B – Head Ht (mm) |
C – A/F Recess (mm) |
D – Washer Diam (mm) |
E – Washers Thk (mm) ② |
F – ① (mm) |
Unit Weight (g) |
| M3 |
0.5 |
5.5 |
3.0 |
2.5 |
7.0 |
— |
3.8 |
— |
| M4 |
0.7 |
7.0 |
4.0 |
3.0 |
9.0 |
— |
4.6 |
— |
| M5 |
0.8 |
8.5 |
5.0 |
4.0 |
10.0 |
— |
5.6 |
— |
| M6 |
1.0 |
10.0 |
6.0 |
5.0 |
12.5 |
— |
5.9 |
— |
| M8 |
1.25 |
13.0 |
8.0 |
6.0 |
17.0 |
— |
7.8 |
— |
| M10 |
1.5 |
16.0 |
10.0 |
8.0 |
21.0 |
— |
9.4 |
— |
| M12 |
1.75 |
18.0 |
12.0 |
10.0 |
24.0 |
— |
11.1 |
— |
| M14 |
2.0 |
21.0 |
14.0 |
12.0 |
28.0 |
— |
13.1 |
— |
| M16 |
2.0 |
24.0 |
16.0 |
14.0 |
30.0 |
— |
15.1 |
— |
① Distance before full thread starts. ② Washers in compressed state (to be confirmed per size). All dimensions in mm.
Unit Weight (g): to be updated. Contact FULLERKREG for weight data sheets.
D column reflects DIN 125 standard washer outer diameter. Head dimensions per ISO 4762.
Recommended Tightening Torque – Class 12.9 (Reference Values)
| Thread |
M3 |
M4 |
M5 |
M6 |
M8 |
M10 |
M12 |
M16 |
| Torque (N·m) |
2.5 |
5.5 |
11 |
19 |
46 |
91 |
158 |
390 |
Reference values for lubricated conditions (μ = 0.12). Always verify torque requirements against joint design and mating material specifications.
Part Number Call-Out Guide
| Field |
Description |
Example |
| Drive Type |
Hex Socket (Allen Key) – ISO 4762 |
Socket Cap Head |
| Washer Config |
FW = DIN 125 Flat Washer; LW = Lock Washer |
FW + LW |
| Material |
40Cr Alloy Steel, Class 12.9 |
12.9 |
| Finish |
Black Oxide (Blackening) |
Black |
| Thread Size |
Metric thread, e.g. M6 |
M6 |
| Length |
Underhead length in mm |
16 mm |
Available Lengths: 6 mm · 8 mm · 10 mm · 12 mm · 16 mm · 20 mm · 25 mm · 30 mm · 35 mm · 40 mm · 45 mm · 50 mm
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Application
FULLERKREG Class 12.9 Alloy Steel Socket Cap Head SEMS Screws with black oxide finish are engineered for the most demanding high-strength fastening applications where maximum tensile performance, vibration resistance, and assembly efficiency are simultaneously required:
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Automotive & Motorsport — engine block assembly, cylinder head fastening, suspension components, brake caliper mounting, and drivetrain assemblies where Class 12.9 strength and vibration resistance are mandatory.
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Heavy Machinery & Industrial Equipment — press frames, hydraulic cylinder end caps, gearbox housings, and structural machine joints subject to high static and dynamic loads.
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Tooling & Fixture Building — precision jigs, fixtures, and machining centres where high clamping force and dimensional stability are critical; black oxide finish reduces light reflection in optical measurement setups.
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Robotics & Automation — robot arm joints, servo motor mounts, linear guide rail fastening, and actuator brackets requiring high preload and vibration resistance in dynamic operating conditions.
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Aerospace Ground Support Equipment — high-strength structural fastening in ground support and test equipment where Class 12.9 performance is specified.
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Construction & Structural Steel — steel structure connections, curtain wall brackets, and heavy-duty anchor bolt assemblies requiring high-strength metric fasteners.
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Mining & Quarrying Equipment — crusher frames, conveyor drive units, and vibrating screen assemblies subject to extreme shock and vibration loads.
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Precision Instruments & Optics — black oxide finish minimises light reflection in optical benches, laser systems, and precision measurement equipment housings.
Why FULLERKREG?
- 🏭 Direct Manufacturer — domestic production capability with full quality control from raw material to finished fastener; Class 12.9 material and mechanical property certificates available.
- 📐 Engineering-Grade Data — complete dimensional data, hardness test reports, tensile test certificates, and material traceability documentation available on request.
- 📦 Bulk & Wholesale Supply — competitive pricing for high-volume orders; custom packaging, kitting, and labelling available for automotive and industrial OEM customers.
- 🌍 Global Distribution — experienced in international logistics and export documentation; supply to automotive OEMs, Tier 1 suppliers, and industrial equipment manufacturers worldwide.
- ✅ Compliance — RoHS compliant black oxide finish; ISO 4762 head dimensions; ISO 898-1 Class 12.9 mechanical properties; full material traceability available.
- 🔧 Custom Specifications — special dimensions, lengths, thread pitches (fine thread available), and alternative finishes available on request.
- ⚠️ Installation Note — Class 12.9 fasteners require accurate torque control. Always use a calibrated torque wrench and refer to the tightening torque table above. Do not re-use Class 12.9 fasteners in safety-critical joints after removal.
