Hex Socket Head Cap SEMS Screw FW + LW - Alloy Steel 12.9 Zinc
FULLERKREG Class 12.9 Zinc-Plated 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 an electrolytic zinc plating surface treatment, these screws deliver the highest strength class available in the SEMS range - minimum tensile strength of 1220 MPa - combined with enhanced corrosion resistance from the zinc coating and the assembly efficiency of a pre-assembled captive washer system. The bright zinc finish provides a clean, professional appearance suitable for general industrial, automotive, and machinery applications where both high strength and moderate corrosion protection are required.
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 across M3 to M16 thread sizes.
In the Class 12.9 zinc-plated variant, the screw body is manufactured from 40Cr chromium alloy steel, heat-treated to achieve the exceptional mechanical properties of property class 12.9, then electroplated with zinc to provide corrosion protection superior to black oxide while maintaining the full structural performance of the 12.9 grade. Note: zinc plating of Class 12.9 fasteners requires strict hydrogen embrittlement relief baking per ISO 4042 to maintain mechanical integrity.
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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Enhanced corrosion resistance vs. black oxide - electrolytic zinc plating provides significantly better corrosion protection than black oxide finish, offering resistance to atmospheric corrosion, humidity, and mild chemical exposure in general industrial environments.
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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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Bright professional appearance - the zinc-plated finish provides a clean, bright silver appearance suitable for visible assemblies and customer-facing equipment where aesthetics matter.
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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.
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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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Hydrogen embrittlement risk - Class 12.9 high-strength alloy steel is susceptible to hydrogen embrittlement during electroplating. Zinc-plated Class 12.9 fasteners must undergo mandatory hydrogen embrittlement relief baking (typically 200-230 degrees C for minimum 4 hours) per ISO 4042 / ASTM F1941 immediately after plating. Failure to bake can result in delayed brittle fracture under load.
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Not suitable for marine or highly corrosive environments - zinc plating provides moderate corrosion protection; for marine, offshore, or chemical environments, specify stainless steel grades (A2-70 or A4-70).
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Fixed washer configuration - the DIN 125 washer type and size are determined at manufacture and 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 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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Zinc coating adds slight dimensional increase - electrolytic zinc plating adds 5-15 microns per surface; verify thread fit with mating components in precision applications.
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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) |
Electrolytic Zinc Plating |
| Standard Flat Washer (DIN 125) |
Alloy Steel / Carbon Steel |
DIN 125 / ISO 7089 |
Zinc Plated |
| Lock Washer |
Alloy Steel / Carbon Steel |
ISO Standard |
Zinc Plated |
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 |
Zinc Plating vs. Black Oxide - Finish Comparison
| Property |
Black Oxide |
Zinc Plated (This Product) |
| Appearance |
Matte black |
Bright silver |
| Coating Thickness |
Less than 1 micron |
5-15 microns |
| Salt Spray Resistance |
Approx. 2-8 hours |
Approx. 72-200 hours |
| Corrosion Protection |
Mild (indoor dry only) |
Moderate (general industrial) |
| Hydrogen Embrittlement Risk |
None (chemical conversion) |
Yes - baking required per ISO 4042 |
| Dimensional Impact |
Negligible |
5-15 microns per surface |
| RoHS Compliance |
Yes |
Yes (trivalent zinc only) |
All finishes are RoHS compliant. Hexavalent chromium (Cr6+) passivation is not used. Trivalent chromium (Cr3+) passivation only.
Dimensions of Pre-Assembled Washers in SEMS Screws
The FW + LW configuration uses DIN 125 standard flat washers (equivalent to ISO 7089). 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)
| Thread Size |
M3 |
M4 |
M5 |
M6 |
M8 |
M10 |
M12 |
M14 |
M16 |
| d1 - Inner Dia. (mm) |
3.2 |
4.3 |
5.3 |
6.4 |
8.4 |
10.5 |
13.0 |
15.0 |
17.0 |
| d2 - Outer Dia. (mm) |
7.0 |
9.0 |
10.0 |
12.5 |
17.0 |
21.0 |
24.0 |
28.0 |
30.0 |
| t - Thickness (mm) |
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. Note: zinc plating adds 5-15 microns per surface to all dimensions.
Dimensional Data - Metric Series (ISO 4762 Head, Class 12.9)
All dimensions in mm. Head dimensions conform to ISO 4762. Thread tolerance Class 6g (before plating). 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) |
F (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 |
- |
F = Distance before full thread starts. E = Washers in compressed state (to be confirmed per size). All dimensions in mm. Unit Weight: to be updated, contact FULLERKREG for data sheets. Dimensions are nominal pre-plating values; zinc plating adds 5-15 microns per surface.
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 (friction coefficient 0.12). For zinc-plated fasteners, lubrication is recommended to achieve consistent friction coefficient. 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 |
Electrolytic Zinc Plating (Trivalent Passivation) |
Zinc |
| Thread Size |
Metric thread |
M8 |
| Length |
Underhead length in mm |
20 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
Product Showcase
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Application
FULLERKREG Class 12.9 Zinc-Plated Alloy Steel Socket Cap Head SEMS Screws are engineered for high-strength fastening applications where both maximum tensile performance and moderate corrosion protection are required:
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Automotive and Powertrain - engine block assembly, transmission housings, suspension components, and brake system fastening where Class 12.9 strength is mandatory and zinc plating provides protection against road salt and moisture exposure.
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Heavy Machinery and Industrial Equipment - press frames, hydraulic cylinder end caps, gearbox housings, and structural machine joints subject to high static and dynamic loads in general industrial environments.
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Agricultural Equipment - combine harvesters, tractor assemblies, and implement attachments where high-strength fastening with moderate outdoor corrosion resistance is required.
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Construction Equipment - excavator attachments, crane structures, and heavy lifting equipment requiring Class 12.9 clamping force with protection against outdoor atmospheric exposure.
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Tooling and Fixture Building - precision jigs, fixtures, and machining centres where high clamping force and dimensional stability are critical, with zinc plating providing protection during storage and transport.
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Robotics and Automation - robot arm joints, servo motor mounts, linear guide rail fastening, and actuator brackets requiring high preload and vibration resistance.
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Mining and Quarrying Equipment - crusher frames, conveyor drive units, and vibrating screen assemblies subject to extreme shock, vibration, and dusty outdoor environments.
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General Industrial Assembly - any high-strength bolted joint in general industrial environments where atmospheric corrosion protection beyond black oxide is required but stainless steel is not specified.
Why FULLERKREG?
- 🏭 Direct Manufacturer - domestic production capability with full quality control; Class 12.9 material and mechanical property certificates available. Hydrogen embrittlement relief baking confirmed per ISO 4042.
- 📐 Engineering-Grade Data - complete dimensional data, hardness test reports, tensile test certificates, salt spray test reports, and material traceability documentation available on request.
- 📦 Bulk and Wholesale Supply - competitive pricing for high-volume orders; custom packaging and kitting available for automotive and industrial OEM customers.
- 🌍 Global Distribution - experienced in international logistics; supply to automotive OEMs, Tier 1 suppliers, and industrial equipment manufacturers worldwide.
- ✅ Compliance - RoHS compliant trivalent zinc plating (no Cr6+); ISO 4762 head dimensions; ISO 898-1 Class 12.9 mechanical properties; ISO 4042 plating process compliance; full material traceability available.
- 🔧 Custom Specifications - special dimensions, lengths, fine thread pitches, alternative plating thicknesses, and yellow zinc or black zinc finishes available on request.
- ⚠ Critical Installation Note - Class 12.9 zinc-plated fasteners require accurate torque control. Always use a calibrated torque wrench and lubricate threads before installation. Do not re-use Class 12.9 fasteners in safety-critical joints after removal. Verify hydrogen embrittlement relief baking compliance before use in safety-critical applications.
