Home » S-Type Double Washer Socket Cap Head SEMS Screws - Marine Stainless Steel (A4-70)

S-Type Double Washer Socket Cap Head SEMS Screws - Marine Stainless Steel (A4-70)

S-Type Double Washer Socket Cap Head SEMS Screws – Marine Stainless Steel (A4-70)

FULLERKREG S-Type Double Washer Socket Cap Head SEMS Screws in A4-70 marine grade stainless steel (SUS316) combine an ISO 4762 hex socket cap head screw with two permanently captivated washers — a DIN 125 standard flat washer and a lock washer — assembled onto the shank by thread rolling. The A4-70 (316 stainless steel) grade provides significantly superior corrosion resistance compared to A2-70 (304), with enhanced resistance to chloride-induced pitting and crevice corrosion, making these screws the preferred choice for marine, offshore, coastal, chemical processing, and other highly corrosive environments. All components — screw body, flat washer, and lock washer — are manufactured from A4-70 (SUS316) stainless steel with passivated finish.

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 S-Type Double Washer configuration pairs a DIN 125 standard flat washer with a lock washer. DIN 125 (equivalent to ISO 7089) specifies the most widely used standard washer outer diameter, providing a balanced bearing surface suitable for the majority of general industrial and marine fastening applications. The hex socket (Allen key) drive head conforms to ISO 4762, delivering high torque capacity and compatibility with standard hex key tooling across M2 to M10 thread sizes.

In the A4-70 marine grade, every component — screw, flat washer, and lock washer — is manufactured from 316 austenitic stainless steel, ensuring full material compatibility and eliminating galvanic corrosion risk between components. This makes the A4-70 S-Type SEMS screw a complete, corrosion-resistant fastening solution in a single part.

Benefits of Using SEMS Screws

  • Superior marine corrosion resistance — A4-70 (SUS316) stainless steel contains 2–3% molybdenum, providing significantly enhanced resistance to chloride pitting, crevice corrosion, and saltwater attack compared to A2-70 (304) grade.
  • Full material compatibility — all three components (screw, flat washer, lock washer) are A4-70 SUS316, eliminating galvanic corrosion between dissimilar metals within the fastener assembly.
  • 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.
  • Faster assembly — one integrated component replaces three separate parts (screw + flat washer + lock washer), reducing cycle time and operator handling steps.
  • Consistent washer positioning — the washer is always correctly seated and centred under the head, ensuring uniform clamping load and preventing misalignment or tilting.
  • Improved vibration resistance — the integrated lock washer provides reliable anti-loosening performance without additional components or thread-locking compounds.
  • Reduced inventory complexity — one SKU replaces three separate part numbers, simplifying procurement, storage, and BOM management.
  • Automation-friendly — the single-piece design is compatible with automated feeding and driving equipment, supporting high-volume production lines.
  • High tensile strength — A4-70 grade delivers minimum tensile strength of 700 MPa, combining corrosion resistance with structural performance.
  • RoHS and REACH compliant — all finishes meet environmental compliance requirements for global markets.

Limitations of Using SEMS Screws

  • Fixed washer configuration — the DIN 125 washer type and size are determined at manufacture; they cannot be changed or substituted in the field.
  • Higher unit cost than A2-70 — A4-70 (316 stainless) carries a material premium over A2-70 (304 stainless) due to the molybdenum content; specify A4-70 only where the corrosion environment justifies the cost.
  • Higher unit cost than loose components — SEMS screws carry a premium over equivalent loose-component assemblies due to the additional manufacturing step.
  • Minimum plate thickness requirement — the captive washer stack height must be considered when selecting screw length for thin sheet metal applications.
  • Not suitable for all joint configurations — applications requiring non-standard or specialty washers must use separate components or alternative SEMS types.
  • 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 Marine Stainless Steel (SUS316 / A4) A4-70 Passivated (Natural)
Standard Flat Washer (DIN 125) Marine Stainless Steel (SUS316) DIN 125 / ISO 7089 Passivated
Lock Washer Marine Stainless Steel (SUS316) ISO Standard Passivated

A4-70 vs. A2-70: Key Material Differences

Property A2-70 (SUS304) A4-70 (SUS316)
Molybdenum Content None 2–3%
Chloride Resistance Moderate Excellent
Pitting Resistance (PRE) ~18–20 ~24–26
Min. Tensile Strength 700 MPa 700 MPa
Min. Yield Strength 450 MPa 450 MPa
Recommended Environment General industrial, mild outdoor Marine, offshore, chemical, coastal

Finish Options:

  • 0 – Natural: As-manufactured, no additional surface treatment.
  • 1 – Passivated: Electrochemical passivation to enhance corrosion resistance and remove free iron from the surface.
  • 2 – Stayblack: Black oxide finish for aesthetic and mild corrosion protection.

All finishes are RoHS and REACH compliant.

Dimensions of Pre-Assembled Washers in SEMS Screws

The S-Type configuration uses DIN 125 standard flat washers (equivalent to ISO 7089). DIN 125 is the most widely used standard washer series, specifying a balanced outer diameter suitable for general industrial and marine 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 (S-Type – used in S-Type SEMS)

Thread Size M2 M2.5 M3 M4 M5 M6 M8 M10
d1 – Inner Diameter (mm) 2.2 2.7 3.2 4.3 5.3 6.4 8.4 10.5
d2 – Outside Diameter (mm) 5.0 6.0 7.0 9.0 10.0 12.5 17.0 21.0
t – Thickness (mm) 0.3 0.5 0.5 0.8 1.0 1.6 1.6 2.0

All dimensions per DIN 125 / ISO 7089 standard. All dimensions in mm.

Dimensional Data – Metric Series (ISO 4762 Head, A4-70)

All dimensions in mm. Head dimensions conform to ISO 4762. Thread rolled to BS 3643, Class 6g. Washer diameter column (D) reflects the DIN 125 standard washer outer diameter.

Part Number Thread (Class 6g) 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)
115-02(X)-020-(XX) M2 × 0.4 3.8 2.0 1.5 5.0 0.95 2.4 —
115-02(X)-025-(XX) M2.5 × 0.45 4.5 2.5 2.0 6.0 1.1 2.8 —
115-02(X)-030-(XX) M3 × 0.5 5.5 3.0 2.5 7.0 1.5 3.8 —
115-02(X)-040-(XX) M4 × 0.7 7.0 4.0 3.0 9.0 1.7 4.6 —
115-02(X)-050-(XX) M5 × 0.8 8.5 5.0 4.0 10.0 2.2 5.6 —
115-02(X)-060-(XX) M6 × 1.0 10.0 6.0 5.0 12.5 2.2 5.9 —
115-02(X)-080-(XX) M8 × 1.25 13.0 8.0 6.0 17.0 3.0 7.8 —
115-02(X)-100-(XX) M10 × 1.5 16.0 10.0 8.0 21.0 3.2 9.4 —

① Distance before full thread starts.   ② Washers in compressed state.   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.

Part Number Call-Out Guide

Field Position Description Example
SEMS Series 115-02X Metric series with DIN 125 standard flat washer + lock washer (S-Type double washer, A4-70) 115-021 = Hex socket, STD washer + lock washer, Passivated
Finish Code 1 digit 0 = Natural, 1 = Passivated, 2 = Stayblack 1 = Passivated
Thread Code 3 digits Thread size code (see table) 060 = M6
Underhead Length 2 digits Length in mm under head (L ±0.5 mm) 16 = 16 mm
Patch Lock Option Suffix P = Patch lock applied (leave blank if not required) P = with patch lock

Example: 115-021-060-16 P = Metric SEMS S-Type A4-70, Passivated, M6, 16 mm long, with Patch Lock

Available Lengths (L ±0.5 mm): 6 mm · 8 mm · 10 mm · 12 mm · 16 mm · 20 mm · 25 mm · 30 mm

Product Showcase

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Application

FULLERKREG S-Type Double Washer Socket Cap Head SEMS Screws in A4-70 marine stainless steel are engineered for the most demanding corrosive environments where standard A2-70 stainless steel is insufficient:

  • Marine & Offshore Structures — boat fittings, deck hardware, hull assemblies, offshore platform equipment, and subsea instrument housings where continuous saltwater exposure demands A4-70 molybdenum-bearing stainless steel.
  • Coastal & Waterfront Construction — jetties, pontoons, sea walls, and coastal architectural structures exposed to salt spray and high-humidity marine atmospheres.
  • Chemical Processing Equipment — reactors, pipework flanges, pump housings, and valve assemblies handling chloride-containing process fluids or acidic environments.
  • Food & Beverage Processing — hygienic equipment, processing tanks, and conveyor systems where frequent washdown with chlorinated cleaning agents requires superior corrosion resistance.
  • Pharmaceutical & Cleanroom — sterile processing equipment and cleanroom structures where material purity and corrosion resistance are regulatory requirements.
  • Swimming Pools & Water Treatment — pool equipment, water treatment plant structures, and desalination plant components exposed to chlorinated or brackish water.
  • Renewable Energy – Offshore — offshore wind turbine nacelles, subsea cable junction boxes, and tidal energy equipment requiring long-term corrosion resistance in seawater environments.
  • Electronics & Precision Instruments in Harsh Environments — enclosures and instrument housings deployed in coastal, tropical, or high-humidity environments where A2-70 would be susceptible to surface corrosion.

Why FULLERKREG?

  • 🏭 Direct Manufacturer — domestic production capability with full quality control from raw material to finished fastener; A4-70 material certificates available.
  • 📐 Engineering-Grade Data — complete dimensional data, material certifications (mill certs), and corrosion test reports available on request.
  • 📦 Bulk & Wholesale Supply — competitive pricing for high-volume orders; custom packaging and kitting available for marine and offshore projects.
  • 🌍 Global Distribution — experienced in international logistics and export documentation; supply to shipyards, offshore contractors, and EPC companies worldwide.
  • ✅ Compliance — RoHS and REACH compliant; DIN 125 and ISO standard dimensions; full material traceability and heat number documentation available.
  • 🔧 Custom Specifications — special dimensions, lengths, finishes, and patch lock options available on request; A4-80 grade available for higher strength requirements.
  • 🧪 Related Services — Cleanroom Wash & Packaging Service and Vibration Resistant Treatment Service available.
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