Home » M-Type Double Washer Socket Cap Head SEMS Screws - High Tensile Stainless Steel (A2-70)

M-Type Double Washer Socket Cap Head SEMS Screws - High Tensile Stainless Steel (A2-70)

M-Type Double Washer Socket Cap Head SEMS Screws – High Tensile Stainless Steel (A2-70)

FULLERKREG M-Type Double Washer Socket Cap Head SEMS Screws integrate an ISO 4762 hex socket cap head screw with two permanently captivated washers — a DIN 433 small outer diameter flat washer and a lock washer — assembled onto the shank by thread rolling. The compact DIN 433 small washer is specifically designed for confined spaces, closely spaced hole patterns, and applications where a reduced washer footprint is required without sacrificing the anti-loosening benefit of a lock washer. Available in A2-70 (SUS304) stainless steel with passivated finish, these screws eliminate loose washer handling and deliver reliable fastening in space-constrained 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 M-Type Double Washer configuration pairs a DIN 433 small-series flat washer with a lock washer. DIN 433 specifies a reduced outer diameter compared to ISO standard washers, making the M-Type the preferred choice where clearance around the fastener is limited or where a low-profile washer footprint is required. 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.

This design is especially beneficial in electronics assembly, precision instruments, compact machinery, and any application where tight hole spacing or confined access makes a standard or large washer impractical.

Benefits of Using SEMS Screws

  • Compact washer footprint — the DIN 433 small washer outer diameter is smaller than ISO standard, enabling use in confined spaces, tight hole patterns, and applications with limited clearance around the fastener.
  • Eliminates loose washers — the DIN 433 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 + small 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.
  • Low-profile assembly — the small washer diameter minimises the overall fastener footprint, ideal for aesthetic or space-critical applications.
  • Corrosion resistance — A2-70 (SUS304) stainless steel with passivated finish provides excellent resistance to oxidation, moisture, and mild chemical environments.
  • RoHS and REACH compliant — all finishes meet environmental compliance requirements for global markets.

Limitations of Using SEMS Screws

  • Reduced bearing area — the DIN 433 small washer outer diameter is smaller than ISO standard or DIN 9021 large washers; not suitable for soft substrates or oversized holes where load spreading is critical.
  • Fixed washer configuration — the DIN 433 washer type and size are determined at manufacture; they cannot be changed or substituted in the field.
  • Higher unit cost — 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 standard, large, 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 Stainless Steel (SUS304 / A2) A2-70 Passivated (Natural)
Small Flat Washer (DIN 433) Stainless Steel (SUS304) DIN 433 Passivated
Lock Washer Stainless Steel (SUS304) ISO Standard Passivated

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 M-Type configuration uses DIN 433 small outer diameter flat washers. DIN 433 specifies a reduced outer diameter compared to ISO standard (DIN 125A) washers, providing a compact bearing surface suited to confined-space applications. The following table shows the inner diameter (d1), outer diameter (d2), and thickness (t) of the captive DIN 433 flat washers by thread size:

DIN 433 Small Plain Washer (M-Type – used in M-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) 4.5 5.0 6.0 8.0 9.0 11.0 15.0 18.0
t – Thickness (mm) 0.3 0.5 0.5 0.8 1.0 1.6 1.6 2.0

All dimensions per DIN 433 standard. All dimensions in mm.

Washer Size Comparison: DIN 433 (M-Type) vs. ISO Standard (S-Type) vs. DIN 9021 (L-Type)

Thread Size DIN 433 OD – M-Type (mm) ISO Std OD – S-Type (mm) DIN 9021 OD – L-Type (mm) M-Type vs. S-Type
M3 6.0 7.0 9.0 −14% smaller
M4 8.0 9.0 12.0 −11% smaller
M5 9.0 10.0 15.0 −10% smaller
M6 11.0 12.5 18.0 −12% smaller
M8 15.0 17.0 24.0 −12% smaller
M10 18.0 21.0 30.0 −14% smaller

Dimensional Data – Metric Series (ISO 4762 Head, A2-70)

All dimensions in mm. Head dimensions conform to ISO 4762. Thread rolled to BS 3643, Class 6g. Washer diameter column (D) reflects the DIN 433 small 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)
115-04(X)-020-(XX) M2 × 0.4 3.8 2.0 1.5 4.5 — 2.4 —
115-04(X)-025-(XX) M2.5 × 0.45 4.5 2.5 2.0 5.0 — 2.8 —
115-04(X)-030-(XX) M3 × 0.5 5.5 3.0 2.5 6.0 — 3.8 —
115-04(X)-040-(XX) M4 × 0.7 7.0 4.0 3.0 8.0 — 4.6 —
115-04(X)-050-(XX) M5 × 0.8 8.5 5.0 4.0 9.0 — 5.6 —
115-04(X)-060-(XX) M6 × 1.0 10.0 6.0 5.0 11.0 — 5.9 —
115-04(X)-080-(XX) M8 × 1.25 13.0 8.0 6.0 15.0 — 7.8 —
115-04(X)-100-(XX) M10 × 1.5 16.0 10.0 8.0 18.0 — 9.4 —

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 433 small washer outer diameter. Head dimensions per ISO 4762.

Part Number Call-Out Guide

Field Position Description Example
SEMS Series 115-04X Metric series with DIN 433 small flat washer + lock washer (M-Type double washer) 115-041 = Hex socket, DIN 433 small washer + lock washer
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) 12 = 12 mm
Patch Lock Option Suffix P = Patch lock applied (leave blank if not required) P = with patch lock

Example: 115-041-060-12 P = Metric SEMS M-Type, Passivated, M6, 12 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 M-Type Double Washer Socket Cap Head SEMS Screws are engineered for applications where a compact washer footprint, vibration resistance, and assembly efficiency are simultaneously required:

  • Electronics & Precision Instruments — PCB mounting, sensor housings, and compact electronic enclosures where tight hole spacing makes standard or large washers impractical.
  • Compact Machinery & Robotics — servo motor mounting, actuator brackets, and robot arm assemblies where confined access and close hole patterns demand a small washer footprint.
  • Automotive & Transportation — interior trim, dashboard components, and underbody assemblies where space constraints limit washer size but vibration resistance remains essential.
  • Sheet Metal Fabrication — thin panel fastening in enclosures and cabinets where the DIN 433 small washer provides adequate load distribution without overlapping adjacent features.
  • Semiconductor & Cleanroom Equipment — the captive washer design eliminates loose washer contamination risk; available with Cleanroom Wash & Packaging Service.
  • Offshore & Marine — corrosion-resistant A2-70 stainless steel makes these screws suitable for saltwater-exposed structures and marine instrument enclosures.
  • Optical & Medical Devices — compact, low-profile fastening for precision optical mounts, medical device housings, and laboratory equipment where minimal footprint is critical.
  • Renewable Energy — solar inverter enclosures, wind turbine control panels, and electrical cabinet assembly where space-efficient fastening is required.

Why FULLERKREG?

  • 🏭 Direct Manufacturer — domestic production capability with full quality control from raw material to finished fastener.
  • 📐 Engineering-Grade Data — complete dimensional data, material certifications, and test reports available on request.
  • 📦 Bulk & Wholesale Supply — competitive pricing for high-volume orders; custom packaging and kitting available.
  • 🌍 Global Distribution — experienced in international logistics and export documentation for worldwide customers.
  • ✅ Compliance — RoHS and REACH compliant; DIN 433 and ISO standard dimensions; material traceability available.
  • 🔧 Custom Specifications — special dimensions, lengths, finishes, and patch lock options available on request.
  • 🧪 Related Services — Cleanroom Wash & Packaging Service and Vibration Resistant Treatment Service available.
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