Product Overview This product is a pre-assembled three-piece combination bolt — also known as a SEMS screw, washer-assembled bolt, or combination screw — consisting of a hex head or socket head bolt, a spring washer (split lock washer), and a flat washer (plain washer) that are factory pre-assembled into a single captive unit. The washers are retained on the bolt shank by the major diameter of the thread, which is larger than the washer hole, allowing free spinning without detachment. This design eliminates the need for separate washer handling during installation, significantly improving assembly efficiency and reducing the risk of missing or dropped washers on site. The product is available in Property Class 4.8 and 8.8 per ISO 898-1, with M3 to M12 as the standard size range (M14–M36 available on request). Both external hex head and internal hex socket head configurations are offered, with zinc plating as the standard surface finish. The bolt can be manufactured to GB/T 9074.13 (external hex), GB/T 9074.4 / 9074.8 (cross recessed pan head), and DIN 912 / ISO 4762 + washer assembly specifications. Product Type & Design Feature Description Product Type Three-Piece Combination Bolt / SEMS Screw / Washer-Assembled Bolt Head Type External Hex / Internal Hex Socket / Cross Recessed Pan Head Washer Configuration One spring washer (lock washer) + one flat washer, factory pre-assembled Washer Retention Captive on shank by thread major diameter — washers spin freely but do not detach Thread Configuration Full thread or partial thread Thread Direction Right-hand (left-hand available on request) Thread Tolerance 6g (metric) The captive washer design is the defining feature of this product. Unlike loose washer assemblies where washers can be lost during transport or handling, the SEMS design ensures that the spring washer and flat washer remain permanently attached to the bolt. This provides several operational advantages: faster assembly (no separate washer handling), reduced parts count (one SKU instead of three), elimination of missing washer defects in automated and manual assembly lines, and consistent preload because the correct washer combination is always present. The spring washer provides locking action to prevent loosening under vibration, while the flat washer distributes the clamping load over a larger bearing area and protects the mating surface from damage. Together, they form a complete fastening solution in a single unit. Material & Strength Grades Grade Material Tensile Strength (min.) Yield Strength (min.) Hardness Typical Applications 4.8 Low carbon steel (Q235) ≥400 MPa ≥320 MPa ≤ 255 HV General assembly, sheet metal, light structural 8.8 Medium carbon steel (10B21, 45#) ≥800 MPa ≥640 MPa 250–320 HV Machinery, equipment assembly, structural connections Grade 4.8 combination bolts are manufactured from low carbon steel (Q235) and are the standard grade for general-purpose assembly applications. The maximum allowable hardness for 4.8 grade is 255 HV. Grade 8.8 combination bolts are typically manufactured from 10B21 boron steel through cold heading, followed by quenching and tempering to achieve the required mechanical properties. The heat treatment process for 8.8 grade combination bolts includes a de-hydrogenation (baking) step to prevent hydrogen embrittlement of the spring washer, which is a critical quality control point. Grade 8.8 provides approximately 100% higher tensile strength than Grade 4.8. For stainless steel options, SS304 (A2-70) and SS316 (A4-70) are available in equivalent configuration. Standards & Specifications Parameter Details Product Type Combination Bolt / SEMS Screw / Washer-Assembled Bolt Standards GB/T 9074.13 (external hex combination bolt) / GB/T 9074.4 / 9074.8 (cross recessed pan head) / DIN 912 + washer assembly / ASME B18.13.1M (SEMS metric) Thread Size M3, M4, M5, M6, M8, M10, M12 (M14–M36 on request) Thread Pitch M3×0.5, M4×0.7, M5×0.8, M6×1.0, M8×1.25, M10×1.5, M12×1.75 Length Range 5 mm – 200 mm (customizable) Thread Tolerance 6g (metric) Surface Finish Zinc Plated (Blue-White / Clear / Yellow) / Plain / HDG (on request) Product Grade A GB/T 9074.13-1988 specifies external hexagon head bolts with spring washer and flat washer assemblies, with thread sizes from M3 to M12 and lengths from 5 mm to 90 mm. This standard is technically equivalent to DIN 6921 (hexagon flange bolts with washers) in terms of the washer assembly concept. ASME B18.13.1M-2011 covers metric SEMS (screw and washer assemblies) for through-hardened machine screws in Property Classes 8.8, 9.8, and 10.9 and case-hardened tapping screws from 5 mm to 12 mm nominal diameter. This standard provides dimensional and mechanical requirements for captive washer assemblies. Key Dimensions (Reference — External Hex, GB/T 9074.13) Thread Size (d) Pitch (P) mm Width Across Flats (s) mm Head Height (k) mm Flat Washer OD (mm) Flat Washer Thickness (mm) M3 0.5 5.5 2.0 7 0.5 M4 0.7 7.0 2.8 9 0.8 M5 0.8 8.0 3.5 10 1.0 M6 1.0 10.0 4.0 12 1.6 M8 1.25 13.0 5.3 16 1.6 M10 1.5 16.0 6.4 20 2.0 M12 1.75 18.0 7.5 24 2.5 All dimensions are in accordance with GB/T 9074.13 / DIN 6921 standards. Washer dimensions per DIN 125 (flat) and DIN 127 (spring). Mechanical Properties (Reference) Size Grade Tensile Strength (MPa) Yield Strength (MPa) Max Hardness M6 4.8 400 320 255 HV M8 4.8 400 320 255 HV M8 8.8 800 640 320 HV M10 8.8 800 640 320 HV M12 8.8 800 640 320 HV For Grade 8.8 combination bolts, the material is typically 10B21 boron steel, which is cold-headed to form and then quenched and tempered to achieve the specified mechanical properties. The spring washer within the assembly is also heat-treated and de-hydrogenated to prevent hydrogen embrittlement — a critical quality control step that distinguishes high-quality combination bolts from inferior products. Surface Finish & Corrosion Protection Finish Coating Thickness Salt Spray Resistance Recommended Environment Zinc Plated (Blue-White) 8–12 μm 72–144 hours Indoor, general industrial Zinc Plated (Yellow) 8–12 μm 96–168 hours Outdoor, moderate corrosion Zinc Plated (Clear) 5–8 μm 48–96 hours Indoor, temporary protection HDG (on request) 45–85 μm 500–1,500+ hours Outdoor, coastal, marine Dacromet (on request) 5–12 μm 600–1,500+ hours Coastal, offshore, high-corrosion Zinc plating provides sacrificial (cathodic) protection to the underlying steel. The standard blue-white zinc plating is the most common finish for combination bolts and provides good corrosion protection for indoor and general industrial applications. The zinc coating is applied after assembly, ensuring complete coverage of the bolt, spring washer, and flat washer. Important note for Grade 8.8: Electroplated high-strength fasteners (Grade 8.8 and above) are susceptible to hydrogen embrittlement. The heat treatment process for 8.8 grade combination bolts includes a de-hydrogenation (baking) step after plating to drive off absorbed hydrogen and prevent delayed brittle fracture of the spring washer. Buyers should verify that the supplier performs this critical step. Applications Combination bolts (SEMS screws) are widely used across industries: Sheet Metal Assembly — Automotive body panels, appliance housings, HVAC ductwork, electrical enclosures Electronics & Instrumentation — Control panels, terminal blocks, equipment chassis, rack mounting Machinery & Equipment — Motor mounting, pump assembly, conveyor systems, industrial machinery Furniture & Fixtures — Metal furniture assembly, shelving systems, display fixtures Automotive — Interior trim, engine accessories, underbody components Solar & Renewable Energy — Mounting brackets, panel frames, battery module assembly Construction & Infrastructure — Metal roofing, cladding, light structural connections General Assembly — Any application where a bolt, spring washer, and flat washer are required together The primary advantage of combination bolts in high-volume assembly operations is the reduction in part count and handling time. Instead of picking up a bolt, a spring washer, and a flat washer separately, the operator handles a single unit — reducing assembly time, eliminating missing washer defects, and ensuring consistent preload. Quality Assurance All products are manufactured under strict quality control procedures in accordance with GB/T 9074.13, ISO 898-1, and ASME B18.13.1M standards. Material test certificates (EN 10204 3.1), dimensional inspection reports, hardness test reports, and third-party inspection reports can be provided upon request. For Grade 8.8 combination bolts, the following quality control steps are performed on each production batch: Tensile testing of the bolt body Hardness testing of the bolt and spring washer De-hydrogenation verification of the spring washer after plating Washer retention test to confirm the washers remain captive but spin freely Packaging & Customization Export Packing: Carton / wooden case / steel pallet Domestic Packing: Standard corrugated box / bag / bulk carton OEM/ODM: Available — custom sizes, head types, washer configurations (single spring washer, single flat washer, or three-piece), materials (SS304, SS316), and surface finishes can be produced according to drawings or samples MOQ: Negotiable Ordering Information To place an order or request a quotation, please specify: Product type — Three-piece combination bolt (bolt + spring washer + flat washer) Head type — External hex / Internal hex socket / Cross recessed pan head Standard — GB/T 9074.13 / DIN 912 + washer assembly / ASME B18.13.1M Material — Carbon steel (Q235, 10B21, 45#) / Stainless steel (SS304, SS316) Strength grade — 4.8 / 8.8 / A2-70 / A4-70 Thread size and pitch — e.g., M6 × 1.0 or M8 × 1.25 Length — Specify total length (e.g., 16 mm, 25 mm, 40 mm) Surface finish — Zinc plated (specify color) / HDG / Dacromet / Plain Quantity — With any tolerance for over/under shipment Certification — EN 10204 3.1 MTC / ISO 898-1 compliance / SASO (for Saudi Arabia) / EAC (for Kazakhstan) Special requirements — Washer type (spring + flat, spring only, flat only), marking, packaging, etc.