Grade 10.9 Alloy Steel Countersunk Socket Head Screws — M1.6 to M4, 10–500 Pcs

For countersunk screws, the stated length is measured from the top of the head (flush with the surface) to the tip. The length should equal the total thickness of the material being fastened. For example, joining two 5mm steel plates typically calls for a 10mm (≈3/8") screw. Available lengths range from 3mm (≈1/8") to 50mm (≈2") depending on the thread diameter selected — see the specs table above for the full range per size.

Grade 10.9 is an ISO property class designation for alloy steel fasteners, indicating a minimum tensile strength of 1040 MPa (≈150,800 psi) and a minimum yield strength of 940 MPa. This makes them significantly stronger than Grade 8.8 fasteners and suitable for high-stress mechanical assemblies, automotive components, and structural joints where reliable clamping force under load is critical.

Grade 10.9 alloy steel screws are generally finished with a black oxide or phosphate coating that provides limited corrosion resistance, suitable for indoor and dry environments. For outdoor, marine, or high-humidity applications, stainless steel fasteners are typically recommended. If outdoor use is necessary, applying a thread-locking compound or anti-corrosion grease can help extend service life.

These screws follow ISO metric coarse thread standards: M1.6×0.35mm, M2×0.4mm, M2.5×0.45mm, M3×0.5mm, and M4×0.7mm pitch. They are compatible with standard ISO metric hex nuts, nylon lock nuts, brass threaded inserts, and any tapped hole of the corresponding size. They are not compatible with UNC/UNF inch-standard threads.

Hex key (Allen wrench) sizes typically required: M1.6 → 0.7mm, M2 → 1.5mm, M2.5 → 2mm, M3 → 2mm, M4 → 3mm. Always use a properly fitting, unworn hex key to avoid rounding the socket — this is especially important at the high torque values associated with Grade 10.9 fasteners. A ball-end hex driver is useful for angled access in tight assemblies.

Yes. Grade 10.9 alloy steel countersunk socket head screws are widely used in automotive bracket and engine component fastening, CNC machine frame assembly, robotics and servo motor mounting, precision instrument enclosures, and structural panel joints. The countersunk head sits flush with or below the mating surface, eliminating protrusions in moving or load-bearing assemblies — a key requirement in many OEM and industrial specifications.

In high-vibration applications — such as engine mounts, compressors, or industrial machinery — a medium-strength thread-locking compound (e.g., Loctite 243) is generally recommended to prevent loosening over time. For static or low-vibration assemblies, thread-locking is typically optional. Always follow the torque specifications for the specific joint design and material; overtorquing small-diameter screws (M1.6–M2) can cause thread stripping in softer base materials.