M3–M8 Pin Torx Countersunk Flat Head Machine Screws | A2 Stainless Steel

For countersunk (flat head) screws, the length is measured from the top of the head to the tip. Generally, choose a length equal to the material thickness plus the depth of engagement in the receiving material — typically at least 1.5× the screw diameter. For example, an M5 screw fastening 3mm (≈1/8") sheet into a bracket would typically use a 10–12mm (≈3/8"–1/2") length. When in doubt, go slightly longer and use a nut or threaded insert to control engagement depth.

Pin torx screws require a pin torx (tamper-resistant torx) security bit with a center hole to accommodate the pin. Standard torx bits will not fit. Common sizes: M3 typically uses T10H, M4–M5 use T20H or T25H, M6–M8 generally use T30H or T40H — confirm the exact size against your screw before purchasing a bit. These bits are widely available at hardware stores and online.

A2 stainless steel (ISO 3506 property class 70) typically has a tensile strength of around 700 MPa and a proof load stress of approximately 450 MPa. This is generally sufficient for light-to-medium structural and mechanical applications. For heavy-load or high-vibration environments, A4-80 or grade 8.8 carbon steel may be more appropriate. If you are unsure whether A2-70 meets your load requirements, consult an engineer.

A2 stainless steel offers good corrosion resistance and is suitable for most outdoor and mildly wet environments. It generally resists rust in rain, humidity, and freshwater exposure. However, for marine or coastal environments with prolonged saltwater exposure, A4 (316) stainless steel is typically recommended for better chloride resistance. For occasional outdoor use, A2 performs well in most US climates.

These screws follow the ISO metric coarse thread standard (DIN 965 / ISO 7046 profile for countersunk head). M3 has a 0.5mm pitch, M4 uses 0.7mm, M5 uses 0.8mm, M6 uses 1.0mm, and M8 uses 1.25mm pitch. They are compatible with standard metric hex nuts, nylon lock nuts, flange nuts, and threaded inserts of the corresponding size. Always verify the thread pitch of your nut or insert matches the screw before assembly.

Countersunk (flat head) screws are designed to sit flush with or below the surface of the material when installed in a countersunk hole. This is ideal when a smooth, snag-free surface is required — such as in panels, enclosures, furniture, or anywhere a protruding head would interfere with fit or aesthetics. Pan and button heads sit above the surface and are better for applications where countersinking is not practical. If flush mounting matters to your project, countersunk is the right choice.

These are machine screws designed to thread into a pre-tapped hole, threaded insert, or paired with a nut. They are not self-tapping. They work well in aluminum, steel, and other metals with tapped holes or threaded inserts. For plastic or wood, you would typically need a threaded insert first, or use a self-tapping variant instead. In most cases, a countersunk hole should be drilled and chamfered to match the 90° head angle for a flush fit.