M3 Torx Flat Head Countersunk Machine Screws — Carbon Steel & Stainless Steel, Multi-Finish

A general rule is to select a length equal to 2–3 times the thickness of the material being fastened, plus the depth of the countersink. For example, if your panel is 4mm (about 5/32") thick, an M3 x 8mm or M3 x 10mm screw is typically appropriate. Always measure the countersink depth so the head sits flush with the surface. When in doubt, choose the longer option — you can always add a washer or spacer to adjust the thread engagement.

The D= value refers to the outer diameter of the countersunk head at its widest point. D=5.5mm means the head is 5.5mm wide, while D=6mm is slightly larger. Choose based on the countersink diameter already machined or drilled in your workpiece. Using the wrong head diameter will result in the head either sitting proud of the surface or not seating properly in the countersink. If you are machining your own countersink, select the head diameter first and drill to match.

The "With Glue" variant has a pre-applied thread-locking patch on the threads. This patch activates under the friction of installation and helps resist loosening from vibration, functioning similarly to a nylon patch or micro-encapsulated adhesive. It is generally a single-use feature — once the screw is removed, the patch is typically spent. For reinstallation, use a fresh screw or apply a liquid thread-locker such as Loctite 243.

It depends on the finish selected. Stainless steel variants (plain or black zinc coated) offer good general corrosion resistance and are suitable for most outdoor and mildly humid environments. Carbon steel variants with zinc plating (black, white, blue, or blue-white) provide moderate corrosion protection for indoor or sheltered use. For marine or highly corrosive environments, stainless steel is generally the better choice. Bright nickel-plated carbon steel offers a decorative finish with moderate corrosion resistance.

These screws use the ISO metric M3 thread standard with standard coarse pitch (0.5mm). They are compatible with standard M3 metric nuts, threaded inserts, and tapped holes. They are not compatible with UNC or UNF inch-standard threads. If you are tapping your own holes, use an M3 x 0.5 tap.

For M3 Torx countersunk screws, a T10 or T15 Torx bit is typically used depending on the specific head design. Always test-fit the bit before applying torque to avoid damaging the drive recess. Using the correct bit size is especially important with small-diameter screws where the drive recess is shallow. A good-quality Torx bit set with T8, T10, and T15 will cover most M3 applications.

Carbon steel M3 screws are generally rated to property class 4.8 or 8.8 depending on the specific grade, with tensile strengths typically ranging from 420 MPa to 800 MPa. Stainless steel A2 variants are generally rated to A2-70, with a tensile strength of approximately 700 MPa. Actual load capacity depends on thread engagement length, base material hardness, and installation torque. For structural or safety-critical applications, contact us to confirm the exact property class before ordering.