M4 Phillips Flat Head Countersunk Machine Screws — 5 to 45mm, Carbon Steel & Stainless, 9 Finishes

Select a length so that at least 1.5–2 times the thread diameter (i.e., 6–8mm for M4) engages in the mating material. For countersunk applications, also account for the depth of the countersink so the head sits flush with the surface. For example, if your panel is 5mm (about 3/16") thick with a 2mm countersink depth, an M4 x 8mm or M4 x 10mm screw is typically appropriate. When in doubt, choose the longer option and verify fit before committing to a bulk order.

The “With Glue” variants have a pre-applied thread-locking patch on the threads that activates under the friction of installation, helping resist loosening from vibration — similar in function to a nylon patch or micro-encapsulated adhesive. This eliminates the need for a separate liquid thread-locker in vibration-prone assemblies. 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.

The D=8.4 designation refers to the outer diameter of the countersunk head at its widest point — 8.4mm in this case. Standard M4 flat head screws may have a slightly different head diameter depending on the manufacturing standard. If your countersink is machined or drilled to a specific head diameter, select the D=8.4 variant to ensure the head seats correctly and sits flush with the surface. If you are drilling your own countersink, select the head diameter first and drill to match.

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

M4 uses the ISO metric thread standard with a standard coarse pitch of 0.7mm. These screws are compatible with standard M4 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 M4 x 0.7 tap. If you are unsure whether your existing hardware is M4 metric or an inch equivalent, measure the thread diameter and pitch with a thread gauge before ordering.

Carbon steel M4 machine screws are generally rated to property class 4.8 or 8.8, with tensile strengths typically ranging from 420 MPa to 800 MPa depending on the specific grade. Stainless steel 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. Contact us to confirm the exact property class if your application has specific strength requirements.

Carbon steel with zinc plating is a good choice for indoor or sheltered applications where cost efficiency matters and corrosion is not a primary concern. The “With Glue” carbon steel variants add vibration resistance without extra hardware, making them well-suited for automotive panels, appliances, and machinery. Stainless steel is generally preferred for outdoor, humid, or corrosive environments. For decorative visible fastener locations, bright nickel or colored zinc carbon steel options provide a distinctive finish at a lower cost than stainless.