Flat Head Socket Cap Screw

Flat Head Socket Cap Screws — Countersunk, Alloy and Stainless Steel

Shop Flat Head Socket Cap Screws

Flat head socket cap screws — also called flat head Allen screws, countersunk socket screws, or FHSCS — combine a countersunk head with an internal hex drive. The head is machined at an angle that matches a standard countersunk hole, drawing flush to the surface as the fastener is tightened. This makes them the correct choice for assemblies where a protruding head would interfere with a mating surface, sliding component, or enclosure panel.

Inch vs. Metric: Countersink Angle Matters

The countersink angle is not universal. Inch-series flat head socket cap screws conform to ASME B18.3 and use an 82° included angle. Metric flat head socket cap screws conform to DIN 7991 and ISO 10642, using a 90° included angle. The mating countersunk hole must be cut to match — using a metric fastener in an 82° hole (or vice versa) will result in improper seating and reduced clamping force. Confirm your countersink tooling before ordering.

Alloy Steel Flat Head Socket Cap Screws

Alloy steel flat head socket cap screws are heat treated and meet ASTM A574. Minimum tensile strength is 170,000 PSI for sizes #0 through 1/2" and 180,000 PSI for larger sizes. Standard finish is plain (bright); black oxide is available in most sizes. The high tensile rating makes alloy FHSCS suitable for structural joints, tooling plates, machine guards, and OEM equipment assemblies where countersinking is required.

Stainless Steel Flat Head Socket Cap Screws

18-8 stainless steel flat head socket cap screws provide excellent corrosion resistance for outdoor, food service, and washdown environments. Tensile strength is lower than alloy (approximately 85,000–125,000 PSI depending on size) — verify load requirements before substituting stainless for alloy in structural applications. 316 stainless is available for marine and chloride-exposed environments.

Metric Flat Head Socket Cap Screws

Metric flat head socket cap screws follow DIN 7991 (alloy, Class 10.9 or 12.9) and ISO 10642. The 90° countersink angle matches standard metric countersink tooling. Common sizes in stock run from M3 through M16; confirm availability for M20 and larger. Metric sizes are available in alloy steel and 18-8 stainless steel.

How to Measure a Flat Head Socket Cap Screw

Flat head socket cap screws are measured differently from standard cap screws. Because the head is designed to sit flush, length is measured from the top of the head (the flat face) to the end of the shank — the full overall length including the head. This differs from socket head cap screws, where length is measured from under the head. Always confirm measurement convention when cross-referencing with a print or specification.

Common Applications

Flat head socket cap screws are used wherever a flush fastener surface is required: machine guards and covers where mating panels must close flush, panel assemblies with close-clearance sliding fits, precision fixture plates where the head would otherwise interfere with workpiece seating, electronic enclosures and equipment housings, and jig plates and tooling where protruding heads would snag workpieces or fixtures.

Frequently Asked Questions

What is the abbreviation for flat head socket cap screw?
The standard abbreviation is FHSCS (flat head socket cap screw) or FHSC. On engineering drawings you may also see FH SKT CAP SCR. DIN 7991 is the metric standard designation.

What angle is a flat head socket cap screw?
Inch-series: 82° included angle (ASME B18.3). Metric-series: 90° included angle (DIN 7991 / ISO 10642). These are not interchangeable — the countersunk hole must match the fastener angle.

How is a flat head socket cap screw measured?
Length is measured from the top of the flat head face to the end of the shank — the full overall length. This is different from socket head cap screws, which measure from under the head.

Can I use a flat head socket cap screw in a standard wood countersink?
Wood countersinks are typically 82° (matching inch-series FHSCS) but vary by bit manufacturer. Verify the angle before assuming compatibility in finish or trim applications.

What hex key size do I use for flat head socket cap screws?
The same as socket head cap screws of the same thread size: 1/4"-20 uses a 3/16" hex key; 5/16"-18 uses 1/4"; 3/8"-16 uses 5/16"; 1/2"-13 uses 3/8". Metric: M5 uses 4mm; M6 uses 5mm; M8 uses 6mm; M10 uses 8mm.

Flat head socket cap screws are commonly installed alongside flat washers under the nut or in applications where additional bearing surface is needed, and with socket head cap screws in mixed-head assemblies where some fasteners require flush seating and others do not. For applications requiring threaded engagement in thin or non-threaded materials, see our rivet nuts and threaded inserts. The full socket fastener family — including button head cap screws, socket set screws, and shoulder bolts — is available through the Sockets category.