Flat Head Socket Cap Screws
What are flat head socket cap screws?
Flat head socket cap screws are high-strength machine screws with a countersunk head and an internal hex drive. Once installed in a matching countersink, the head sits flush with the surface for a clean, snag-free finish while still allowing high installation torque.
Why choose flat head socket cap screws
- Flush, seamless appearance — countersinks sit level with panels and plates.
- Compact assembly — internal hex drive fits tight spaces and concealed hardware designs.
- High clamp load — consistent head geometry and deep socket support reliable preload.
- Versatile options — inch coarse/fine and metric ISO threads across common sizes.
Materials and finishes for flat head socket cap screws
- Alloy steel black oxide — very high strength for dry interior service.
- Stainless steel 18-8 / 304 — broad corrosion resistance for most environments.
- Stainless steel 316 — added resistance for marine/coastal or chloride exposure.
- Zinc-plated steel — economical corrosion protection on select grades.
How do I size and specify flat head socket cap screws?
Specify diameter × thread × length (e.g., 1/4″-20 × 1″, 10-32 × 3/4″, M6-1.0 × 20 mm). For flat heads, length is measured overall (including the head). Match UNC/UNF or metric ISO to the tapped component and select the correct countersink angle (typically 82° inch / 90° metric) for full head support.
Common applications for flat head socket cap screws
- Machinery, equipment panels, and precision assemblies needing a flush surface
- Automotive and aerospace interiors, fairings, and brackets
- Countersunk joints in fixtures, tooling, and instrumentation
- Structural assemblies where snag-free surfaces are required
Flat head socket cap screws vs other socket styles
- Flat head vs standard socket head — flat sits flush; standard offers the highest drive depth and torque.
- Flat head vs button head — button is low-profile above the surface; flat is fully countersunk.
- Flat head vs low-profile — low-profile saves height but with reduced drive depth; flat requires a countersink.
Installation tips for flat head socket cap screws
- Use the matching countersink angle and depth so the head bears on the cone, not just the top edge.
- Seat the hex key fully and tighten to the specified torque for size/material.
- Consider threadlocker in vibration-prone assemblies to maintain preload.
- Protect finishes with compatible materials and anti-seize where galling or galvanic corrosion is a risk.
Flat Head Socket Cap Screws FAQs
See the accordion below for quick answers on measuring length, countersink angles, material choice, and torque guidance.
Measure the overall length from tip to the top of the head (flat heads are measured overall; standard/button are measured from under the head).
Inch hardware typically uses 82°; metric commonly uses 90°. Match the fastener’s head angle to the countersink for full contact.
Use alloy steel black oxide for maximum strength in dry interiors. Choose 18-8/304 for general corrosion resistance, or 316 for marine/coastal or chloride exposure.
No, bearing is provided by the countersink. Ensure the countersink is correctly sized; washers are generally not used with flat heads.
Yes, if there’s enough thickness for a proper countersink. Consider back-up plates or rivet nuts if base material is too thin for full bearing.
Follow size/material torque guidance and account for lubrication/finish. Over-torque can strip threads or round the socket.
They can. Use a suitable anti-seize and consider isolators to mitigate galling and galvanic corrosion.