Flat Head Socket Cap Screws (Countersunk Allen Screws)
In assemblies where a protruding head is a functional defect—sliding rails, rotating housings, guarding panels, or tight envelope brackets—the real risk is not “bolt strength”, it’s interference, snagging, and clamp loss after the countersink seats. Flat Head Socket Cap Screws solve the geometry problem by pulling the joint into a flush surface while keeping torque transmission inside the head via a hex socket.
In the field, failures often come from the details: a 90° vs 82° countersink mismatch leading to line contact, fretting, and loosening; or poor countersink finish causing embedment and preload decay. We manufacture to ISO 10642 / DIN 7991 (metric, typically 90°) and can supply ASME B18.3 flat head socket screws (inch, commonly 82°) to match the mating design intent—so the screw seats correctly, carries load as designed, and remains serviceable during rework.
- Flush mount; clear moving parts
- Match 90° or 82° seats
- Transmit torque via hex socket
- Offer Class 10.9 / 12.9
- Provide A2-70 / A4-80
- Control friction; stabilise preload
Technical Specifications
Product Name
Flat Head Socket Cap Screws / Hex Socket Countersunk Head Screws
Standards
ISO 10642 (metric hex socket countersunk head), DIN 7991 (legacy equivalent), ASME B18.3 (inch series, flat head socket)
Material
Alloy steel (e.g., SCM435 / 42CrMo / 4140 Q&T), Carbon steel (project-dependent), Stainless steel A2 (304) / A4 (316)
Grades
Metric steel: 10.9 / 12.9 (per ISO 898-1 property classes); Stainless: A2-70 / A4-70 / A4-80 (per ISO 3506, when specified)
Diameter Range
Typical metric: M3–M20 (other sizes on request); inch per ASME B18.3 on request
Surface Finish
Black oxide (indoor), Zinc plated (Cr3), Zinc-Nickel, Geomet/Dacromet (flake coating), Phosphate & oil (process-controlled), Passivation (stainless)
Certifications
ISO 9001:2015, EN 10204 3.1 material certs, RoHS/REACH declarations on request; PPAP/traceability packages for OEM programs
1: Countersink mismatch causes loosening and fretting.
What happens in production: A 90° screw seated in an 82° countersink (or vice versa) contacts on a narrow ring. Under vibration, that ring frets, embedment grows, and preload decays.
Solution: Specify the countersink angle explicitly (metric ISO typically 90°, inch designs often 82°) and source the screw to the matching standard (ISO 10642/DIN 7991 vs ASME B18.3).
2: Preload scatter due to friction and seat quality.
What happens on the line: A rough countersink, coating stack-up, or inconsistent lubrication changes under-head friction. Torque goes up, but bolt stretch (preload) does not—leading to early loosening or joint slip.
Solution: Control surface finish + lubrication state, and validate torque–tension behaviour using a process method (e.g., torque-angle or torque + audit checks). For critical joints, request friction window control and verification (torque–tension testing such as ISO 16047 methodology).
3: Socket stripping and rework failure.
What happens in maintenance: Partially engaged hex keys, ball-end tools used at high torque, or worn drivers round the socket. Once stripped, removal escalates to drilling—high downtime.
Solution: Ensure full driver engagement, use correct key size/tolerance, and specify hardness/heat treatment suitable for service torque (Class 12.9 where higher clamp load is required, with process controls).
4: Soft substrates and head pull-through.
What happens in aluminium/castings: Countersunk heads concentrate stress; soft materials can crush, leading to embedment and preload loss.
Solution: Use controlled seating, consider countersunk washers (90° seat) or inserts where applicable, and increase engagement length in softer alloys.
Example dimensions for “Dimensions” search intent. Verify against the specified standard edition and drawing tolerances for release.
| Thread d | Pitch P (coarse) | Hex socket s | Head Ø dk (typ.) | Head height k (typ.) | Thread length b |
|---|---|---|---|---|---|
| M3 | 0.5 | 2.0 | 6.0 | 1.7 | ≈ L (full thread) |
| M4 | 0.7 | 2.5 | 8.0 | 2.3 | ≈ L (full thread) |
| M5 | 0.8 | 3.0 | 10.0 | 2.8 | ≈ L (full thread) |
| M6 | 1.0 | 4.0 | 12.0 | 3.3 | ≈ L (full thread) |
| M8 | 1.25 | 5.0 | 16.0 | 4.4 | ≈ L (full thread) |
| M10 | 1.5 | 6.0 | 20.0 | 5.5 | ≈ L (full thread) |
Dimension checks that prevent field issues
dk + countersink OD: ensure seat fully supports head without edge contact.
Socket size s: tool compatibility and stripping risk.
k: confirms flush requirement and countersink depth.
Reach target preload while maintaining flush seating and socket serviceability.
Countersink control (angle + finish)
Match screw to seat: 90° (ISO 10642/DIN 7991) vs 82° (common ASME designs).
Machine countersinks with controlled surface finish; burrs create false seating and preload loss.
Hole Clearance (ISO 273)
For clearance holes in through joints, follow ISO 273 (close/normal/large series) to avoid shank interference that prevents proper seating.
For tapped holes, ensure adequate thread engagement (rule-of-thumb: ≥1×d in steel, ≥1.5×d in aluminium—validate for load case).
Torque, Preload, Lubrication (don’t mix conditions)
Torque is friction-dominated. Define the assembly condition: dry / oiled / coated.
Practical relationship: T = K × F × d (K varies with finish/lube).
Indicative K ranges: dry 0.18–0.25, light oil 0.14–0.20, anti-seize 0.10–0.16.
Illustrative examples (K=0.20, lightly oiled):
M6 Class 10.9: preload ~12 kN → torque ~14 N·m
M8 Class 10.9: preload ~21 kN → torque ~34 N·m
M10 Class 10.9: preload ~34 kN → torque ~68 N·m
Use these only as starting points; confirm by torque–tension testing for your joint stack.
Socket stripping prevention
Avoid ball-end keys for final torque.
Ensure full socket engagement depth; replace worn drivers early (rounding often starts from tool wear, not screw hardness).
When to add washers
Flat head screws normally seat directly into the countersink. Use countersunk washers when the substrate is soft, coated, or oversized, and you need a stable bearing surface without crushing.
Related Products
FAQ
What is a flat head socket cap screw?
A flat head socket cap screw is a countersunk screw with an internal hex drive designed to sit flush in a matching countersink. It’s used where protruding heads would interfere with motion, clearance, or safety.
What is the difference between ISO 10642 and DIN 7991?
ISO 10642 is the current international standard for metric hex socket countersunk head screws, while DIN 7991 is the legacy German standard commonly used as an equivalent reference. For interchangeability, specify the required standard and verify key dimensions (dk, k, socket size).
Are flat head socket screws 90° or 82°?
Metric ISO countersunk socket screws are typically 90°, while many inch flat head designs are 82°. Using the wrong angle can cause ring contact, fretting, and preload loss, so match the screw to the countersink.
How do I prevent the hex socket from stripping?
Prevent stripping by using the correct key size, ensuring full engagement depth, avoiding ball-end tools for final torque, and controlling torque based on lubrication and finish. Driver wear is a common root cause of rounded sockets.
Can I use flat head socket cap screws in aluminium?
Yes, but aluminium is prone to seat embedment and thread strip-out if the joint is not designed properly. Use adequate thread engagement, consider countersunk washers, and validate preload to avoid crushing and long-term clamp loss.