Hex Flange Bolts (DIN 6921 / ISO 4162)
In vibration-prone joints, a “good bolt” is the one that keeps preload stable after real cycles—not the one that only looks correct on paper. Hex flange bolts integrate a bearing surface (flange) under the hex head to distribute contact pressure and reduce embedment-related preload loss. In assembly lines and field maintenance, they help cut rework caused by joint settling, paint/coating creep, and loosening under transverse vibration.
For applications where rotation resistance is needed at the head interface, we offer serrated flange (ribs/knurling under head) options; for sensitive mating surfaces (aluminum, coated brackets, soft washers), we also supply non-serrated flange versions to avoid surface damage and galvanic initiation sites.
- Integrate flange; enlarge bearing area
- Reduce embedment; stabilize preload
- Offer serrated or plain flange
- Support Grade 8.8 / 10.9 / 12.9
- Enable zinc / Geomet / Dacromet finishes
- Provide PPAP-ready traceability options
Technical Specifications
Product Name
Hex Flange Bolts (Hexagon Flange Bolts / Hex Head Flange Bolt)
Material Grades
Carbon steel (e.g., SCM435 / 35CrMo for quenched & tempered), Alloy steel (Q&T), Stainless steel (A2 / A4 on request)
Diameter Range
Typical: M5–M20 (custom up to M24 on request)
Standards
DIN 6921 (hexagon flange bolts, commonly serrated option), ISO 4162 (hexagon flange bolts), JIS B 1189 (on request)
Grades
Metric property class 8.8 / 10.9 / 12.9 (per ISO 898-1 for steel fasteners); Stainless classes A2-70 / A4-70 / A4-80 (per ISO 3506, if applicable)
Surface Finish
Zinc plated (Cr3), Zinc-Nickel, Phosphate & oil, Geomet/Dacromet (flake coating), Black oxide (indoor use)
Thread
Metric coarse by default; fine thread available for higher clamp stability in thin sections
Certifications
Material cert EN 10204 3.1, RoHS/REACH declarations on request; PPAP/IMDS support for automotive programs
Preload loss after tightening (joint settling / embedment).
What happens in practice: Painted brackets, hot-dip coated parts, or soft aluminum interfaces compress after initial torque. Clamp load drops, and the joint starts to slip under service loads.
How hex flange bolts help: The integrated flange increases the bearing area, reducing contact stress and localized crushing. This typically improves preload retention versus a small washer footprint—especially when production tolerances and coatings vary.
Loosening under vibration (transverse slip).
What happens in practice: In machinery frames, vehicle sub-assemblies, and HVAC mounts, micro-slip at the bearing surface accelerates self-loosening (Junker-type transverse vibration).
Solution options:
Serrated flange (ribs/knurling under head): increases resistance to head rotation by mechanically biting into the mating surface. Useful when loosening risk is high and the joint face is robust (steel, thick brackets).
Plain flange: preferred when the mating surface is soft (aluminum), cosmetically critical, or coated where serrations could damage the layer and create corrosion initiation points.
Galvanic and corrosion-driven failures at the interface.
What happens in practice: Stainless fasteners on aluminum, or mixed-metal stacks, can see galvanic coupling—especially when coatings are compromised during tightening.
Engineering response: choose compatible materials, specify barrier coatings (e.g., zinc-nickel or flake coatings), and control lubrication to avoid galling on stainless.
Why buyers search “hex head flange bolt dimensions”
Because interchangeability matters: standards define head geometry, flange OD, and thread engagement expectations. We support standard-compliant builds and provide inspection records (AQL/CPK plans as required) so engineering teams can approve without guesswork.
Below is a standard example table to capture “dimensions” searches and help engineers do quick checks. For program release, confirm against the latest DIN/ISO issue and your drawing tolerance stack.
| Thread d | Pitch P (coarse) | Hex s | Head height k | Typical flange OD (dw) | Thread length b (example) |
|---|---|---|---|---|---|
| M6 | 1.0 | 10 | 6.0 | 14.2 | 18 (for L ≤ 40) |
| M8 | 1.25 | 13 | 8.0 | 17.9 | 22 (for L ≤ 45) |
| M10 | 1.5 | 15 | 10.0 | 21.8 | 26 (for L ≤ 60) |
| M12 | 1.75 | 18 | 12.0 | 26.0 | 30 (for L ≤ 65) |
| M14 | 2.0 | 21 | 14.0 | 29.9 | 34 (for L ≤ 80) |
| M16 | 2.0 | 24 | 16.0 | 34.5 | 38 (for L ≤ 90) |
Notes engineers care about
d, P drive tapped hole design and strip-out margin (especially in aluminum/cast iron).
s, k affect tool access and socket clearance.
Flange OD impacts bearing pressure and paint/coating crush.
b (thread length) matters for nut engagement and “full thread in shear plane” risks.
Torque vs. Preload (don’t confuse them)
Torque is only an indirect proxy; friction scatter dominates. Use controlled friction systems when clamp consistency matters (e.g., critical structural joints).
A simplified relationship often used on the floor:
T = K × F × d
T torque, F preload, d nominal diameter, K nut factor (depends on finish & lubrication).
Typical K-factor ranges (indicative):
Dry steel-on-steel: 0.18–0.25
Light oil: 0.14–0.20
Moly/anti-seize: 0.10–0.16
Engineering takeaway: specify lubrication state on the drawing/process sheet, or torque specs will not transfer between suppliers/lines.
Lubrication control (especially for Grade 10.9/12.9)
High-strength bolts are sensitive to over-preload if friction drops unexpectedly. If you use zinc-nickel + topcoat or flake coatings, confirm friction coefficient windows with supplier test reports.
Washers: when to use / when to avoid
Flange bolts often reduce the need for washers by design.
Use washers when:
Joint face is soft or slotted
You need a controlled bearing surface for torque-angle tightening
You must protect coatings or isolate galvanic couples
Hole Clearance (ISO 273 reference)
For consistent seating and to prevent edge loading under the flange, follow standard clearance hole recommendations (close/normal/large series per ISO 273). Misfit holes create bending in the shank and accelerate fatigue.
Serrated flange caution
Serrations increase under-head friction and can damage coatings. If corrosion performance or paint integrity is critical, use plain flange and consider a separate locking strategy (prevailing torque nut, threadlocker, wedge-lock washer) based on validation testing.
Related Products
FAQ
What is a hex flange bolt?
A hex flange bolt is a hex head bolt with an integrated flange under the head that acts like a built-in washer to spread load and improve seating. It reduces localized bearing stress and can improve preload retention in joints with coatings or softer materials.
DIN 6921 vs ISO 4162—what’s the difference?
DIN 6921 and ISO 4162 both cover hexagon flange bolts, but details like flange geometry, marking conventions, and legacy tolerances can differ by revision and supplier practice. For interchangeability, reference the exact standard edition on the drawing and verify critical dimensions (flange OD, head height, under-head features).
When should I choose a serrated flange bolt?
Choose a serrated flange bolt when you need increased under-head resistance to rotation in vibration-prone joints and the mating surface can tolerate serration bite. Avoid serrations on soft aluminum, cosmetic surfaces, or corrosion-critical coatings unless validated by testing.
Are hex flange bolts better than using a washer?
Not universally—hex flange bolts reduce the need for a washer in many steel joints, but washers are still useful for slotted holes, soft materials, or when you need a controlled bearing surface. The decision should be based on bearing pressure, coating integrity, and clamp consistency requirements.
What clearance hole should I use for metric flange bolts?
Use ISO 273 clearance hole guidance (close/normal/large series) based on alignment tolerance and assembly method. Too tight increases assembly scrap; too large can cause flange edge loading and joint slip—so pick the minimum clearance that still supports your process capability.