Hex Flange Bolts
Manufacturer
India · Mumbai
What a Hex Flange Bolt Actually Is — And Why the Flange Matters
A hex flange bolt is a hex head bolt with a wide circular flange integrally forged under the head — combining the function of a bolt and a flat washer into a single component. The flange diameter is typically 1.7–2× the bolt’s nominal thread diameter, spreading the clamping load over a significantly larger bearing area than a standard hex bolt head alone.
This single design change eliminates the most common assembly-line error: the missing or wrong-size washer. With a hex flange bolt, the washer cannot be forgotten, cannot be the wrong diameter for the application, and cannot fall out of alignment during assembly. This is why hex flange bolts are the standard specification across the automotive, agricultural machinery, and general engineering industries — anywhere high-volume assembly and consistent joint clamping force matter more than the marginal material cost of the larger head forging.
The flange underside is available in two forms: serrated (a series of radial or circumferential ribs pressed into the flange face) and non-serrated / plain (a smooth flat flange face). This single choice — serrated or plain — is the most important specification decision when ordering hex flange bolts, and it depends entirely on the surface the bolt will be tightened against.
Why Hex Flange Bolts — Assembly Efficiency and Joint Reliability
One Component Instead of Two — Assembly Speed
Every hex bolt + flat washer combination is two separate parts that must be picked, oriented, and assembled correctly. A hex flange bolt is one part. On high-volume production lines (automotive, appliance, agricultural equipment), this halves the fastener component count, eliminates washer-drop assembly errors, and measurably speeds torque-down cycle time. This single-part efficiency is the primary reason automotive OEMs standardised on hex flange bolts decades ago.
Wider Bearing Area — Reduced Surface Pressure
The flange diameter (typically 1.7–2× the thread diameter) spreads clamping force over a significantly larger contact area than a standard hex head alone. This reduces surface pressure at the joint face, which matters directly for softer materials — aluminium castings, cast iron, and thin sheet steel — where a standard hex bolt head can locally deform or crack the substrate under high preload. The flange distributes the same clamping force over more material.
Serrated Flange — Mechanical Anti-Vibration Locking
The serrated flange’s radial ribs bite into the mating surface as the bolt is tightened, creating a mechanical interlock that resists rotational loosening under vibration — without a separate lock washer, thread locking compound, or nylon insert nut. This is why serrated hex flange bolts are the default fastener on engine blocks, transmission housings, and any vibrating machinery assembly where self-loosening under cyclic load must be prevented without adding a second locking component.
Plain Flange — Coated and Cosmetic Surface Protection
Non-serrated (plain) flange bolts are specified wherever the serrations of a serrated flange would damage a painted, powder-coated, plated, or otherwise finished surface — the ribs of a serrated flange will bite through paint and coating, exposing bare metal and creating a corrosion initiation site. For visible cosmetic assemblies and any coated substrate where surface integrity must be preserved, plain flange is the correct specification.
Serrated vs Non-Serrated Flange — When to Specify Each
Serrated and non-serrated hex flange bolts look almost identical but perform completely different jobs. Specifying the wrong type either sacrifices vibration resistance or damages a coated surface.
Serrated Flange
Radial or circumferential serrations pressed into the flange underside bite into the mating surface as torque is applied, generating a mechanical friction lock that resists loosening from vibration, thermal cycling, and impact loading. The serrations require a bare metal or lightly finished mating surface to be effective — they are designed to dig in, not slide.
Non-Serrated (Plain) Flange
A smooth, flat flange face that distributes clamping load evenly without biting into the mating surface — preserving paint, powder coating, zinc plating, or any other surface finish on the joined components. Plain flange bolts rely on standard bolt preload and thread friction for joint retention, the same as a standard hex bolt plus washer combination.
Hex Flange Bolt Material & Strength Options
From general-purpose Grade 8.8 carbon steel through Grade 10.9 high-tensile and stainless A2/A4 corrosion-resistant grades.
Hex Flange Bolt Size Chart — M5 to M20
Flange diameter, head height, and across-flats dimensions per DIN 6921. All sizes available in every material grade and both serration options.
| Thread Size | Flange Dia. (mm) | Head Height (mm) | Across Flats (mm) | Pitch (Coarse) |
|---|---|---|---|---|
| M5 | 11.8 | 5.0 | 8 | 0.8 |
| M6 | 14.2 | 6.0 | 10 | 1.0 |
| M8 | 17.9 | 7.5 | 13 | 1.25 |
| M10 | 21.8 | 9.0 | 15 | 1.5 |
| M12 | 25.6 | 10.8 | 18 | 1.75 |
| M14 | 28.2 | 12.8 | 21 | 2.0 |
| M16 | 31.6 | 14.8 | 24 | 2.0 |
| M20 | 39.8 | 18.0 | 30 | 2.5 |
Hex Flange Bolts Application Areas
Automotive, agricultural, general machinery, and structural assemblies — wherever assembly speed, vibration resistance, and reduced part count matter.
Automotive
serrated for vibration
Agricultural Machinery
high-vibration duty
General Machinery
plain flange for coatings
Structural Steel
fewer parts, faster erection
Compressors & Pumps
vibration-resistant joints
HVAC Equipment
plain flange, coated panels
Appliance Manufacturing
high-volume production
Rail & Transport
high-vibration serrated
Hex Flange Bolt Standards Coverage
Same ISO 9001:2015 quality system as our complete fastener range. Every serrated and non-serrated hex flange bolt lot is dimensionally verified against the applicable standard before dispatch.
Complete Documentation Package
Grade / Standard Stated
Tensile · Hardness
Flange Dia. Confirmed
Conformance
On Request
Related Bolt Types & Materials
Hex flange bolts for integral washer convenience. Standard hex bolts for general use. Carriage bolts for wood and flush-head applications. Full alloy range for corrosion-critical service.
Full Bolt Range
Complete bolt manufacturing capability at Akbar Fasteners — standard hex, flange, carriage, socket, and all specialty head forms across carbon steel through exotic alloy. ASTM, DIN, ISO, IS, and custom drawings all covered from a single Mumbai facility.
View Bolt Range →Carriage Bolts
For wood-to-metal and flush-head applications where the self-locking square neck (rather than a flange) prevents rotation during tightening. The correct alternative to hex flange bolts when the substrate is timber rather than metal or when a smooth exterior head profile is required.
View Carriage Bolts →High Tensile Fasteners
For applications where hex flange bolts in Grade 10.9 are still insufficient — Grade 12.9 standard hex bolts (not typically available in flange form due to HDG and coating limitations at this strength class) for the highest-load structural and machinery connections.
View High Tensile →Stainless Steel Fasteners
For hex flange bolt applications requiring corrosion resistance beyond carbon steel + coating — SS A2 (304L equivalent) and A4 (316L equivalent) hex flange bolts for food, chemical, marine, and architectural applications where the integral washer benefit is combined with stainless corrosion resistance.
View SS Range →Need Hex Flange
Bolts?
Serrated or plain · grade · size · standard (DIN/ISO/JIS) · quantity. Akbar Fasteners Pvt. Ltd. responds within 24 hours.
Export: +91 8696 32 4646 · Support: +91 800 9315 333 · export@akbarfasteners.in