Technical Foundation
What Is a Flange Bolt?
A flange bolt combines a hex (or sometimes 12-point) bolt head with an integral, wider flange directly beneath it, functioning like a bolt with a built-in washer that spreads clamping load over a larger bearing surface than the head alone would provide. This eliminates the need for a separate flat washer in many applications, simplifying assembly and reducing the chance of a missing or wrong-size washer in the field.
Serrated flange bolts add small radial teeth (serrations) on the underside of the flange that bite slightly into the mating surface as the bolt is torqued, providing additional resistance to loosening under vibration — common in automotive, engine and equipment applications with sustained vibration exposure. Non-serrated flange bolts are specified where the mating surface finish must not be marred by serration teeth, such as on painted, coated, or cosmetically visible surfaces.
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| Spec | What It Means | Why It Matters |
|---|---|---|
| Flange Type | Serrated vs. non-serrated (smooth) flange | Serrated resists loosening under vibration; non-serrated protects finished/coated mating surfaces |
| Thread Size & Pitch | UNC/UNF inch sizes, metric coarse/fine | Must match the tapped hole or nut thread exactly — not interchangeable across standards |
| Grade / Property Class | SAE Grade 5/8, metric Class 8.8/10.9 | Determines tensile and yield strength for the joint's clamping load requirement |
| Flange Diameter | Larger than a standard hex head, model-specific | Wider bearing surface reduces point-load and the need for a separate washer |
| Material & Finish | Steel (zinc, black oxide), stainless steel | Determines corrosion resistance and compatibility with the mating material |
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Applications
Where Flange Bolts Get Used
Flange bolts are specified anywhere a wider load-bearing surface is needed under the bolt head, or where vibration resistance from a serrated flange offers a simpler alternative to a lock washer.
Automotive & Engine Assembly
Serrated flange bolts securing engine, exhaust and drivetrain components subject to sustained vibration.
Agricultural Equipment
Flange bolts assembling implements and machinery exposed to continuous vibration and shock loading.
Industrial Machinery Assembly
Flange bolts reducing washer inventory and assembly steps on production equipment builds.
HVAC Equipment Assembly
Flange bolts securing ductwork brackets and equipment panels in vibration-prone mechanical rooms.
General Fabrication
Flange bolts specified wherever a wider bearing surface is needed without adding a separate washer.
FAQ
Flange Bolts Questions, Answered
What's the advantage of a flange bolt over a standard hex bolt with a separate washer?
A flange bolt's integral flange provides the wider bearing surface a washer would, without requiring a separate part — this simplifies inventory and assembly, and eliminates the risk of a missing or mismatched washer in the field. It also ensures the washer-equivalent surface is always properly sized and centered under the head.
When should I use a serrated flange bolt instead of a non-serrated one?
Serrated flange bolts are specified where vibration resistance matters, since the serrations bite slightly into the mating surface to resist loosening — common in automotive and engine applications. Non-serrated flange bolts are used where the mating surface must stay smooth or unmarred, such as on painted or finished surfaces where serration marks would be undesirable.
Are flange bolts interchangeable between inch and metric thread standards?
No — flange bolts follow the same thread standard rules as any other fastener, so a UNC inch-thread flange bolt will not fit a metric tapped hole or nut, and vice versa. Always match thread size, pitch, and standard exactly.
Do flange bolts come in different strength grades like standard bolts?
Yes, flange bolts are manufactured in the same range of strength grades as standard hex bolts — SAE Grade 5 and 8 for inch sizes, Class 8.8 and 10.9 for metric — and should be selected based on the joint's clamping load requirement just as with any other structural fastener.
Can a serrated flange bolt damage a soft or coated mating surface?
Yes — the serrations are designed to bite slightly into the mating material for vibration resistance, which can mar soft, painted, or coated surfaces. For those applications, a non-serrated flange bolt or a standard bolt with a separate washer is the better choice.
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