Flange Bolts
What Are Flange Bolts?
Flange bolts are hex-head fasteners with an integrated washer under the head. The built-in flange increases bearing surface and distributes clamping force more evenly than a standard hex bolt alone, often reducing the need for a separate flat washer and speeding installation.
Serrated vs. Non-Serrated Flange Bolts
- Serrated flange bolts: The underside of the flange has radial serrations that bite into the mating surface to help resist loosening from vibration. Use where a locking action is desired and surface marring is acceptable.
- Non-serrated flange bolts: Smooth bearing surface under the flange for applications where you want the load distribution of a flange but prefer not to mark the surface or where parts need to be repositioned.
Full-Thread vs. Partial-Thread Flange Bolts
- Fully threaded (FT): Threads run the full shank—best for thin stacks and maximum thread engagement.
- Partially threaded (PT): Includes a smooth shoulder that can aid alignment and provide shear/bearing area under the head.
Materials & Finishes
- Carbon/alloy steel: Offered in common inch grades (e.g., Grade 2, 5, 8) and metric property classes (e.g., 8.8, 10.9, 12.9).
- Stainless steel: 18-8/304 for general corrosion resistance; 316 for harsher or marine environments.
- Coatings: zinc or yellow zinc for interior service, hot-dip galvanized (HDG) for heavy-duty outdoor protection, and black oxide for a matte appearance (limited corrosion resistance).
Flange Bolt Sizes & Threads
Order by diameter × thread × length (e.g., 3/8"-16 × 1-1/2" or M10-1.5 × 40 mm). Inch threads are typically UNC (coarse) or UNF (fine). Length is measured from under the head to the tip.
Common Uses for Flange Bolts
- Automotive, equipment, and machinery assemblies subject to vibration
- Steel framing, brackets, and base plates where fast installation is valuable
- Applications where a wide bearing surface is needed without a separate washer
Installation Tips for Flange Bolts
- Match nuts and any washers to the bolt’s grade/material and thread.
- For HDG fasteners, use compatible HDG nuts to account for coating thickness.
- On painted or delicate surfaces, prefer non-serrated flanges or add a protective washer.
- Use appropriate torque; for stainless, a suitable anti-seize can reduce the risk of galling.
- Where vibration is severe, combine with prevailing-torque nuts or approved thread-locking methods.
Related Hardware
- Flange nuts (to mirror the increased bearing area)
- Hex nuts, lock nuts, and washers (when required by spec)
Flange Bolts FAQs
See the accordion below for answers about serrations, washers, torque, and material selection.
This content is general guidance. Always follow product datasheets and project specifications.
The built-in flange saves parts and time, provides consistent bearing area, and helps center the load. Serrated versions add a locking action against vibration.
Use serrations when you want extra resistance to loosening and the surface can accept light marking (e.g., machinery, automotive). Avoid on soft or cosmetic surfaces.
Often no, because the flange acts like a washer. You may still use a washer for delicate finishes, oversized holes, or when required by the specification.
Serrations can mar or break coatings. Choose non-serrated flanges or add a protective washer when appearance or coating integrity matters.
Yes, common inch grades include 2/5/8; metric property classes include 8.8/10.9/12.9. Thread pitch must match the mating nut or tapped hole.
Yes. 18-8/304 is common for general corrosion resistance; choose 316 for marine or more aggressive environments.
Both fully threaded and partially threaded versions exist. Pick fully threaded for thin stacks and maximum engagement; choose partial thread when a shoulder is beneficial.
Torque depends on size, grade/material, lubrication, and finish. Use the product’s torque chart or the project’s engineered procedure.
Structural connections typically require heavy-hex structural bolts per project spec. Use flange bolts only where permitted by the design.
Yes—flange nuts mirror the wider bearing surface and are common in vibration-prone assemblies.
They’re often reused if undamaged, but follow your maintenance procedure. For safety-critical joints, replace per spec.
Use nuts tapped for HDG (or otherwise specified) so the thicker coating doesn’t interfere with fit.