How to Choose the Right O-Ring for Your -8 ORB Fittings
By ACUITY Instruments: Performance Automotive Products | Published: 2026-10-22
Category: Buying Guides
Learn how to select the correct o-ring for -8 ORB fittings, including material compatibility, sizing, and when to use a -908 FKM o-ring for fuel systems.
Quick answer
For -8 ORB fittings in fuel systems, use a -908 size FKM o-ring (75 durometer) rated for gasoline, ethanol, and E85. This single seal handles nearly all performance fuel applications without guessing.
Why the -908 size matters for -8 ORB fittings
O-ring boss (ORB) fittings seal with a captured o-ring inside a machined groove. The dash number (-8) refers to the thread size, not the o-ring. For -8 ORB ports, the correct o-ring is a -908. Using a different size can cause leaks or extrusion under pressure.
The -908 o-ring fits snugly into the groove and compresses evenly when the fitting is torqued. This creates a reliable seal without relying on thread sealant or tape.
Material choice: why FKM over Buna-N
Standard Buna-N (Nitrile) o-rings work with gasoline but degrade quickly with ethanol blends like E10 or E85. FKM (Viton) resists both gasoline and ethanol, making it the safe choice for any fuel system that may see alcohol-based fuels.
The -908 FKM o-ring (75 durometer) offers a good balance of flexibility and compression set resistance. It seals well at typical fuel system pressures without becoming too hard or brittle over time.
When an adapter changes the o-ring size
If you need to step from -8 ORB to -6AN hose, the adapter itself contains its own o-ring. For example, a -6AN to -8 ORB adapter includes an ethanol-safe FKM o-ring pre-installed. In that case, you do not need a separate -908 o-ring for the adapter body.
However, the adapter still seals into the -8 ORB port using the same -908 o-ring. Always check whether the adapter ships with its own o-ring or expects you to supply one.
Decision rules for -8 ORB o-rings
- Identify the o-ring size: For -8 ORB fittings, always use a -908 size o-ring. The dash number of the fitting (-8) does not match the o-ring size (-908). Write down -908 as the o-ring size for any -8 ORB port.
- Choose FKM for fuel systems: Select FKM (Viton) material if the system may see gasoline, ethanol, or any ethanol blend. Buna-N is cheaper but fails quickly with ethanol. FKM is the default for fuel rails, injector bungs, and regulator ports.
- Check the adapter's included seal: When using a -6AN to -8 ORB adapter, verify whether it comes with its own o-ring. If it does, you only need the -908 o-ring for the port itself. Avoid double-ordering o-rings for adapter-equipped ports.
- Inspect the o-ring groove: Before installing, ensure the groove is clean and free of burrs. A damaged groove will leak regardless of o-ring quality. Clean the port with a non-residue solvent before assembly.
Products mentioned
- '-908 FKM O-Rings for use with -8 ORB Fittings (5-pack): FKM material, 75a durometer, compatible with gasoline, ethanol, and mixtures like E10 and E85. This single o-ring covers nearly all fuel types, so you don't need to stock different seals for different blends.
'-908 FKM O-Rings for use with -8 ORB Fittings (5-pack) - '-6AN to -8 O-Ring Boss (ORB) Adapter: Supplied with an ethanol-safe FKM o-ring; adapts -8 ORB ports to -6AN lines. When using -6AN hose with a -8 ORB fuel rail, this adapter includes the correct seal, simplifying your parts list.
'-6AN to -8 O-Ring Boss (ORB) Adapter
Who should use this approach
Best for
- Building or servicing a fuel rail with -8 ORB ports
- Converting a -8 ORB fuel system to -6AN hose using an adapter
- Anyone working with ethanol-blended fuels who wants a single o-ring solution
Not for
- NPT or push-lock fittings that use different sealing methods
- Systems requiring o-rings smaller than -904 or larger than -916
