What size fitting do I need for a fuel line?
Fitting size is determined by the hose or line ID and the connection type you’re joining. Measure the hose ID or the port diameter and match it to a compatible barb, flare, NPT, or AN/metric fitting that can handle the fuel and the system’s pressure.
In practice, there isn’t a single universal size. This guide walks you through how to identify the correct size, what sizes are commonly encountered on different fuel systems, and how to verify compatibility with your vehicle. From carbureted setups to modern direct-injection systems, the basic steps—identify ends, measure, and match to the right standard—remain the same.
Identify your fuel system and connection type
Knowing the type of fuel system and the port you’re connecting to is the first step. Carbureted and port-fuel-injected systems use relatively low pressures and simple barb or flare fittings, while modern direct-injection systems use higher pressures and specialized high-rate fittings. Always confirm compatibility with the manufacturer’s specifications before purchasing.
Common connection styles
Fuel line connections fall into several broad categories, and the size you need depends on which category your components belong to. The most common options are barb (hose-to-barb), flare, NPT (pipe thread), and AN (Army-Navy) or metric fittings used on performance and European vehicles.
Before you buy, confirm whether you’re working with a hose with a barb, a flare port, a threaded NPT port, or an AN/metric port, and then match the fitting size to that port and to the hose ID.
Below are typical steps to determine the right size and a quick reference to common sizes you’ll encounter.
Steps to determine the right size
- Identify the type of connection at each end (barb, flare, NPT, AN, or metric port).
- Measure the inner diameter (ID) of the hose you’re connecting to; this is the most common determinant for barb fittings.
- Check the port or thread size on the vehicle component (fuel pump, rail, carb, or rail adapter) to determine the correct threaded size (for example, NPT or AN size, or a metric equivalent).
- Choose a fitting that matches both ends (e.g., barb-to-barb with the correct ID, or a threaded adapter that matches the port and the hose ID).
- Verify the fuel type compatibility and the system pressure rating for safety and reliability, and use clamps or adapters rated for gasoline or ethanol blends as appropriate.
Following these steps will help you select a fitting that matches both ends and maintains a safe, leak-free connection.
Common sizes and typical uses
These guidelines reflect typical hose IDs and common pairing with fittings in everyday automotive work. Always double-check with the component manufacturer or product specs before purchasing.
- 1/4 inch ID hose — common on small, low-pressure carbureted lines. Use a 1/4 inch barb fitting or a 1/4 inch NPT adapter when connecting to threaded ports.
- 5/16 inch ID hose — widely used on many carbureted and some early EFI feeds. Pair with a 5/16 inch barb fitting; some setups use 1/4 inch NPT adapters to connect to threaded components.
- 3/8 inch ID hose — a frequent choice for modern EFI feed lines and aftermarket performance kits. Often found with -6 AN fittings in performance assemblies; ensure the hose and fittings are rated for the system pressure.
- 1/2 inch ID hose — used for higher-flow needs in some EFI and custom installations. Commonly associated with -8 AN fittings in performance line kits.
- 5/8 inch ID hose — less common in stock applications but found in some high-flow or diesel-type setups; frequently paired with -10 AN fittings in aftermarket builds.
Note: -6 AN lines commonly pair with roughly 3/8" ID hoses, -8 AN with around 1/2" ID hoses, and -10 AN with about 5/8" ID hoses in many aftermarket configurations. Always verify with the specific manufacturer’s chart for exact mappings, as there can be variations by brand and application.
System pressures to keep in mind
Fuel system pressure varies by application and fuel delivery technology. Carbureted systems operate at relatively low pressures (roughly a few psi). Port- and direct-injection systems run higher pressures, with typical port-injection systems around 40–60 psi, while direct injection can exceed 2,000 psi in the rail. For high-pressure lines, use components specifically rated for the fuel type and pressure, and avoid using low-pressure barb fittings for high-pressure lines.
Safety and best practices
Fuel system work carries fire and health risks. Always depressurize the fuel system before disconnecting anything, work in a well-ventilated area, wear eye protection, and use ethanol-compatible hose if you’re running E10+ fuels. When in doubt, consult a qualified technician or the component manufacturer for the exact fitting size and specifications for your vehicle.
Summary
The right fuel-line fitting size hinges on the hose ID, the port type, and the system’s pressure rating. Start by identifying the connection style, measure the hose ID, and match to the correct barb, flare, NPT, or AN/metric fitting. Common small IDs include 1/4" and 5/16", while higher-flow EFI lines often use 3/8" or larger with corresponding AN fittings. Always verify specifications from the manufacturer and prioritize fuel compatibility and safety.
