Hard Fuel Line Connectors & Fittings

What Are Hard Line Fittings?

Hard line fittings are the metal parts that join sections of rigid tubing and connect that tubing to pumps, filters, gauges, and reservoirs. Unlike rubber hose ends, they seal on solid metal line, so each type does a specific job. Here are the main types you'll shop in this category:

• Tube sleeves that slide over the tube and seat against the flare to form the sealing surface.
• Tube nuts that thread onto the fitting and press the sleeve tight against the mating cone.
• Olives, also called ferrules, which are compression rings that bite onto the tube in compression fittings.
• Compression fittings that seal by squeezing a ferrule around the tube, with no flaring required.
• Flare and AN adapters that convert a 37-degree AN flare to NPT pipe threads or another size.
• Unions that join two lines of the same size in a straight run.

You'll find these grouped under Hose, Hardline & Tubing, part of the broader Fittings & Hose department. Our guide on how to assemble AN fittings and hose assemblies covers the basics.

How Tube Sleeves and Tube Nuts Work Together

Tube sleeves and tube nuts work as a pair to seal a flared hard line. First you flare the end of the tube to a 37-degree cone (the AN standard), which spreads the metal into a sealing face. The tube sleeve slides onto the line behind that flare, and the tube nut threads onto the fitting body. As you tighten the nut, it pushes the sleeve forward so the flare clamps between the sleeve and the fitting's cone. That forms a metal-to-metal seal without any gasket. That's why the two parts are sized and ordered together by AN size. Match the sleeve, nut, and flare angle to the same standard, or the joint won't seal. If you're new to flaring, our walkthrough on how to flare tubing and brake lines covers 37-degree single and 45-degree double flares. You can also browse straight hard line and tubing to match your fittings.

Choosing the Right Size and Material for Hard Line Fittings

Start by matching the AN size of the fitting to the outside diameter of your tube. AN sizing sets both the thread and the flare dimensions. AN sizes in this category run from -3 through -12. The number is the tube's outside diameter in sixteenths of an inch, so -6 AN fits 3/8-inch line. Our guide to AN fittings and hose sizes breaks down how those numbers translate to real measurements. Material is your next choice. Aluminum fittings are light and easy to tighten, and they suit low-pressure fuel and oil lines. Steel and stainless hard line fittings resist corrosion and handle the higher pressure of brake systems, which is why many builders run stainless on brake lines. Brands like Earl's, Fragola Performance Systems, and Russell offer sleeves, nuts, and adapters across these sizes and materials, so you can keep one standard through the whole system.

Applications for Fuel, Brake, Oil, and Transmission Lines

Hard lines carry fluid on the parts of a vehicle where rigid tubing beats flexible hose: along the frame, near heat, and anywhere abrasion is a risk. On fuel systems, hard line runs from the tank to the engine bay, then transitions to flexible line at the last few inches. Brake systems rely on hard line for its high-pressure strength, and stainless fittings keep those joints corrosion-free. Oil and transmission cooler lines use hard line and adapters to route fluid to coolers and gauges without leaks. Where a hard line meets a flexible section, you'll often finish the job with rubber hose and secure it with hose clamps. Plan each run end to end so every fitting, sleeve, and nut matches the same AN size and flare angle. When you know your sizes and materials, shop hard line fittings here to complete your fuel, brake, oil, and transmission connections.