A Guide to Automotive Fluid Lines, Connections, and Fittings

Consider the fact that there are more than 30 quarts of fluid pumping or pressing throughout your vehicle at any given time – and that’s not even including the fuel system. That is a lot of fluids to keep sealed in their respective circuits, and it’s extremely important as some of those fluids are flammable while others could cause system issues or failures on the road if left unchecked. Not to mention just the annoyance of a puddle on the garage floor.

To keep all of these different fluids sealed tight, every hose, fitting and connection must be prepared and used correctly, and there are a lot of different clamps, flares, adapters, and fittings to consider. We decided to take a look at the tools and products available to help you plumb and seal the brakes, fuel systems, fluid lines, and other hoses on your car or truck to keep each system functioning properly and safely.

Most automotive hard line connections revolve around two style of flares, a 45-degree or a 37-degree. The 45-degree inverted flare is an SAE (Society of Automotive Engineers) standard found on hard lines for brakes, transmission lines and older fuel systems. These are double flared (a two-step process) and use an inverted flare fitting. The 37-degree flare derived from use in the Army and Navy, hence its name – the AN fitting – and it has a single flare. Obviously, these two flare styles and their fittings shan’t be intermixed.

To create hydraulic pressure, the brake system must be 100-percent sealed tight, which is why it is so important to use the correct fittings when building brake lines. Wilwood offers a number of fittings and adapters to help you complete your custom brake lines correctly, including chassis adapters, banjo fittings, flex lines and NTP-to-AN adapters. Learn more at wilwood.com/LineKits.

The great thing is that with a little effort on your part, you can produce both style of connections at home to fix a pesky connection or build an entirely new brake system, fuel system, or oil cooler lines. At first, shopping for fittings or flares may seem intimidating, but each manufacturer can break it down to the parts you’ll need for your application. Once you set about the bending, flaring, fittings and routing of one fluid system, you may find yourself looking to update another part of your car!


Flex and Fittings

For obvious reasons, one can never underestimate the need for proper connections throughout the brake system – and there are a lot of connections! When it comes to getting brake fluid to the caliper, Wilwood recommends running new stainless steel hoses for the best hydraulic fluid connection.

Whether you’ve stepped up to one of Wilwood’s disc brake setups or not, the company offers a variety of vehicle-specific braided stainless steel brake line kits. These premium-grade flex lines are durable and provide an improved brake pedal feel compared to original rubber lines. The -3AN lines are terminated with various inverted flares, metric, banjo, or AN ends to mate factory hard lines with Wilwood calipers or adapter fittings. Each line kit is supplied with all the required fittings to complete the installation on a specific vehicle – check out wilwood.com/Linekits for all the details.


Summit Racing Equipment

Flare Like a Pro

If you’re going to custom plumb your fuel system hard lines or perhaps a complete brake system, do yourself a favor and invest in a professional flaring tool. The consistency and quality of each flare will make it worthwhile knowing everything will fit and seal properly.

Summit Racing Equipment offers a couple different Pro Flaring Tools to form either 37- or 45-degree single-, double-, or bubble flares on 3/16-, ¼-, 5/16-, and 3/8-inch diameter OE steel, stainless steel, and soft metal tubing. The ‘turret-style’ design allows you to make precision flares and the T-handle screw clamp secures the line in place while forming. The dies are mounted right on the turret—no fumbling around for small parts or worrying about missing dies and the tools come in a sturdy storage case to keep everything together when not in use.


Sidebar – Fittings

Here are some different fittings and their common uses that you’ll find within high pressure fuel systems, transmission cooler lines or other oils (left to right):

A – AN 90-degree hose fitting. This fitting is used on the end of a stainless high pressure line. The fitting pointing down is installed to the hose, the female fitting pointing right connect to male AN fitting. All AN fittings are 37-degrees

B – NPT to male AN on top, NPT to a hose nipple below. Both are common on fuel systems and oil coolers in order to adapt NPT female ports to accept an AN fitting or hose end.

C – O-Ring Boss to AN. The O-Ring Boss fitting shares the same straight threads of an AN fitting but seals against a flat, machined surface with an O-ring to produce the seal. Installation is simple and effective. Common uses include performance fuel rails, water or fuel pumps and fuel tank modules.

D – OEM fuel rail to AN adapter. When using a factory fuel rail on an engine swap, you’ll likely need an adapter to go from the OE quick-connect fuel rail to an AN fitting.

Vintage Air

EZ AC Hoses

One thing that slows a lot of people down when installing an A/C system is when it comes to crimping the hose fittings in place. In most cases, special dies are required to crimp the lines in place to seal in the refrigerant and high pressures. Vintage Air developed a connection and crimp design that not only seals the hose and fitting, but also made it so you can build custom hoses in your garage.

The E-Z Clip refrigerant hoses are 30-percent smaller in outside diameter than traditional rubber hoses and feature an extremely durable textured finish. The nickel-plated steel O-ring fittings incorporate stainless steel cages and clips that effectively crimp in place with a simple hand tool. This combination delivers a positive, leak-free seal that can even be completed inside the car or under, or even after the hoses have been routed through the firewall or core support. The E-Z Clips will keep the cool in!



AN Prep Kit

When it comes down to securing high-pressure hoses, AN fittings are a solid choice. There is a bit more to assembling an AN fitting and it may take a couple practice runs to get it right, but once they’re done, you’ll be happy you did the extra work.

You can get away with typical hand tools to assemble AN fittings, but if you’re plumbing your whole car, do yourself a favor and invest in the tools to do the job right while making it easier. Speedway Motors offers a kit that comes with everything you’ll need including wrenches for AN -3 through -20 fittings, a heavy-duty cutting tool for crisp, clean cuts, and three assembly dies from Koul Tools to help easily assemble -4, -6 and -8 lines. Speedway also includes their reference template to match AN or NPT fittings to AN hose and specs on brake or fuel lines.


How many different fluid connectors are in this one pic? There’s a metal radiator tube with two small hoses and clamps, A/C lines that have been crimped and two hoses with shrink clamps, plus down on the bottom there are two push-lock style fittings going into the oil breather catch can.


Complete Fuel Lines

Whenever you make a fuel line system, it’s almost guaranteed that you will at some point need another fitting, which stops all progress until you run to your local speed shop. FiTech set out to solve this dilemma with their new Stainless Steel Hose Kits.

The kits are available with 20- or 40-feet of high-pressure stainless hose in natural or black finishes so you’ll have plenty to plumb a complete feed and return fuel system. Each kit is supplied with three 90-degree and 45-degree fittings along with six straight fittings (all -6AN size). They even include a 40-micron fuel filter and inline check valve to complete the job at hand! The fuel line is safe for pump gas, ethanol, or alcohol and has a chemical-resistant braid that will look good for miles to come.


Champion Cooling Systems

Filtering Coolant

Fuel filters and oil filters get all the glory and attention, but have you ever thought about a coolant filter? Debris can find a lot of hiding places in an engine block and when that crud finally breaks free, it can cause serious issues, including overheating.

Champion Cooling Systems has developed an effective filter that will protect your radiator and heater core from debris. The filters are made of an impact-resistant glass tube for easy coolant inspection at a glance along with a washable filter element. The filters are available in 1.25-, 1.5- and 1.75-inch sizes in black, chrome or gray.


Koul Tools

Barbs Made Easy

If you’ve ever installed an EFI system, you’ve likely encountered the barbed fittings and heavy-duty fuel hose needed to handle the higher fuel pressure. Without a special tool, you’ll build up your biceps like Popeye before you’re done with half the fuel system. The lines and fittings work great, but they can be a chore to build. Thankfully, Koul Tools has a handy solution.

The company’s EZ-ON Hose Press, PN 409B, makes installing ¼- to 1-inch barbed fittings a breeze! The unique tool features a patented lead screw system that requires only a ratchet or small impact gun to assemble hoses quickly and safely. Simply secure the fitting in the vice side of the tool, clamp the hose in the traveling carriage, and turn the drive screw system to install the hose. No heating or uneven hose installations – and no muscle aches.


Master Power Brakes

Finding Fittings

Whether you’re repairing a single brake line or fabricating a complete brake line system, you can bet that your local parts shop is not going to have the right fittings and adapters you’ll need. Instead, you should turn to experts who live and breathe brake systems every day, like the team at Master Power Brakes.

MPB offers a handy 60-piece Deluxe Brake Fitting Kit that provides many of the normal fittings and several of the odd fittings you may need for your brake system. It’s loaded with fittings, couplers, adapters, a couple plugs, and even a couple T-fittings. You may not use them all, but it will definitely save a trip (or two) to the store. If this kit is a little overkill, they also offer a 22-piece kit ideal for front disc swaps and smaller jobs.


Sidebar – AN Assembly

AN fittings are commonly used on high pressure fuel lines but can also be used for any fluid system under the hood. They take a little more work to build, but are worth the extra effort, especially for connections you take on and off.

We stopped by Teegarden Motorsport Specialties in Rancho Cordova, California, to get some pro tips when it comes to assembling AN fittings. Their most important tip is to stick with one brand of fittings to finish your project and always use the hose recommended for the fittings.


Mr. Gasket

AN Wrenching

If you’re using a lot of AN fittings on your car, it’s best to have a complete set of AN-specific wrenches handy such as this seven-piece set from Mr. Gasket. The kit includes wrench sizes from -3 to -20 that are CNC machined from billet 6061-T6 aluminum then stamped and anodized different colors to help when selecting a tool for the job at hand.

Each wrench has contoured edges for a comfortable grip and a precise, wide surface to keep from scratching your fittings. The wrenches are also short to ease working in tight spots and to reduce the chance of over-tightening the fittings (a common mistake!). A quality set of AN wrenches should always be used rather than standard wrenches to do the job right.



Fuel Hose Upgrade

Whether you’re stepping up to an EFI system or sticking with your tried and true carburetor, it may be time to consider new fuel hose. Today’s blended fuels are absolutely brutal on older rubber hoses, especially standard old replacement lines. Earl’s Performance Plumbing offers Vapor Guard hose that is designed for high fuel pressure and to withstand the corrosive properties of modern fuels including E-85, 100-percent methanol, diesel, and bio diesel.

The Vapor Guard rubber hose is a multi-layer design with the innermost primary barrier providing resistance to heat and aggressive fuels. The middle barrier is a thermoplastic layer that provides superior fuel isolation and virtually eliminates vapors from escaping through the hose, while the exterior layer provides extreme tolerance to heat and ozone. The hose is available in 5/16- and 3/8-inch diameters in different lengths. When running a rubber hose, always use the correct clamps that grip the hose, not cut into it.


If you’re running the factory fuel rails of a late model engine, you’ll need an adapter to fit them to an AN-style fitting. This is example is available from Earl’s.


Smooth Bends

There is nothing stock about most hot rods, so when it comes to routing brake lines, fuel, or transmission lines, you’re likely going to be bending your own. The best way to keep from kinking a line is with the aid of a quality tubing bender such as the ones offered by Snap-on.

A couple different models are available, with the model shown easily working ¼-, 5/16- and 3/8-inch OD soft copper or aluminum tubing. The tool is lightweight yet very durable, with cushioned handles and an offset lever to ease making up to 180-degree bends. There are also 15-degree increments marked off to help create consistent forms. You’ll be amazed at just how handy a small tubing bender will be to have in your toolbox.


Todd Ryden is first and foremost a car guy and admits to how lucky he is to have been able to build a career out of a hobby that he enjoys so much. He’s owned muscle cars and classics, raced a bit and has cruised across the country. With over 25 years in the industry from the manufacturing and marketing side to writing books and articles, he just gets it.