Everything You Need To Know About Automotive Plumbing

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By Ron Ceridono – Photography by the Author

Building a hot rod or classic truck requires many unique disciplines. Often overlooked is the vehicle’s plumbing, this includes engine coolant, brakes, vacuum ports, etc. There are many variables, so we built this article to define basic terms and provide a better understanding of plumbing your classic truck.

Someone once said if any part of building a classic truck looks simple, you’ve got to be overlooking something. That applies to every step along the way, including a straightforward task such as plumbing the chassis. When it comes to installing brake and fuel lines there are a variety of hard line materials, hoses, and fittings that must be compatible to operate safely and reliably. There are also a number of tools available that will make the job easier to accomplish and more professional looking when you’re done.

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Here are some pointers on selecting automotive plumbing components and build your truck properly:

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Pipe Threads

These are found everywhere from under kitchen sinks to vehicles of all sorts. The common variety used on cars and trucks is the American National Standard Pipe Threads or National Pipe Threads (NPT). Pipe threads differ from fasteners and other fittings as the threads are tapered—they screw together until they tighten due to the taper. To create a leak-free seal, NPT threads use some sort of sealer such as Teflon tape or a leak-preventative paste.

Inverted Flare Fitting

One of the most common types of fittings found on automobiles are SAE (Society of Automotive Engineers) inverted flares. These are unique because the tubing used with these fittings has a double flare for increased strength and resistance to cracking. When assembled, the flared end of the tube is clamped between the nut on the tube and the seat in the fitting. The result is a leakproof connection that will withstand extremely high working pressure and no sealer is required. Typical inverted flare fitting applications are brakes, power steering, fuel, and transmission cooler lines.

SAE 45 Degree Flare

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These see limited automotive use and may be found on instruments and other low-pressure applications. Use caution with single flares as they can crack due to vibration or over tightening.

Compression Fittings 

These are intended for low- and medium-pressure applications. Often found on instruments and some air systems, compression fittings are used on a variety of tubing materials, such as copper, steel, and plastic. They should never be used on brakes.

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AN Fittings

The reference “AN” stands for Army/Navy and refers to a system devised by the government during World War II to ensure interchangeability and compatibility of parts made by various manufacturers. AN lines and fittings use 37-degree single flares and are identified by a dash and a number for each size of metal tubing, hose, and the corresponding fittings. The number after the dash indicates the internal size of the components in increments of 1/16 inch. As an example, a -6 line is 6/16, or 3/8 inch.

Another important distinction when using AN fittings are the port threads, they are not NPT, rather they have straight threads (like any normal fastener) and use an O-Ring Boss (ORB) to provide a leakproof seal.

Fitting Adapters

There are a variety of adapters available to connect different types of fittings. They make it possible to mix and match inverted flare, pipe, and AN fittings as well as connect standard and metric sizes.

Clamp Style Hose Fittings

Clamp style hose fittings are used for low-pressure applications (maximum of 35 psi) with flexible hose, most often on carbureted fuel systems. These fittings have a raised end to keep the clamp and hose from slipping off. Slipping hose over a piece of tubing without a raised section, which is often done on fuel systems and transmission coolers, relies on the force of the clamp, only making it easy for the two to separate.

Push-On Hose Fittings

Push-on hose fittings are manufactured for low and medium pressure (maximum pressure 200 to 250 psi). These are quick disconnect fittings and have high resistance to vibration damage. When used in combination with the proper hose, no clamps are required. Because of the tight fit they can be challenging to assemble.

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Teflon Tape

Compared to every other product we can think of, Teflon tape is likely the most often misused. When applied correctly Teflon tape can prevent leaks, prevent thread galling, and protect fittings from corroding. However, it is sometimes used on fittings that shouldn’t leak. As an example, it should never be used on inverted flare fittings as they are designed to seal without it. Never use Teflon tape on an engine’s electrical senders as it will prevent the proper grounding necessary for accurate gauge operation. Surprisingly, Teflon tape should not be used on the cooler line fittings in automatic transmissions as it can make the pipe fittings tighten so easily that the tapered threads can split the case.

Pipe Thread Sealing Compound

Thread sealant is made from a material that fills the gaps in threaded connections preventing fluids from leaking out. Besides creating fluid-tight seals, pipe thread sealants also lubricate the threads, making them easier to assemble.

Automotive Tubing 

One of the decisions that has to be made when plumbing any vehicle is what kind of tubing to use. For brake lines there are only three options: steel (usually tin or with a plastic-like coating to prevent rust), stainless steel, or NiCopp (seamless nickel-copper alloy tubing that is DOT approved for hydraulic brake systems)—which is easier to bend and form than steel tubing and non-annealed stainless steel.

Keep in mind that in operation a brake system may produce well over 1,000 psi, so any other type of tubing, such as copper or aluminum, should never be used. In addition, long lengths of flex line, including the braided variety, should not be used, as a spongy pedal will result.

On the subject of brake lines, there are a couple of common misconceptions we should clear up. One is there is no relationship between brake line size and hydraulic pressure. The hydraulic pressure in the brake system is created by the master cylinder, the lines simply deliver the pressurized fluid to each wheel. In most cases 3/16- or 1/4-inch-diameter brake lines are used.

While there will be no pressure difference between the two, there may be a difference in the amount of fluid delivered. The bigger tubing will carry more volume, so 1/4-inch line may be preferable in some instances, such as when using disc brake calipers with large piston displacements. The second misconception about brake lines is that stainless steel tubing cannot be double flared for use with standard automotive fittings. While it is true that some stainless steel tubing will crack in the process of double flaring, appropriately annealed stainless steel tubing will.

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As for fuel lines, steel and stainless steel are recommended for most applications. Aluminum tubing is often found on race cars because it’s light, but it chafes easily and needs more support with line clamps than steel or stainless steel, so it’s not recommended for “street driven” vehicles. Another consideration when plumbing a fuel system is flexible hose. The additives in today’s fuel can cause hoses to deteriorate rapidly. Always buy top-grade hose that is compatible with alcohol or buy braided hose with the proper inner liner.

Flex Lines 

When measuring for flex lines, movement must be taken into account. For brake lines, the front wheels must be able to turn lock-to-lock and the suspension on both ends of the truck must be able to go through their full range of travel without putting stress on the hoses. That means also ensuring that the hoses are long enough so when the truck is jacked up the hoses won’t be pulled apart. For flexible fuel lines leading to the engine provide enough length to accommodate movement between the engine and chassis.


This is simple enough—keep brake and fuel away from moving parts, sharp edges, and sources of heat. Make sure they are secure by using clamps every 18-24 inches so they can’t move or vibrate, which can cause damage resulting in a leak.

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Take a look at the following photos for our assortment of proper plumbing pointers that will help your classic truck perform safely and reliably without any puddles of critical fluids on the ground.

002 Steel brake line with a double flare for brake line applications
This is a typical steel brake line with a double flare and a tubing nut commonly used for brake and fuel line applications.
003 Double flare tube
Double flared tubing has the flare folded back onto itself to make a thick seat that is unlikely to crack. The angle of the sealing surface is 45 degrees.
004 AN fitting hydrualic fittings
In this example the tubing is stainless steel with a single flair at 37 degrees for an AN fitting (another application of 37-degree flares will be found on JIC hydraulic fittings).
005 Flared fitting for AN fittings
Flares for AN fittings use a reinforcement sleeve and a nut with female threads.
006 AN fitting guide chart
This chart shows the “dash” sizes for AN fittings and the diameters and thread counts for pipe.
007 Speedway Motors flaring tools
Speedway Motors offers a variety of flaring tools, including this handy tool that makes perfect 45-degree double flares. Tools for 37-degree flares are also available.
008 Double flaring tool for hydraulic lines
Another 45-degree double flaring tool is this hydraulic version available from Summit Racing.
009 Single flare bending tubing tool
For single 45-degree flares this simple set was picked up at a swap meet years ago. The springs are supposed to help bend tubing—they don’t.
010 Tube cutter for steel and copper tubing
Tubing cutters for steel and copper tubing come in several shapes and sizes. However, these cutters will often work harden so a 32-tooth per inch saw blade should be used. Regardless of the method, the tubing should be cut off squarely and any burrs in the end should be removed before flaring.
011 Bending tubes with a tube bending tool
Bending tubing takes planning, patience, and a good tool. This is our old faithful bender—we found it works best by using some silicone spray on the tubing to help the tool slide.
012 Male hose fittings with barbs
These are examples of a male and female hose fittings with barbs to help retain the hose. Others have a raised portion on the held to keep the hose and clamp from slipping off.
013 Hose fitting with barbs for classic truck
These are examples of a male and female hose fittings with barbs to help retain the hose. Others have a raised portion on the held to keep the hose and clamp from slipping off.
014 SAE inverted flare fittings for hot rods
SAE inverted flare fittings have a sealing surface that matches the 45-degree angle of the tubing. These types of fitting do not require thread sealer.
015 Tubing adapters with female inverted flare seat from Speedway Motors
Tubing adapters come in a mind-boggling array of shapes and sizes. This 90-degree fitting has a female inverted flare seat on one end with male pipe threads on the other. This and the following examples are from Speedway Motors.
016 AN adapter for fuel and cooling system
This AN adapter has a 37-degree male connector on the left with male pipe threads on the right (note the taper of the pipe threads). These are often used in applications such as fuel and cooling systems and remote oil filters.
017 Brake fitting with double flared end brake line
A common adapter is this brake fitting that connects 3/16 double flared steel brake line to a -3 AN hose. The slot is for a clip to hold the fitting in a frame-mounted bracket.
018 AN Hose for a wheel cylinder
Another common brake adapter, this is used to connect an AN hose to the inverted seat connection found in many wheel cylinders.
019 Banjo fitting for a AN line to master cylinder
This can be a real problem solver when space is an issue—it’s a banjo fitting to connect an AN line to a master cylinder.
020 Compression fittings for oil pressure or vacuum line
Compression fittings use a nut and a ferrule to seal the tubing to the main body. This example has male pipe threads and would typically be used on a mechanical gauge, such as oil pressure or vacuum.
021 Compression fittings for low pressure application
After compression fittings are tightened, the ferrule grips the tubing, which in this case is copper. Compression fittings are for low-pressure applications and should never be used on brakes.
022 Koul Tools Surseat lapping tool from speedway motors
Not all flares are created equal. Koul Tools offers the Surseat lapping tool that removes irregularities in 37- or 45-degree flared tubing to prevent leaks. Koul Tools are available from Speedway Motors.
023 Koul tools fitting repair tool to repair AN fittings
Another lapping tool from Koul Tools, this Fitting Repair Tool one dresses the male ends of damaged 37-degree AN fittings.
024 Push lock hose tool Koul Tools EZ-ON hose press tool
Assembling push lock hoses can be difficult. We started using a Koul Tools EZ-ON hose press and found it simplifies installing push lock hoses onto barbed fittings and cuts down considerably on the amount of swearing done during the process.
025 Teflon Tape threads from Speedway motors
Teflon tape is applied to male fitting starting on the second thread to keep from contaminating the system. Looking from the end of the fitting, wrap three to five layers around the threads in a clockwise direction.
026 ARP teflon tape paste thread sealer
Thermo-Tec’s Express Sleeve can be used to protect brake and fuel lines from radiant sources of heat. It can be installed with the lines in place and then secured with the integral hook and loop closures.

027 Thermo-Tec Express Sleeve for brake lines and fuel lines

(800) 826-3045

Summit Racing Equipment
(800) 230-3030

Speedway Motors
(855) 313-9173

(800) 274-8437

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