Automotive Fluid Connections Explained

Radium Engineering has extensive experience with automotive fluid connections. This guide explains the most popular fluid connections currently used for plumbing fuel, oil, coolant, air, and other needs. It will also explain how to properly utilize connections to ensure a leak-free operation. To properly and safely install our products, understanding connections is of the utmost importance. Most of the following information can be found with research. However, we decided to consolidate it in one place to discuss it's relation to automotive applications. Brake line related fittings such as SAE flare, inverted flare, hard lines, etc will not be covered. All listed information is simply a general guide. Please consult with hose and fitting manufacturers before selecting components for safety critical systems.


Beaded/Barbed Stem
Beaded or barbed stem fittings work as a hose-to-fitting connection. This is simply a pipe with a slight flare in it. These are the most common connection types and are found on practically all vehicles. They are inexpensive, effective, and do not require any special end fitting for the hose. Hoses slide on and secure with a hose clamp. An example of a plumbing manifold with beaded stem connections is pictured below.

When using beaded or barbed stems and rubber hose for fuel injection systems, high pressure EFI rated fuel hose should be utilized. Fuel injection hose also requires smooth rolled edged EFI hose clamps as shown below.

Beaded/Barbed Stem Summary
Common Automotive Uses: fuel lines, coolant lines, radiator hoses, PCV/Emissions hoses.
Benefits: inexpensive, easy servicing, very common, no hose fitting required, no special tools.
Drawbacks: not for extremely high pressure, not for high temperature, not as robust as other fluid connections, requires clamp, aesthetics.


SAE Quick Connect
SAE quick connections are commonly found on all modern OEM fuel systems after the millenium. There are no threads involved, just o-rings and locks. 

The most popular sizes used in fuel systems are 3/8" and 5/16" (or 8mm shown below). However, we have used less common sizes such as 1/2" and 12mm. 

SAE quick connect fittings follow an industry standard that is used in all automotive markets (USDM, JDM, EDM, etc.). While the physical dimensions associated with sealing (diameters, lengths, radii, etc.) is standardized, the locking portion of the connector varies greatly. 

For instance, the European market commonly use a short area past the bead (Porsche shown above) that does not cater to most aftermarket adapter fittings. Some OEM SAE quick connectors require a special tool for disconnecting while others use multiple locks (Ford Focus RS shown below) that can be simply disassembled by hand. These locking versions might be more service-friendly, but they are also more susceptible to breaking.

SAE Quick Connect Summary
Common Automotive Uses: fuel lines, PCV systems, EVAP, etc.
Benefits: inexpensive, easy servicing, very common.
Drawbacks: many require a special tool, some plastic lock variations can easily break.


Push-Lok Hose Ends
Push-Lok hose barbs have a means to attach a hose-end to a rubber hose. Several aggressive barbs lock into the inside diameter of the hose and create a seal. Below is an example of a push-lok hose end.

Depending on hose type, these hose ends can be slightly difficult to assemble and may require lubricant. Once installed, they cannot be pulled apart and can withstand several hundred PSI. The hose must be cut to remove and preserve the fitting. Most hose manufacturers offer a hose type specifically made for Push-Lok fittings. Push-Lok hose names may vary (i.e. Aeroquip labels them SOCKETLESS fittings, Russell labels them TWIST-LOK, Summit labels them TWIST-TITE).  When these hosee nds are used with the correct hose, clamps are not required. When using the fitting with a non-Push-Lok rated hose, it is suggested to use a clamp. This combination would only be appropriate for low-pressure applications and one must be careful to ensure the Push-Lok fitting does not damage the hose when a clamp is used. The barbs have the potential to slice through the walls of non-Push Lok rated hose.

Push-Lok Hose End Summary
Common Automotive Uses: aftermarket fuel systems, vacuum/boost hoses.
Benefits: reusable, high pressure rating, will not leak, low cost, can be easy to assemble, no hose clamp required.
Drawbacks: Requires special hose, can be difficult to assemble, hose must be cut for removal.


Tapered Pipe Threads
In the US, pipe threads are commonly called NPT (National Pipe Thread). NPT is a fitting-to-fitting connection. They work in a simple, crude, but effective manner. As the tapered threaded are screwed together, the male fitting wedges into the female fitting. The flanks of threads compress into each other forming a seal. See diagram below. Pipe threads are commonly called out by a fractional size such as 1/4 NPT or 3/8 NPT. More information on sizing can be found here

When using NPT fittings, attention must be paid to material compatibility. One of the worst case scenarios is screwing together 2 stainless steel pipe thread fittings. This combination threaded together can cause "galling". In this case, anti-seize should be applied while paying careful attention not to over-torque. In general, NPT threads in all materials deform when tightened. For this reason, they may not seal as well the second time they are used. This is especially true for soft non-ferrous materials. Besides NPT, it is important to note there is another family of tapered pipe threads called British Standard Pipe Tapered (BSPT). They work exactly in the same way and look almost identical when similar sizes are compared; however, the threads are slightly different. An NPT fitting will not properly seal in a BSPT fitting. Many Japanese engine engines (Nissan, Toyota, Subaru, etc) use 1/8 BSPT oil pressure switches. When using this size for an aftermarket turbocharger kit, it is often mistakenly for 1/8 NPT fitting. Mixing the two sizes will create a slow leak over time.

For installing, Teflon (or PTFE equivalent) in the form of paste or tape MUST BE USED. This lubricates the interface to prevent damage and make sure the fittings can be unscrewed in the future. Next, hand tighten the fitting. Finally, add another 1.5-3 turns using a wrench.

NPT Fittings Summary
Common Automotive Uses: valve cover PCV fittings, threaded fluid holes in engine blocks (oil, water etc), sensors, gauges, etc.
Benefits: inexpensive, very common, no special tools required, can permit ideal orientation (or clocking) of components.
Drawbacks: permanently deforms threads, not a positive guaranteed seal, sensitive to installation, some fittings cannot be reused, do not swivel, requires PTFE lubricant.


-AN Conical Seal (aka J.I.C.)
-AN (Army/Navy) style fittings are commonly spotted in engine bays in red and blue anodized colors. They are very popular to use for all fluid types from oil to fuel to brake lines. History of this style of fitting can be found online. They are also known as J.I.C. fittings (mostly in the hydraulic industrial world). They both use a 37 degree conical seal but technically have different threads standards. However, JIC and -AN fittings can be interchanged with each other. This style of fitting originated in military applications and over the years have made their way into the automotive aftermarket realm. Authentic MIL-spec -AN fittings are constructed to exacting tolerances and are very expensive. Luckily, many automotive aftermarket companies exist today that manufacture -AN "style" fittings that may not be MIL-spec but still work great for our needs. These would include Russell, Earl's, Aeromotive, Radium, Red Horse Performance, etc..

Example of -AN hose and fittings on a Nissan SR20DET engine.

These fittings work by mating a tapered conical surface on the male fitting into a reverse tapered surface in the female ftting, shown below. When the B-nut (female side, blue) is tightened it draws the two gold and gray parts together and a metal-on-metal mechanical seal is formed between the two conical surfaces. THE THREADS DO NOT PROVIDE A SEAL.



Over-tightening the B-nut can create a leak. It takes suprisingly little torque to fully tighten an -AN fitting. Many auto parts retailers sell -AN wrenches that are aluminum so they do not marr the fitting surface. These wrenches are also very short, helping in tight spaces while preventing over tightening. DO NOT USE ANY TYPE OF TEFLON, PTFE, LOCTITE, SILICONE SEALANT, ETC ON -AN FITTINGS! They are designed to screw together dry. A small amount of light oil may be used to assist screwing the B-nut down in some applications. The male and female tapered conical surfaces need to be clean and free of scratches or dings, otherwise a leak can occur. More information can be found here.
The 37 degree -AN/JIC flare should not be confused with a 45 degree SAE flare. These two are not compatible and will not seal.

Shown above, special tools can be used to form a 37 degree flare in hard tubing which allows a leak free way to transition from hard tubing to hose using all 37 degree flare fittings.

The chart below explains how -AN fittings are referenced. So when you hear "I need a dash six ninety hose end" you know the person is working with 3/8" ID hose.

These fittings are available in a variety of different colors and materials. Visit your local high performance parts retailer to see all the options.
-AN hose ends are installed on a hose by different means. The Push-Lok hose ends are commonly used to convert a rubber push-lok compatible hose to be used with an -AN fitting.

However, -AN hose ends can be assembled using a screw down collar as well.

Another way to attach a hose end to a hose is by crimp. Below is a photo of a -AN hose ends that are crimped by a special machine. The hose type in this picture is PTFE teflon with a black sheath covering the stainless steel braid material.
Why so many different types of hose? Many factors influence what hose to select for your application. Pressure rating, operating environment, aesthetics, cost, ease of assembly, fluid type, etc. Contact the manufacturer if you are unsure what hose is best for your application.

-AN Conical Seal (J.I.C.) Fittings Summary
Common Automotive Uses: aftermarket plumbing of fuel, air, coolant, and oil, used on racing vehicles of all types.
Benefits: positive seal, uninterrupted flow, wide variety of fitting and hose configurations available, easily serviced, suitable for high pressures and temperatures, very robust connection, can be easy to disassemble, most can swivel after assembly.
Drawbacks: high cost, assembling hose-end to hose can be labor intensive.


O-Ring Port
O-Ring port fittings is commonly called ORB for short which stands for o-ring boss. They are an excellent way to attach a -AN hose to a threaded hole. The fittings seal by simply compressing an O-ring making them serviceable. The female threaded hole MUST have an SAE specific chamfer at the opening in order for the O-ring to work properly. If there is no chamfer, the O-ring will be damaged from extreme compression and the fitting will never fully tighten. The male and female threads are both straight threads. The threads are the same as the threads on -AN 37 degree fittings shown above. This is handy as an O-ring can be installed onto an -AN fitting and thread it into an O-ring port assuming there is clearance. There are a wide variety of ORB adapters available from most fitting manufacturers. No PTFE or sealant is used on the threads of these fittings because the O-ring is providing the seal, not the threads. However, the O-ring must be lubricated with a small amount of petroleum such as engine oil before assembly. This helps prevent damage to the O-ring when tightening the fitting.

Below is an example of a distribution block with ORB ports.

O-Ring Port Fittings summary
Common Automotive Uses: aftermarket oil pumps, catch cans, fuel rails, fuel pump ports, fuel filters, etc.
Benefits: Positive seal, wide variety of adapters available, easily serviced, suitable for high pressures, robust connection, easy to disassemble.
Drawbacks: None.


There are more connections used in the automotive industry, but these are the ones that are dealt with the most by automotive builders and enthusiests. Understanding what products to use and how to properly use them will result in a leak-free reliable connection. If you are unsure, contact us for assistance or consult with your builder.

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