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Honda K-Series Engine Swaps: AN Plumbing

Almost any K-series engine swap exposes you to specialty fluid components, such as steel-braided hose and anodized-aluminum fittings, and they might lead you to the home improvement warehouse’s plumbing aisle in search of enlightenment. At first, it all seems simple: Like plumbing underneath a bathroom sink, your car’s fuel, cooling, and oiling systems simply displace fluid from one place to the next. Any differences between the two seem inconsequential but, as it turns out, they really aren’t.

For one thing, home improvement hardware isn’t exposed to corrosive, 200-degree-Fahrenheit fluids and doesn’t undergo the sort of movement that, for example, a radiator cooling hose might. Neither can be said of whatever’s flowing through your engine right now. Caustic fluids, high temperatures, and moving parts all make its fluid transfer systems exponentially more complex than anything in your home. Thread sizes, thread pitches, and sealing surfaces only add to the complexity.

This Tech Tip is From the Full Book, HONDA K-SERIES ENGINE SWAPS. For a comprehensive guide on this entire subject you can visit this link:


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AN (Army/Navy) plumbing, as it’s commonly known, is exactly the sort of stuff you should be considering for your car and is exactly the sort of stuff that you won’t find at a hardware store. AN plumbing was developed just before World War II by the aerospace industry as a military standard but has moved into the high-performance realm; its usefulness in a K-series engine swap is significant. Understanding the good and the bad of all of this as well as what is and isn’t compatible with one another is important if you really want to separate what’s appropriate for the home as opposed to your engine bay.

Braided Hose

The poster-child of competition plumbing is the stainless steel braided hose. A closer look reveals that it’s more than just a shiny cover over any old rubber hose. Internally, it’s made of synthetic rubber or Teflon.

Rubber-based versions consist of a steel-braided sheath that’s embedded into the hose itself and a second that wraps about its exterior. Hoses such as these are best suited for oil, fuel, or coolant and can handle temperatures as high as 300 degrees Fahrenheit at relatively moderate pressures.

Teflon-based steel-braided hose is able to withstand pressures as high as 4,000 psi, making it ideal for custom hydraulic clutch lines, which are required of almost every K-series engine swap. When assembling hoses and hose ends, be sure to pressure test them appropriately before installation and subjecting them to use.

Teflon-based hose is made of extruded Teflon tubing that’s also covered with a protective steel braiding. Such hoses are capable of withstanding pressures as high as 4,000 psi, which makes them the obvious choice for hydraulic systems, such as those for brake and clutch assemblies as well as for various exotic fuels that typically don’t play nicely with rubber. Steel-braided hose isn’t always the best choice, though, and isn’t always better than whatever was in your car originally. Surprisingly, the synthetic rubber typically found in steel-braided applications isn’t always as versatile as automakers’ modern hose compounds. Depending on whatever’s flowing through it, it can crack over time and without warning because of its protective covering.

Steel-braided hose can also be difficult to route because of its large radiuses and when rubbed against softer materials can damage them from abrasion. Steel-braided hose also isn’t compatible with slip-over barb fittings or hose clamps (regardless of how often you see it done), which means the appropriate and equally expensive AN hose ends and adapters must always be used, of which several different types are available, depending on the hose.

Hose selection doesn’t end with the steel-braided variety. A number of Kevlar- and nylon-covered rubber-based hoses are available as well as rubber-based hoses that don’t feature any sort of protective sheathing at all. Applications vary and there’s a material that’s compatible with just about any fluid, temperature, or pressure.

Not all rubber hoses are created equal. When choosing a hose (steel-braided or not) be sure that it features a woven-fabric core, which increases its strength and durability, and that it’s suitable for whatever fluid you’re expecting it to carry.

Hose Ends

Figuring out what hose ends you need isn’t any less complicated. The fact that they’re available in configurations specific to each type of hose in a variety of shapes and radiuses make the process even more muddled. Each serves a purpose. For example, the most common single- and double-nipple tube-style hose ends are generally less expensive but are susceptible to damage from vibration or leverage.

Forged hose ends are typically stronger and feature a lower profile but can lead to fluid cavitation because of their harsh bends. Some hose ends can swivel, which allows them to be reoriented even after assembly for precise fitment. Stationary hose ends are less expensive. Choosing the proper shape and type of hose end can typically reduce the need to bend the hose itself and provide for an overall tidier appearance.

All hose ends designed for use with steel-braided hose are based on the same principles. Here, a threaded socket slips over the hose. The hose’s male-threaded end screws into the socket, wedging the hose into the small space between the socket and the nipple; it creates an interference fit that’s exponentially stronger than any clamp.

In some cases, a crushable brass, aluminum, or stainless steel olive is also used, such as with a Teflon-based hose. Most other hoses that don’t feature protective sheathings, such as steel-braided ones, can be used with barb adapters that can be pushed into place with a small amount of lubricant and retained using a hose clamp.


All hose ends hook up to the appropriate hose but don’t expect its other end to magically connect to anything on your car. The appropriate adapter(s) must be used to transition from the male- or female-threaded connection or nipple on, for example, your engine to the hose end’s AN threads. Scores of adapters with the appropriate AN threads are available for transitioning from NPT (National Pipe Thread), BSPT (British Standard Pipe Thread), banjo, barb, metric, or just about anything else you can imagine to whatever hose end you use. Simply figure out what exactly it is you’re trying to adapt to, what size hose end you’d like to adapt, and source the appropriate adapter(s) to make it all work. In some cases, more than one adapter may be needed depending on location, angle, and thread orientation.

AN components, such as steel-braided hose, use a unique measurement system that’s based off of 1/16-inch increments, preceded by a dash. For example, -8 hose features an internal diameter of approximately 8/16 inch. Common AN sizes include -3, -4, -6, -8, -10, -12, and -16 although larger sizes and oddballs, such as -2 and -5, are available but are also more expensive because of their rarity. The complexities don’t end there, though.

You would think that whatever size hose you’re considering is a measurement based on the hose’s internal diameter but you’d be wrong. Instead, an individual AN size is a measurement of the hypothetical tube’s outer diameter that would presumably flow the same. As you’d expect, hypothetical tubes that presumably flow a certain amount can vary in size, which means one manufacturer’s -8 hose is almost always just a little bit smaller or larger than another manufacturer’s.

Hose ends may be connected to their respective hoses by crimping, swaging (a form of crimping that compresses a ferrule), or by hand, depending on the style used. Swaging and crimping require expensive hydraulic tooling, which makes assembling them by hand the method you’ll likely choose.

First, the hose must be cut to size before doing anything. Most hose can be cut with hose cutters or a blade; however, steel-braided hose must be cut using a toothless-blade chop saw. A fine-tooth hacksaw may also be used but the chances of a poor cut and possible leaking increase.

AN hose and fitting sizes are typically represented as a number preceded by a dash. To determine a particular AN size, multiply the advertised number by 1/16 inch. For example, -4, -6, and -8 can be interpreted as 1/4, 3/8, and 1/2 inch, respectively.


A rubber-based steel-braided hose assembly typically includes the hose itself, a hose end, and a threaded socket that interfaces the two components. When assembled properly, the AN connection can be removed from its corresponding adapter and reused almost forever; it requires no form of sealing compound.

Regardless of the method chosen, wrap the area to be cut with masking tape to avoid fraying and cut perpendicular to the hose; a square cut is critical to the seal.

Once cut, trim any frayed edges and thread the hose end’s socket onto the end of the hose.

Apply anti-seize compound to the hose end’s threads and nipple before assembly and thread it into the socket. AN components should never be assembled dry; stripping may occur or, over time, their threads can seize.

Although most of the hoses mentioned feature tremendous pressure ratings, the truth is that they’re only as capable as their hose ends and their assembly method. A poor connection or inappropriate adapter is generally the cause of failure, not the actual hose.

Improper sealing surfaces and incorrect thread compatibility are two other common causes of failure. Before selecting a hose end, adapter, or fitting, you’ve got to understand the difference between the two most popular threads: AN and NPT. First, they aren’t compatible with each other, which is why there’s an abundance of AN-to-NPT adapters. AN threads feature a straight design and seal by means of a 37-degree-flared seat or O-ring. NPT threads are tapered for an interference-fit and require some sort of sealant, such as Teflon paste or tape. It’s important to know that applying any such sealant to AN threads only increases the likelihood of leaking.

Adapters such as these and any other AN components aren’t typically available from the local hardware store. Instead, look to manufacturers including Earl’s, XRP, or Aeroquip.

Frequently, AN fittings must be adapted to a dissimilar NPT component. A number of adapters are available that make this easy; however, the first step is recognizing the difference between the two. NPT threads (left) are tapered, and when threaded together with another NPT fitting or adapter, they create an interference fit. AN threads (right) are straight and seal by means of flared ends that compress against each other when tightened. NPT threads require some sort of sealant; AN threads do not.

Not all flared fittings that seal by compression are the same. Typical hardware store fittings designed for home improvement projects feature a 45-degree flare. AN fittings are based on a 37-degree design. It’s important to never mix and match the two. Combining the two may seem as if it’ll work at first; however, their incompatibility with each other almost always results in failure.


Not all flared fittings are the same, either, which is what makes your trip to the home improvement store either a waste of time or an experience that ends with something underneath your hood catching on fire. For example, although the plumbing aisle’s flared fittings may look like what you need, they aren’t.

Home improvement plumbing is based on a 45-degree flare that, when matched with a 37-degree AN fitting, seals only along a razor-thin surface. This may work for a short time but soon fails; it either makes a mess of a home improvement project or all sorts of trouble underneath your hood.


Written by Aaron Bonk and published with permission of CarTech Books


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