Chances are, you’ve in some way meddled with any of your car’s fluid systems and, chances are, all of that’s led you to the home improvement warehouse’s plumbing aisle in search of enlightenment. At first, it all seems like a very good idea: like plumbing underneath a bathroom sink, your hatchback’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, your toilet isn’t exposed to corrosive, 200-degree Fahrenheit fluids. At least we hope it isn’t (and if it is, we recommend you stop eating that crap). And unless your throne is something special, it stays put. 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 your latrine’s. Thread sizes, thread pitches and sealing surfaces only add to the confusion. AN (Army/Navy) plumbing, as it’s commonly known, can be a complex thing, but 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 on the water heater aisle. AN plumbing was developed just before World War II by the aerospace industry as a military standard but has since transcended into the high-performance realm; its usefulness in terms of whatever it is you drive 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 John as opposed to your engine bay.
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 slid over any old rubber hose, though. Internally, it’s made up 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 like 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 hose is made up 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, like 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 your car was originally built with. Surprisingly, the synthetic rubber typically found in steel-braided applications isn’t always as versatile as modern OEM 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 radius and, when subjected against softer materials, can damage them from abrasion. Steel-braided hose also isn’t compatible with slip-over barb fittings or hose clamps—no matter 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. There are a number of Kevlar- and nylon-covered, rubber-based hoses 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, either. 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.
Figuring out what hose ends you’ll 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, though. 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. There are also hose ends that can swivel, which allow them to be reoriented even when assembled for precise fitment, as well as stationary ones that 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 off of the same principles. Here, a threaded socket slips over the hose. The hose end’s male-threaded end screws into the socket, wedging the hose into the small space between the socket and the nipple, creating an interference fit that’s exponentially stronger than any clamp. In some cases, like with Teflon-based hose, a crushable brass, aluminum, or stainless-steel olive is also used. Most other hoses that don’t feature protective sheathings, like 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.
Whatever hose end you plan on using, by design, will 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 plan on using. 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, like 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 like -2 and -5 are available but are also more expensive because of their rarity. The complexities don’t end there, though. You think that whatever size hose you’re considering is a measurement based off of 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 swaging, crimping 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.
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. No matter which method is 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 whether or not everything will 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; here, 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 you’ll encounter: AN and NPT. First, they aren’t compatible with one another, 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, like Teflon paste or tape. It’s important to mention that applying any such sealant to AN threads will only increase their likelihood of leaking.
AN Versus the Hardware Store
Not all flared fittings are the same either, which is what’ll make your trip to the home improvement store a waste of time or an experience that’ll end 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 off of a 45-degree flare that when matched with a 37-degree AN fitting, seals only along a razor-thin surface, which may work for a short period of time, but will soon fail, and will either make for a mess of an outhouse or all sorts of trouble underneath your hood.