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Written by Robert Bowen
From the Dec. 2003 issue
Posted in Drivetrain
The last time you checked out a pro’s car at your local autocross or road course, or even in a magazine, you probably noticed the many stainless steel braided hoses. Few car builders would be caught dead using black rubber hose and clamps anymore, as even show cars sport braided stainless fuel lines and the associated red and blue aluminum fittings.
While it sure looks good, there is more to this stuff than appearance. True braided hoses and the appropriate end fittings are simply the most reliable way to transfer fluid from one place to another, and they are a lot easier to assemble and use than they look.
The technology has been around long enough that hose manufacturers have made their fittings easier to assemble and use, while introducing hose types for every need and budget. With the right information and a few tools, you can design and make your own tough, good-looking lines for any fluid transfer need.
The high-performance plumbing seen on race cars is usually referred to as “AN” hose and ends. This refers to the Army-Navy standard of aircraft tubing, hoses and flare fittings designed in the pre-WWII era to facilitate easy interchange and repair of aviation fluid systems. The system was designed to be light, simple and foolproof.
These characteristics and the easy availability of AN parts on the surplus market in the 1950s and 1960s encouraged postwar racers to use these lines on their cars. When the supply of military surplus parts dried up, dealers like Earl’s Performance Products and Russell Performance started to market AN hose and fittings directly to racers, followed soon after by Aeroquip, the largest aviation hose manufacturer. New designs and materials intended for high-performance automotive use have been introduced since, but the basics have not changed much in the past few decades.
The size of each line and end is referenced by a “dash number,” such as -3, for example. This dash number is the final portion of a full AN part number referring to the type of hose or fitting. The system is based on the outside diameter of thin-wall hard tube in 1/16-inch increments. For example, -3 tubing measures 3/16-inch in diameter.
Every tubing size has a corresponding compression fitting thread diameter and pitch, as well as a corresponding hose size. The inside diameter of the hose is not the same as the outer diameter of the hard tube; rather, it is a size that is considered to have the same flow capacity. This results in the slight differences in hose inside diameter from the expected even increments.
The tube compression fittings form the basis of the flexible hose end system and are interchangeable with them. For this reason, AN hose ends seal on a tapered seat, the angle of which is 37 degrees.
The most common general-purpose hose used with AN fittings and ends has a stainless outer braid, but there are numerous variations. For one thing, there are several different grades of hose that have the same braided external appearance.
The premium hose from reputable manufacturers, such as Earl’s Perform-o-flex, Russell ProRace and Aeroquip AQP, has a multilayer construction. A smooth inner layer of synthetic rubber is surrounded by a partial metal braid embedded in rubber, with a full outer braid bonded to the surface of the hose.
This construction method strengthens the hose from the inside out, and models made this way will withstand more than 1000 psi and significant vacuum without issue. It is preferred for critical lines in all applications; it can resist vacuum well enough for dry sump systems and will not degrade with use.
There are a couple of cheaper alternatives to this premium hose for use when true aviation quality is overkill. Earl’s Auto-Flex and Russell ProFlex hose have the same inner rubber liner as the top-of-the-line stuff, but have fabric reinforcement rather than stainless under the outer braid. This hose is perfectly adequate for most non-critical lines and any street car plumbing needs. It looks as good as the racing-grade hose but comes without the high price.
The least expensive approach to flexible plumbing, however, is hose without an outer stainless braid, such as Earl’s Super Stock, Aeroquip non-braided AQP and Russell Twist-Lock hose. This “push-on” hose has an inner synthetic liner, a cloth or other fiber reinforcing braid, and an outer layer of rubber. It is lightweight and easy to use, but it lacks the bulletproof characteristics of braid-covered hose.
Despite this cost-effective construction, push-on hose has a lot to recommend it for water and other low-pressure lines in protected environments. For one thing, non-braided hose has the advantage that cheaper barbed hose ends can be used. Be sure to use it with the recommended ends, though, as others may not hold securely and can blow off.
The final type of general-use, high-performance hose features an outer Kevlar braid instead of stainless steel. This hose is extremely lightweight and just as tough as stainless braided hose. In addition, the Kevlar outer braid makes the hose very heat resistant. The most widely available hose of this kind is Aeroquip Starlite, which requires specialized fittings.
Starlite hose is exceptionally lightweight; with the special crimp-on ends sold by Aeroquip, it can weigh nearly half that of stainless line and aluminum ends. The only problem with this hose is the extreme cost, coming in at two to three times the price of a comparable steel-reinforced hose. While this puts it out of the reach of mere mortals, there is no denying the attraction of less weight without impacting strength and usefulness.
There are also several specialized hoses and ends used in the high-performance world. The first of these is stainless-braid-covered Teflon tube used for brake and clutch lines. This hose is designed for the high pressures encountered in hydraulic systems and is available from Earl’s (Speed Flex), Aeroquip (TFE hose) and Russell (Powerflex). Goodridge offers a complete line of ready-to-install brake hose sets for a wide range of cars.
Another specialized hose is intended for power-steering systems. It is specifically designed for the deep vacuum and high pressures encountered here. Power-steering hose has a fabric outer braid with several layers of steel reinforcement within the hose. Again, this hose has specialized ends that cannot be used with other hose types.
Finally, Aeroquip makes a special hose intended for air-conditioning systems seen in street rods. While this really is a “looks only” application, the line is specifically intended for R-134 refrigerant and comes with a complete assortment of end fittings designed for air-conditioning use.
Having the right hose is only part of the solution, as the correct fittings are needed to secure the hoses in place.
The most widely employed hose ends are reusable aluminum, but steel and brass are available through industrial hose suppliers. Most fittings have an outer threaded socket that holds the hose firmly on the nipple, except for push-on hose, which uses barbed ends that do not need a socket. Nonreusable, swaged-end fittings that are assembled in a large crimping tool do not use threaded sockets, either.
There are two kinds of socketed hose ends: those with a single hose nipple and those with a double nipple. All double-nipple and most modern single-nipple hose ends are swiveling, meaning that the ends do not have to be “clocked” in relation to each other and can be rotated after assembly.
Push-on socketless hose ends have barbs that prevent the hose from blowing off when they are used with non-braided hose. These ends can be difficult to assemble because of the force needed to get the hose over the barbs, but they are much cheaper than equivalent socketed ends. Earl’s Super Stock ends are nicer looking than most, since they incorporate an aluminum cover for the end of the hose.
Single-nipple ends are the simplest and least expensive socketed hose ends, although they do not seal as well as double-nipple ends. A single-nipple hose end has an outer socket that compresses the hose over a straightforward tapered nipple. The socket has female threads that mate with male threads on the base of the nipple, sealing the assembly and holding the hose in place.
Double-nipple ends have an additional, concentric outer nipple that cuts into the rubber hose under the stainless outer braid. This provides a non-threaded barrier for fluids in the hose and separates the retention and sealing functions of a hose end. These ends are more reliable than single-nipple ends and do not tend to weep after installation.
Special hose calls for special ends, and all three major hose manufacturers offer steel ends for their hydraulic and steering hose. Ends such as Aeroquip’s Little Gem and Earl’s Speed Seal incorporate an aluminum ferrule that supports the Teflon hose and prevents it from blowing off the end fitting. The ferrule is the only part of the assembly that cannot be reused. Power-steering fittings are heavy-duty versions of standard single-nipple ends, with extra-long sockets and steel construction. In addition to these, Aeroquip makes a line of air-conditioning hose ends that incorporate charge ports just like OEM lines.
Aeroquip hose ends have the additional ability to seal on both AN (37 degree) and SAE (45 degree) adapter fittings, which have the same threads. While this is usually a nonissue if assembling your own plumbing from new parts, it can become a problem if you like to scavenge for parts at surplus stores or have your hoses made by a hydraulic/heavy equipment shop.
The AN system of flexible lines and end fittings is versatile and able to tackle nearly every need. General-purpose hose can be used for oil, water and fuel lines, while specialty hose takes care of the rest.
Fuel lines are perhaps the best possible use for braided-stainless AN lines, since a fuel system leak is very dangerous. Most fuel system plumbing is done in -6, and Aeroquip, Earl’s and Russell all make adapter fittings for most popular fuel system components, including Weber, Holley and Mikuni/Solex carburetors.
Attaching braided hose to EFI fuel rails can be difficult, but aftermarket rails with AN or pipe thread fittings are available for popular engines. Aeroquip makes AN adapter fittings for GM O-ring fuel rail fittings, and these work great if you have this type of system. Other OEM fuel rails can be adapted to braided lines by brazing steel AN fittings to the tubes, or by flaring the ends of the feed and return tubes for AN tube nuts and flare fittings.
Oil systems are perhaps the most straightforward to plumb. Oil ports on most engines are threaded with pipe threads, whether standard, BSP or metric, which can be adapted easily with off-the-shelf fittings. Most oil pressure lines use -8 or -10 hose and ends, although dry sump scavenge lines are usually larger. Some oil coolers (such as Earl’s) have AN fittings already attached, while others have female pipe thread ports.
One place where it definitely does not pay to use cheaper or inferior parts is in brake and clutch hydraulic systems. Use the best parts you can find and get a professional to assemble them if you do not feel comfortable doing it yourself.
Many manufacturers offer assembled brake lines with swaged ends for popular cars, as well as a complete selection of universal lines with standard AN female ends. The universal lines can be combined with adapters for almost any combination of caliper and frame fittings. Be sure to inspect any line before installing it—poor crimps or kinks in the line instantly disqualify it.
Brake lines are usually made from -3 hose to keep the system volume low and pressure high, although some circle-track racers use -4 for the added fluid flow with huge single-piston calipers.
Clutch lines should usually be -4 because the fluid displacement is greater than brake systems and deflection is less of an issue. (Minimizing deflection reduces brake pedal travel and makes more force available at the calipers.) Teflon lines are better than OEM-type, fabric-reinforced hoses because they do not swell as much.
The only downside to these Teflon lines is that they cannot tolerate any kinking or stretching and should be inspected yearly for damage. In addition, many of these lines are not DOT approved. While this does not necessarily mean they are not good or not safe, it can be a factor for some street-driven vehicles.
Power-steering lines present a particular challenge because of the pressure involved. The outlet side of a steering pump can easily hit 2000 psi, which is right on the edge of the recommended pressure of standard AN hose and fittings.
Power-steering hose and fittings are designed for these conditions, however, and will not collapse under the vacuum seen on the intake side of a steering pump. The only drawback to power-steering lines is the limited availability of adapter fittings. They are available for GM and Ford pumps and racks, but not much else. Some fittings can be brazed or TIG welded onto the ends of OEM lines, or directly to pumps or reservoirs if adapters are not made.
Using braided stainless lines for air-conditioning and heating systems definitely falls under the category of “form before function” since they are not necessary for the pressures and temperatures seen in these systems. One exception to this rule might be the heater lines found in a rally car, but even then these are not likely to need the extra strength or expense of braided stainless lines.
Using braided lines for smaller coolant lines has a purpose, however, since it can make them bulletproof and reduce cooling-system-related failures.
The most important factor in hose design is alignment and installation. For example, when a hose must meet a component at an angle, there are two ways to solve the problem: using an angled hose end or using an angled adapter fitting. The second method is cheaper, since angled adapters cost less than angled hose ends, but the internal restriction of the adapter fitting can be an issue with flow-critical systems. Overall, the angled hose end is the more elegant solution, since the smoothly bent tube presents no obstruction to flow and just looks better than an angled adapter.
As much as we hate to think about it, another important factor in the design stage is price. Braided hose assemblies can be quite expensive, but the durability of AN plumbing makes the cost/performance tradeoff more attractive. A single hose assembly might last for many seasons, while end and adapter fittings can be reused infinitely.
The cost of a given assembly depends on length, diameter, type of end fittings and type of hose. Hose and fittings become more expensive as they get larger in diameter: Fabric-braided -4 hose can be as inexpensive as $2 per foot, while -32 stainless braided performance hose costs upward of $25 per foot. As we mentioned, 90-degree hose ends are more expensive than straight ends ($20 vs. $7 for -6), although they flow better than equivalently sized 90-degree adapter fittings. Straight pipe-thread adapter fittings are fairly inexpensive, ranging from $3 to $15, but oddball ports such as carburetor or fuel injection adapters can cost much more to connect to.
Once a plumbing system has been designed, installation is pretty straightforward. There are only a few points to keep in mind, including making sure that all lines are located away from heat, each other, and adjacent parts of the car. Flexible hoses should be supported every few inches and protected from chafing. While the line itself is unlikely to be damaged, the stainless braid will rub through just about anything that gets in the way.
Lines should be longer than the shortest distance between the end fittings to keep them from being stretched. According to the “Aviation Mechanic’s Handbook,” the difference should be at least five to eight percent. In addition, flexible hose cannot bend very tightly, so use elbow fittings to eliminate kinks and keep bend radii generous.
The simplicity and reliability of the AN system of plumbing explains why high-performance car builders are using the same system that has kept aircraft defying gravity (and leaks) for the last 60 years.
With a few simple parts and specialized adapter fittings, AN hose can be used to plumb any automotive system, including a few that don’t really need it. This is definitely a case where looks and performance go hand in hand; AN stainless braided hoses look as good as they work, and they work very well. Proper use of AN fittings to plumb a race car can significantly increase your chance of finishing, and as we all know, you have to finish in order to win.
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