1.8t Fueling FAQ
Note: I have some terminology that references brake systems. Brake systems are similar to fuel systems in that 1) they are critical and not allowed to fail for safety reasons and 2) they are high pressure lines filled with material that must be treated special (compared to water or similar). As such, brake line methodologies can be carried across to fuel line methodologies very nicely.
In case any of the image links ever decide to break, I've put this in a PDF for all future posterity (updated October 20, 2010)
- AN vs. NPT: Understanding port threads, adapter fittings and line sizes.
What is AN?
The designation AN stands for Army/Navy and calls out mil/spec (military specifications) for dimensional standards of hydraulic lines, hose-end connectors and port adapter fittings. AN specifications are a popular standard met by all companies that manufacture AN style performance fuel hose and accessories. For many there has been much confusion about the subject of AN lines, NPT and ORB ports, and how all of this works together. Here are the answers for those wanting to know.
The flare angle used to seal AN connections is required to be SAE, 37 degree, as apposed to the 45 degree flare commonly found on household plumbing adapters. This angle can be found on the male point of the port adapter fitting and on the female inside the hose-end nut. AN port threads are not NPT or “pipe thread” but instead utilize straight threads (like any normal fastener) and SAE O-Ring Boss (ORB) technology for sealing. AN lines, ORB ports and the appropriate port adapter fittings are measured in inch/fractional sizes.
AN 'dash' sizing
A dash (–) size in AN “speak” refers to the I.D. of a standard, thin wall, hard line as the basis to construct a comparable flexible hose that may be used in it’s place. A 1/2”, thin wall, hard line measures .500” on the outside diameter (O.D.), has an inside diameter (I.D.) of 0.440”, and a wall thickness of 0.030”. An appropriate, flexible replacement line would be –8 AN, with a minimum 0.440” I.D. Depending on line construction, rubber with stainless steel or nylon braid, or Teflon with stainless steel braid, the line’s wall thickness and O.D. may vary.
AN line sizes will have a dash (-) preceding the line size. The number after the dash refers to the number of 1/16 of an inch O.D., thin wall, hard line to which the flexible line will compare. For example, calling for a –8 AN line would mean the engineer or system designer requires a flexible line, made of certain materials suitable for the application, that would have the minimum I.D. of an 8/16” (1/2”) O.D. hard line. The actual line construction is dictated by the application with regard to line flexibility, vacuum and pressure capability, abrasion resistance and chemical compatibility, etc. Regardless, the engineer knows a -8 line of any construction will have a minimum I.D. equal to 1/2” hard line (.0440”), and be able to support similar flow rates.
Modern, Best Practices
Modern, high performance fuel systems are predominately fitted with safer, better sealing, higher flowing, AN-ORB ports. These ports require a straight thread adapter fitting, with a sealing O-Ring installed over the threads, up to the hex, that disappears into the port when properly installed. No additional thread sealer is required or recommended.
NPT, AN, and adapters
National Pipe Thread (NPT) ports, AN Ports and port adapter fittings:
Over the years, in low-pressure hydraulics, NPT has been a popular thread for ports and adapter fittings. When NPT ports are used in a fuel system with AN line, an adapter fitting to convert from NPT to AN is required. NPT was designed for use with thick walled pipe, typically black pipe, used in fixed structures like buildings, to handle distribution of water and natural gas. Black pipe isn’t particularly bendable, flexible or lightweight and hardly desirable for plumbing a high performance fuel system. As a result fittings that adapt NPT ports to AN line are common to allow flexible AN lines to be utilized in performance automotive fuel systems.
Unlike AN thread, which is straight, NPT ports and fittings are both tapered. NPT male to female adapters start loose, threading easily but get tight and harder to turn well before the hex touches the port. When threaded together, the NPT design creates a wedging effect, binding the thread in order to seal. The use of a thread sealant is common and required with NPT, as it does not consistently create a positive seal on it’s own, like an O-Ring configuration. It’s common to see a number of threads showing on the adapter fitting when NPT is sufficiently tight, making NPT assemblies bulkier and less clean appearing than a similar AN assembly.
NPT ports are commonly adapted to AN lines, but the NPT size designation is confusing, identifying the pipe I.D. rather than the O.D. Black pipe has a much thicker wall than hard line, so the pipe/port O.D. is much larger than the NPT size would seem to indicate. For example, a 3/8” NPT port will have an outside diameter of 5/8”, allowing for a wall thickness of 1/8” (0.125”). As a result, NPT port sizes allow use of a one step larger AN line than their indicated size would seem to support. As long as the wall of the adapter fitting is not overly thick, the following NPT Port to AN adapters will provide a common I.D. through-hole:
JIC fittings, defined by the SAE J514 and MIL-F-18866 standards, are a type of flare fitting machined with a 37-degree flare seating surface. JIC (Joint Industry Council) fittings are widely used in fuel delivery and fluid power applications, especially where extremely high pressure is involved. The SAE J514 standard replaces the MS16142 military specification, although some tooling is still listed under MS16142. JIC fittings are dimensionally identical to AN (Army-Navy) fittings, but are produced to less exacting tolerances and are generally less costly. 45-degree flare fittings are similar in appearance, but are not interchangeable.
JIC fitting systems have three components that make a tubing assembly: fitting, flare nut, and sleeve. As with other flared connection systems, the seal is achieved through metal-to-metal contact between the finished surface of the fitting nose and the inside diameter of the flared tubing. The sleeve is used to evenly distribute the compressive forces of the flare nut to the flared end of the tube. Materials commonly used to fabricate JIC fittings include forged carbon steel, forged stainless steel, forged brass, machined brass, Monel and nickel-copper alloys.
AN -> NPT port sizing
Maximum AN line for NPT port size:
1/4” NPT is compatible with up to -6 AN (3/8” hard line)
3/8” NPT is compatible with up to –8 AN (1/2” hard line)
1/2” NPT is compatible with up to –10 AN (5/8” hard line)
3/4” NPT is compatible with up to -16 AN (1” hard line)
Connecting large AN fittings to smaller NPT fittings WARNINGS
Adapter fittings are available for connecting larger than recommended AN lines to the above NPT ports. Beware, the inside diameter of the adapter fitting will necessarily be smaller on the NPT side, creating a flow restriction that many racers and hotrod enthusiasts overlook. This is a poor practice and should be avoided, but when no alternative is available, consider sourcing a steel NPT to AN adapter from a good hydraulic supplier. Steel adapters will have a thinner wall than aluminum, due to the increase in material strength, leaving a larger I.D. to support higher flow on the too small, NPT side of the adapter.
- SAE J514, Straight Thread O-Ring Boss (ORB)
This straight thread connection uses the same threads as the JIC 37°. However the 37° flare has been removed and an o-ring has been added. When mated with a female o-ring boss port the o-ring is trapped in a special tapered counter bore to affect the seal.
See here for more information
- Common army/navy (AN) line and thread specifications
AN | Metal Tube (Hard Line) OD | Port & Fitting (Diameter-TPI)
2 | 1/8" | 5/16-24 SAE
3 | 3/16" | 3/8-24 SAE
4 | 1/4" | 7/16-20 SAE
5 | 5/16" | 1/2-20 SAE
6 | 3/8" | 9/16-18 SAE
8 | 1/2" | 3/4-16 SAE
10 | 5/8" | 7/8-14 SAE
12 | 3/4" | 1-1/16-12 SAE
16 | 1" | 1-5/16-12 SAE
20 1-1/4" | 1-5/8-12 SAE
24 | 1-1/2" | 1-7/8-12 SAE
28 | 1-3/4" | 2-1/4-12 SAE
32 | 2" | 2-1/2-12 SAE
- A note about AN Flares versus SAE Flares:
Automotive flares and flare tools are 45-degree flares. “AN” flares are 37 degrees. It is not “technically correct” to use a 45-degree flared tube with the 37 degree AN fittings, and for military and aviation use, this is not allowed: The 37-degree flares are designed to operate safely in systems up through 3,000 psi, so a flared angle mismatch is not allowed under these conditions. We never see such pressures in auto fuel systems. In actual testing that I have done, I have found that the 45/37 combination works safely and reliably in pressure systems up to 250 psi. In applications above 250 psi, the line-interface created by the angle mismatch can start to leak if the fittings are not re-torqued. In automotive fuel systems operating at 4 to 8 psi, the 45/37 interface works reliably and flawlessly, without the need to re-torque. Much more reliably than a rubber fuel line with a clamp… However, if you want to create a “perfect” flare interface, 37-degree flare tools are available from aviation supply houses.
You will not notice any difference in system reliability between the two flare angles in this automotive fuel system application.
Example of AN Flare
- Where are the different flares used on our cars?
There are four (more or less) flaring styles in common use for brake systems. British cars have a bubble flare (aka Girling flare) backed up with a male swivel nut or a 45 degree double flare backed up with a female swivel nut. Metric cars have ISO bubble flares, where the pipes and threads are metric sizes rather than inch. Detroit iron has a 45 degree double flare backed up by a male threaded nut. Most (non-British) race cars are plumbed with AN (aka JIC) type single flares - a 37 degree single flare with a backup sleeve and inch threaded swivel nut (some people make a double flare here, which is useless overkill and may lead to failure; see below). Lastly, some brake fittings use tapered pipe fittings.
Take the easy one first: tapered pipe fittings are not really a positive seal under adverse conditions. They may do the job for a street car, but they certainly have no place on a race car.
The bubble flare is used with a male swivel nut, and seals at the bottom of a drilled and tapped hole, with a nice angled bottom. While it can usually be resealed, it has a limited lifetime - there's no good way to get back the deformation that was crushed out for the first seal, short of remaking the flare from scratch.
Making such a flare is easy: if you are in possession of a standard 45 degree double flaring tool, the bubble flare is what results after the first half of the operation. Simply stop there, and you have the bubble flare which will seat nicely at the bottom of the hole. If you continue, inverting the form tool and finishing the job, you then have the more familiar double flare used by Girling and the US automotive industry.
The SAE 45 degree double flare usually has a male-threaded tube nut that bears directly on the OD of the flared tube- so you need a double flare to help control galling that can result in stress cracking right at the flare. In short, you need "give" there. Problem is, the deformation that results is kind of irreversible, so the next cycle or two will result in your having to use astronomical torques to keep the flare from weeping. Worse is trying to use a single flare in an SAE flare nut and seat, and worse still if the seat is brass- the flared tube is sqaushed from both sides, even as it is deformed by the nut galling on it. The brass seat deforms and work hardens. It may seal once, with a ton of torque and some luck. It's not recommended practice - it's not even a good idea.
Racers (and aircraft, which is where the system originated as the "Army-Navy" or AN standard in WWII) use the single 37 degree flare. The AN single flare is still a concave flare, but its 37deg angle seals by stretching, not squashing. The tube is supported by a separate sleeve that the female-threaded tube nut bears upon. This isolates the flare from the torques imparted by the nut. So rather than trying to get a seal despite the presence of rotating torques and the resulting galling, you press the flare between precisely-machined (steel!) seat and precisely-machined support sleeve. The sealing area under compression is at least double that of the SAE flare. An additional bonus is that the OD of the nut is a lot larger than the 3/8" of an SAE nut, which means you won't kill as many trying to get the proper sealing torque. (Even so, you should always use a proper flare nut wrench on any tube nut.)
The SAE stuff was designed to go together once on the assembly line, and then be "immortal", as defined by Detroit. It's pretty good at it, too! The AN stuff is designed for field serviceability, long fatigue life, and a level of bulletproofness the SAE never considered. A further, Very Strong, recommendation is that single flared AN fittings are the only thing that Carroll Smith will suffer to put on his race cars.
The reason for harping on repeatability and multiple mate-demate cycles is that, to the best of my knowledge, I have never once put something on the car, and had it stay put on. I always forgot something, or broke something during the season, or needed to swap out something because it was at the end of its service life. While the double flare has that nice "squish" feeling as you tighten the flare nut the first time, the AN fitting has the same torque requirement for the second mating cycle as the first. That's where the reliablity across multiple cycles comes from.
Never use compression fittings for brake plumbing.
I know racers that have gone so far as to purge all the SAE double flare fittings out of the plumbing of their track cars, except possibly the fittings at the master cylinder. The rationale is two-fold: first, the above-mentioned repeatability, and second, the fact that the AN parts are universally available at every race shop in every backwater in the country.
I have not gone quite so far on my British cars, which are plumbed with Girling flares. It is a happy coincidence (at least, it seems like a coincidence, perhaps it is by design) that the female Girling flare hardware will mate comfortably with a male AN-3 fitting; I have thus replaced the fancy fittings on the rubber hoses with the appropriate AN-3 male-to-male bulkhead fittings, and the hoses themselves with -3 female fittings on Teflon -3 line. My master cylinders all have machined flat surfaces, so the outlets are easy to adapt to -3 hoses as well (-4 for the clutch, typically).
A few words about Teflon lines: they may not have a place on your street car. For starters, the Teflon lines you make up at home don't have DOT approval, because they haven't been through the DOT approval process and you're not a certified manufacturer. Even the Teflon lines you can buy pre-built are probably not DOT approved (though there are a few coming onto the market) for the same certification reasons. The main reason for this lack of approval is that manufacturers of racing gear don't care to spend the money - they're building the hoses for racers. The next reason is that the DOT expected lifetime for such brake hoses is five years, and the prudent racer will replace his or her brake hoses every other season, just to be sure. (This is a good reason to make your own - then you can reuse the fittings and just buy new olives and hose.) There are some concerns about using Teflon line in cold environments, though Aircraft Spruce is willing to sell you lines custom made from Aeroquip Teflon hose and rate them to -40 degF, so I'm not so sure that's a concern. It may well be that the intended application for those lines is not subject to as much repeated deflection as the flex line snaking through your front suspension out to the caliper.
- Other Sealing Methods
Other sealing methods
There is one other kind of sealing involved in braking systems, usually at caliper or cylinder entry and often when converting over from one kind of flaring system to another: a washer face at the top of a hole. This requires a crush washer of soft copper, aluminum, or in extreme cases, a Stat-O-Seal, which is a combination of O-ring and aluminum crush washer. To make this an effective seal, a precision machined surface is required around the drilling, exactly perpendicular to the axis of the threaded hole. If this is not there, you must use a bottom sealing fitting. Conversely, if the hole does not have a nicely formed drill point form at its bottom or the fitting will not reach the bottom, you must use a sealing washer (and, of course, provide the machined washer face for it to seal against).
A neat trick I learned a while back is that copper crush washers can be reused - IF you anneal them. Place the washer in question on a brick and play your propane torch across it until it glows orange. Let it cool. It's now as soft as it was originally and will re-crush and seal perfectly.