Here's the scenario: It's Saturday morning and you begin a project/update/repair on your truck that you've told yourself will only take half an hour or so to complete. Before you know it, an hour has passed and you're still searching through several coffee cans full of fittings for the last one needed to complete the chore, but nothing seems to fit. In desperation, you jam a couple of fittings together with enough Teflon tape to bandage your head, but your rod now leaks like the Valdez. So you're off to the local parts house where the kid behind the counter wants to know the make, model, and year of the car, and if it has air conditioning. He then points you to a cabinet full of fittings, most of which are in the wrong bins, and now the challenge is to figure out what fits with what, so you waste even more time trying to make things fit. If something like this has ever happened to you, your pals at Custom Classic Trucks want to help.
When it comes to automotive fittings, the possible combinations will melt your brain, but thanks to the help of Gary Clausen and Jason Hofman of Pure Choice Motorsports, we're going to explain the most common fittings and show what they look like.
Pipe Thread
There are actually several types of pipe threads, but the most common in automotive use is the American Standard Pipe Taper Thread, or NPT. Unlike the threads on a screw, or other fittings for that matter, pipe threads are tapered. Because of the taper, a pipe can only screw into a fitting a certain distance before it begins to jams.
When dealing with pipe fittings, figuring out the proper size can be confusing. At one time, size was determined by the pipe's internal diameter-a misconception that continues today, even among some of those who sell the stuff for a living. The correct method to determine pipe size for cars is now determined by the Society of Automotive Engineers' guidelines for Dryseal American Standard Tapered Pipe Threads (NPTF). With this system, pipe size does not agree with either the id or od, or thread pitch. The proper means to determine pipe size (up to 11/4-inch) is to measure the diameter of the threads and subtract 1/4-inch. For example, subtract 1/4-inch from a 1-inch thread od to obtain the nominal pipe size of 3/4-inch.
While NPTF fittings are supposed to make a leak-proof mechanical seal, some sort of sealer-such as Teflon tape or Teflon-based paste-is usually advisable. When using either, use caution so as not to get either inside the assembly. The best way to do this is to stay off the first two threads.
Inverted Flare
One of the most common types of fittings found on automobiles is the SAE double inverted flare. It is unique in that the flare is doubled, or folded back on itself, for increased strength and resistance to cracking. The tube flare is clamped between the nut and flare seat of body when screwed together so when the assembly is tightened, a leak-proof metal-to-metal joint is created and no sealer is necessary.
Typical inverted flare applications are brakes, power steering, fuel, and transmission cooler lines. These connections are vibration resistant and when used with the proper tubing will withstand working pressure up to 2,000 psi with burst pressures up to 5,000 psi.
SAE 45-degree Flare
Found on instruments, power steering, and some hydraulic (not brakes) and fuel lines, these differ from inverted design in that a single flare is used. Care should be used during installation as over tightening can cause the tubing to split.
Compression Fittings
Intended for low and medium pressure applications, compression fittings are easy to assemble because they do not require flaring or soldering. A sleeve is used over the tubing, and when the nut is tightened, the wedging action of sleeve between body and nut create a tight, leak-free connection.
A variety of tubing is used...
A variety of tubing is used in classic trucks, and it takes an even wider variety of fittings to hook it all together. Only rigid steel or stainless should be used with plumbing brakes; aluminum may be used for fuel lines if it's secured properly to prevent vibration; and copper or plastic may be used to connect some mechanical instruments.
Here's a look at a double-flare;...
Here's a look at a double-flare; note the uniform diameters-the sign of a well-formed flare.
For a tight, leak-free seal,...
For a tight, leak-free seal, the tubing nut for an inverted flare has a seat that matches the angle of the flare.
This is a female inverted...
This is a female inverted flare fitting. Note the seat to accommodate the flared end of the tubing.
The same inverted flare fitting...
The same inverted flare fitting showing the male pipe threads. This example is 3/16-inch inverted flare (the size of the line used) and 9/16-inch male pipe.
Two sealing methods are at...
Two sealing methods are at work here: an inverted flare and pipe threads. This combination shouldn't need sealer or Teflon tape.
Fittings come in all sorts...
Fittings come in all sorts of shapes and angles; this is a 90-degree 3/8-inch inverted flare (size of the line) and 3/8-inch pipe.
An inverted flare coupler...
An inverted flare coupler is used when connecting two lines together.
Often used on instruments, and some air and hydraulic lines, compression fittings are used on a variety of tubing materials; however, they should never be used on brakes.
Brake-fitting Adapters
Hydraulic brake-fitting adapters can be used to connect brake lines to different size ports in a dual master cylinder, combine different types of fitting-such as inverted flare, pipe and AN-or to mix standard and metric fittings.
AN Fittings
Conventional automotive lines and fittings use 45-degree double flares; AN lines and fittings use 37-degree single flares. The reference "AN" stands for Army/Navy and it was a system devised by the government to ensure interchangeability and compatibility of parts made by various manufacturers. With the AN system, a dash number is assigned to each different size metal tubing and the corresponding fittings, and the same numbers are also assigned to the hose and their ends. Another application of 37-degree flares will be found on JIC hydraulic fittings.
AN fittings are often found on street rods in combination with pipe and inverted flare fittings, which means special adapters are required. Suppliers, such as Pure Choice Motorsports, can supply such fittings as well as lines, individual parts, or complete kits.
Adapters
Look long and hard enough and you're likely to find an adapter to connect just about any two fittings together. However, keep in mind that when it comes to brakes, only 45-degree double-flare or 37-degree AN components should be used.
Clamp-style Hose Fittings
Clamp-style hose fittings are manufactured for low-pressure applications with multipurpose hose (worm-style clamps are good for a maximum of 35 psi). A hose clamp is required between the raised end of the fitting and the end of the hose.
Push-On Hose Fittings
Push-on hose fittings are manufactured for low and medium pressure (maximum pressure 200 to 250 psi). These fittings are quick connecting and have high resistance to vibration damage. When used in combination with the proper hose, no clamps are required.
Teflon Tape
In most cases, properly flared and installed fittings should seal by themselves, however, Teflon tape or Teflon-based sealing compound is often used on pipe threads.
Teflon tape can prevent leaks and make it easier to tighten fittings and disassemble them later, and it can reduce/eliminate thread galling and protect fasteners made from corrosion. However, there are some places it should never be used, such as the engine's electrical senders and the cooler line fittings in automatic transmissions.
Used on senders, it will insulate them, preventing the proper grounding necessary for accurate gauge operation. On transmission lines, it can make the pipe fittings tighten so easily that the tapered threads can split the case.
So Many Choices
If we included every fitting available in the form of tubing and fitting, we wouldn't have room for anything else in the magazine this month, so we've tried to cover the most common items a home builder will come across. Next time we'll show how to properly flare tubing, use sealants, and build solid and rigid lines, and a bunch more.
Another common junction is...
Another common junction is the inverted flare branch tee. It has two inverted flare connections and one male pipe. Branch tees are also available with female pipe threads.
A simple pipe nipple uses...
A simple pipe nipple uses male threads on both ends.
This is a properly done 37-degree...
This is a properly done 37-degree AN flare. Notice the sleeve that is used.
SAE 45-degree fittings use...
SAE 45-degree fittings use a single flare. This cutaway drawing shows how they seal.
The complete AN connection...
The complete AN connection consists of the flared line, sleeve, and nut.
AN fittings are sized by "dash"...
AN fittings are sized by "dash" numbers. Each number refers to 1/16-inch. As an example, -3 equates to a 3/16-inch line.
A common combination: -6 AN...
A common combination: -6 AN loine and a -6 to 1/4-inch pipe adapter.
What we have here is an -8...
What we have here is an -8 AN (1/2-inch) to 1/2-inch tube compression fitting. Note the sleeve that slides over the tubing; it grips the tube when the nut is tightened.
Compression fitting such as...
Compression fitting such as this one will only go together properly one way. Note how the sleeve protrudes through the nut.
Often found on oil pressure...
Often found on oil pressure gauges, this is another style of compression fitting. Note the male end is pipe.
After the nut is tightened,...
After the nut is tightened, the sleeve compresses and grips the tubing. The tube protrudes slightly because it seats in the other portion of the fitting.
Often called a Ford nut, this...
Often called a Ford nut, this is a compression fitting without a sleeve. It was used on early Ford vacuum lines.
Referred to as a banjo design...
Referred to as a banjo design because of its shape, these fittings can swivel to make connections easier. They are often found in the brake system. Gaskets are used on both sides of the banjo to prevent leaks. This one accepts an AN fitting.
In a banjo fitting, fluid...
In a banjo fitting, fluid flows through the center of the attachment bolt (note the hole) and an annular groove cut into the housing.
This is a cool piece. It accepts...
This is a cool piece. It accepts a standard 3/16-inch inverted flare brake line and connects to a -3 flex hose. The groove is for a brake line clip to lock the fitting securely in a mounting tab.
This adapter would be used...
This adapter would be used to connect a female -3 AN line to a wheel cylinder or caliper that has a female 3/16-inch inverted flare port.
Another handy adapter: a tee...
Another handy adapter: a tee to allow the use of an 1/8-inch pipe hydraulic stoplight switch in -3 AN lines.
Barb fittings are commonly...
Barb fittings are commonly found in fuel lines. They accommodate different size hose, and may have male or female ends.
While this looks like an ordinary...
While this looks like an ordinary barb fitting, it is unique because it has the unusual threads found in Stromberg 97 and early Ford Holley carburetors.
There are a variety of adapters...
There are a variety of adapters for later-model carburetors that allow the use of AN lines rather than the OEM-style inverted flare style.
On the subject of Stromberg...
On the subject of Stromberg 97s, Dick Crawford makes these cool combination needle and seat/barb fittings that swivel to make connecting single or multiple carburetors easy. They also come oversize so stripped float bowls can be saved.
Teflon tape has many legitimate...
Teflon tape has many legitimate uses; unfortunately, it's often seen as a cure-all for mismatched components and improperly formed flares. In addition, care must be used when applying the tape to ensure it doesn't end up inside the system being sealed.
A simple trick to keep copper...
A simple trick to keep copper oil pressure lines from fracturing is a vibration loop (plus it looks cool). Wrap the tubing a turn or two around anything cylindrical.