Here's the scenario: It's Saturday morning and you begin a project/update/repair on your street rod 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 is compatible with what, so you waste even more time trying to make things screw together. If something like this has ever happened to you, your pals at STREET RODDER 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 Threads
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 jam.
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 inside diamter (id) or ouside diameter (od), or thread pitch. The proper means to determine pipe size (up to 1 1/4-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.
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.
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 A variety of tubing is used...  A variety of tubing is used in street rods, 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...  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. |
 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. |
 Two sealing methods are at...  Two sealing methods are at work here: an inverted flare and pipe threads. This combination should only require sealer or Teflon tape on the pipe threads. |
 An inverted flare coupler...  An inverted flare coupler is used when connecting two lines together. |
 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. |