It's surprising how many people that work on their own cars don't know the difference between different kind of fittings. I'm going to start off simple...



The above picture shows examples of two kinds of fittings that I'm going to identify and explain. The top half of the fitting is a MALE JIC/AN connection. You can identify this by the flare. Speaking of the flare, the flare does the sealing, NOT THE THREADS. With that said, this style connection DOES NOT require ANY sealant on the threads. The proper torque for this style fitting is hand tight, and 1-2 hex flats (1/6th to 1/3rd of a turn).

The bottom half of the fitting in that picture is a male NPT (National Pipe Taper) fitting. Unlike the AN/JIC side, the THREADS of the NPT fitting do the sealing. With that said, you should be using a teflon tape/paste when assembling these kinds of connections.

Note- AN/JIC fittings (almost all of your automotive aftermarket) have 37 degree flares, where as your OEM flare's are typically going to be a 45 degree inverted flare.

Here is a decent read on the subject-

http://www.inlinetube.com/install%20...aring%2045.htm