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AN fitting and 45 degree fitting comparision
45 degree fitting flare angle
AN fitting flare angle
All flared fittings aren't created equal. In the United States you will find 45 degree flared fittings in your local hardware store. You will find 37 degree AN fittings at your local aircraft parts store.

An easy way of telling if the flare angle is 45 or 37 degree flare is shown in the pictures.

Not all 37 degree flared fittings are the same. There are two types:
  1. JIC or industrial and,
  2. AN or aircraft.
Both use a 37 degree flare and both use a screw thread but this is where the similarities end. The AN fitting is stronger than the JIC fitting.

99% of the time when you are working on aircraft the hoses have a 37 degree flare and you don't think about the flare angle. Then someone screws a hose with a 45 degree industrial fitting into a 37 degree aircraft.  Not good.

You might also run across an air conditioning hose that has a 45 degree industrial fitting on one end and an aircraft 37 degree on the other.  Be sure that 45 mates to 45 and 37 mates to 37. Both male and female need to be the same angle for a leak-tight seal.

AN and JIC fittings are sized using a dash size concept.
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