Manufacturers often differ on the type of thread they make their fittings with - some make their male threads tapered and their female parallel, while others make their males tapered and female tapered too. Make sure you know what kind of thread your fitting has before you try thread it in to another fitting. Taper and parallel threads look almost exactly the same to the naked eye, but they do have a difference. A taper thread's base starts off wide and ends much narrower at the far end of the thread, meaning the diameter of the thread gets less the further down the thread you go. A parallel thread, however, has the same diameter all the way along from the base to the end.
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