Inch (Imperial) Threaded Fittings

In terms of threading these kinds of fittings together, you need to be careful. A parallel male threaded into a taper female will jam up after about one thread, which doesn't give you a satisfying seal. A parallel male threaded into a parallel female will go right the way to the bottom on the inside of the female fitting and try to seal on the bottom of the thread rather the thread itself, and so has a chance of leaking. A taper male threaded into a parallel female will go in nicely and will lock before it gets to the bottom of the thread, about four or five threads in, providing a good seal. A taper male threaded into a taper female will go in about four or five threads as well, lock up there, and provide a good seal.

People often think that for a good seal your male should thread all the way in to the bottom of the female thread. However, if that happens and the thread completely bottoms out, it means the thread hasn't bitten anywhere and the result is a leaky fitting. Most pumps and pieces of equipment come with a female taper if it is a female thread used. If you have a male thread, you'll find it will be a male taper - unless they have a special O-ring fitting that goes on top. However, in general, if it's just a threaded piece of equipment that comes to you, it will be a tapered thread in it.