Drownerdowner t1_jeh40oc wrote

I'm a licensed plumber all you ever need to do is look at the shape of the thread to determine what makes the seal. The threads we create on steel pipes when screwed together are tapered. This means that the threads diameter increases as the thread goes which creates a joint so tight water can't force its way through. Threads that are straight usually require some sort of washer rubber etc to create a seal.