Jim said it very well.
Pipe threads are tapered. With teflon tape on the threads, you don't have to tighten the plug very tight, and in fact, you should not. You don't want to crack the fitting. If you chase the threads with a tap, don't cut metal, just clean the threads. You never run a pipe tap all the way in, again, because it's a tapered thread that relies on interference to seal. About two to three wraps of the yellow style teflon, three wraps of the blue style, or about 6 wraps of the very thin Harbor Freight style tape is all you need. Keep a roll of the blue or yellow in your tool box to fix all kinds of things. It makes great O rings, a good anti seize, good gaskets for hose to barb fittings, reduces corrosion in dissimilar metals and can act as a thread locker, just to name a few.
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I only exaggerate enough to compensate for being taken with a grain of salt.
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