I have purchased a few 2" shower drains but the are a little to small and do not thread or they are too large and can not crew in. The one too small seem to have a dia (around threads of 2-7/8) and the one 2 big seems to have a dia of a little over 3 inches. It seems as if the correct size is 3". Is this common that a 2" drain can have this many different sizes and how do I find my size"

Common drain for shower pan is the one that fits 2" pipe. Yes, it is a bit smaller than actual pan drain opening but with rubber seal under the pan you will get positive seal. I am enclosing photo of average shower drain. Let me know if that's the one you have. Back to you. Milo

What type of drain pipe and you connecting to. Metal or plastic? Is this a custom tile shower with a shower pan or is it a plastic shower base and enclosure? Back to you, TomBack to you, Tom