plumbingwater
The adapter I use to get standard 3/8” water connections on the 1/2” pex in my home (to replace a faucet for example) always comes with a little copper piece (on the right of the picture) inside the connection.
What is this? Should I be using it? I sort of assumed that it and the nut there are for if you're running your own custom line, but now I’m getting paranoid that I’m messing up. Thanks!
See this thread about how much to tape to wrap. Two wraps might be too little while 20 seems pretty extreme.
Some people like tape, some like compound and some like both. I gave up on reading about which is better because it just seems to be a matter of opinion.
Both are for lubricating and sealing. The "Pipe Dope" has a substance in it (sometimes hemp) that expands with moisture to further help prevent leaks.
Regarding how tight, you can definitely over tighten but if they are leaking I wouldn't hesitate to give another bit of a turn. You need to be particularly careful with compression fittings since they can be more easily over-tightened.
You'll need to install a drain assembly. In a bathroom, pop-up drain assemblies are common.
Here's an image of the drain assembly, with all the parts labeled.
(source: naturalhandyman.com)
The tailpiece of the drain assembly, should attach to the trap assembly. If the trap is too low, an extension can be used.
When installing the drain, put a bead of plumber's putty around the drain hole of the sink. Then bed the drain flange in the plumber's putty. From below, install the washer and nut, and thread on the tailpiece. When you tighten it down, the excess plumber's putty will squeeze out. Use you finger, or a rag to remove whatever oozes out.
Best Answer
That looks like an olive for use with a copper pipe.
The pex fittings I use have a split collar as they are designed with the water seal internally onto the brass spigot and the nut/collar is to secure the pipe into place.