Leak at Cold Water Connection to Shower Valve

[AHB]

We completed construction on an ADU on our property here in western NC around 6 weeks ago. We converted a woodshop into a dwelling. We had inspections (rough-in and final) and got a CO. Our daughter moved in. On Monday she told us that the end seams of the planks in the brand new hardwood floor were turning black near the plumbing wall (I think this is tannin pull because the floor is white oak and there is a lot of iron in the water here--the dirt in these mountains is red). My husband found water leaking down into the crawlspace under the wall where the shower valve is located. He opened the wall behind the shower valve and discovered a leak in the cold water connection to the valve.

The valve was connected with tape and pipe dope. As a temporary fix until we could get supplies, he put an O-ring into the connector, and the leak stopped. We got Blue Monster tape and sealant, he removed the O-ring, and applied tape and dope to the threads on the male fitting per directions and tightened with a wrench, but the connection still leaked. He put the O-ring back in, and the leaking stopped. We're putting an access panel in that wall so we can keep checking and make future repairs, but here's my question:

We're both architects and my husband is an experienced DIYer, but we're not plumbers. I've read about the different types of connectors, and that NPT connectors require tape and pipe dope, and connectors with straight threads require O-rings/gaskets. The shower valve specs say that the mail connector on the valve is NPT. But the female Pex to valve connector looks like it has a flat shoulder at the bottom (see photo), and when my husband tightened it he said there was a hard stop, which I don't think would happen with an NPT connector. Can anyone tell me what's going on here? Is it okay to leave the O-ring in this connector, or do we need to go back and try again with tape and pipe dope until we get a tight seal?

For what it's worth, the hot water connection isn't leaking (yet).

Thanks for any light you can shed on this!

 

[Sylvan]

Disconnect the C X F adapter use Teflon tape 3- 5 turns and use pipe dope if you feel you want a little more protection

Make sure the solder did flow inside the joint with no gaps

 

[Sponsor]

 

[Breplum]

What the heck is an o-ring doing in a FIP (aka NPT) fitting. That makes no sense. As noted, a normal threaded fitting should not grind into a hard stop because tapered threads tighten up at less-than-bottom on a fitting. Something is wrong. Maybe you ended up buying or were given a bad or wrong fitting. like Sylvan said, proper direction of tape wrap and should be good. could the tape have been wrapped in the wrong direction...that is VERY common with DIY

 

[John Gayewski]

Any number of things could've happened, doesn't matter. You should buy from plumbing supply houses and not big box stores as big box stores often put things in the wrong bin or just plain carry low quality product.

Start over with another adapter do both sides to be safe. Tape behind the first two threads, work the tape into the base of those threads, then dope over the tape.