Success! No leaks - finally!
As you can see, I followed
@wwhitney 's idea of converting the male copper threads to brass, using a FIP coupling and a close hex nipple. I used PTFE tape and dope on the appropriate threads, and also on the overflow tank. I did
not use any on the four nipples, again since the washer is supposed to be making the seals not the threads. I also think the type of flex pipe I used is very important here. As you can see in the photo, I used stainless lines this time, but the material isn't relevant here. Rather, the corrugation pattern is what matters. I selected these (Brasscraft parts SWB00-24N and SWB00-18N) because the thin corrugates suggested that they might be far more flexible than the units with thick spiral corrugates commonly found at blue and orange retail giants, and I was correct. These are MUCH easier to bend, and this would have also been true with the Falcon lines I mentioned earlier. This is significant because it was much easier to make sure that the flex lines thread onto the nipples perfectly straight, as I was able to make adjustments as I screwed them in. I felt much more confident that the washers were being compressed evenly, which I can't say is true for the thick-corrugate flex lines I wasted earlier. I also started with a heavy hand tight, then adjusted until drips stopped (probably about 1/2-3/4 of a turn with the wrench). If I had to do it over, I probably would have used a 24" pipe on the cold side as well, rather than the 18", because I think looping it (like you would for a heat trap) would make it even easier to align things perfectly.
The cold side nipple was a challenge, and frankly a bit of a roll of the dice as to whether the job could be done without serious damage to the WH. As I've read elsewhere, on my A.O. Smith Signature 100 it's a one-piece unit (dielectric heat trap nipple + dip tube), and it's REALLY tight in there, as it's installed with an impact from the factory. I used a 13" pipe wrench with a cheater bar and frankly lucked out that it popped out without breaking something else. I had a helper hold the WH (at appx 75% capacity) to prevent it from moving, and the nipple itself deformed before breaking loose, making this a do-or-die job as there would be no hope of re-threading the flex pipe back on after such damage. While it did break loose finally, the top of the tank buckled slightly as a result of the torque:
The dip tube looks like it's flanged and captive to the inside of the old nipple:
Not sure if that would pull out with force, but I suppose that one could just use a traditional brass nipple and a separate dip tube (40" on this model) as a replacement in lieu of the expensive one-piece unit from A.O. Smith (which every supply house seemed to have on backorder).
The hot side nipple had a little bit of rust on the top, as the previous flex pipe seems to have pressed down the plastic top leaving a metal-to-rubber connection. I opted to clean this up with a brass wire brush instead of replacing it, because I was worried that I wouldn't be so lucky in getting this one out. It's frustrating that something as simple as a nipple can make a WH less of a serviceable appliance.
Thank you all again for your time and advice. This is such a great community.
