Okay, I was wrong. I only got a short amount of time out of the system before it started leaking. I had to cut the pipe out and start over. I went back to the 3/8-20 tap with no issues in 2 days. I will say that the nipples don't look like they go all of the way in, but they seal tight...
Thanks for this post, just helped me finish my new waterer project. I tried a 3/8-20 tap first, but got better results and a tighter fit when I stepped up to a 7/16-14. Went in tight and seated all the way in. No leaks yet.