Repaired a Central shower mixing valve that had a union that was cracked on the FPT side. While buying a new one a salesman (retired master plumber w/34 years in the trade) told me that's a common problem. He said the way to do it right is to clean and flux the FPT and MPT, hand tighten them and solder the joint together. Worked beautifully. Can't tell you the number of times I tightened threaded joints and soldered the adapter just to have it leak at the threaded joint. Thought I might pass this on.