Hi, We have a very old Ex-trol ST5 Expansion tank for that has failed on the lines of small electric water heater in the basement a small office. Maybe over 45 years old? Is that possible? Whatever it is... it no longer made under that exact brand. The cross-reference replacement is an Amtrol that has according to the specs has a 3/4 inch thread. One is ordered an on its way. The old tank is so rusted out that I can really study the connection threads, but it did spin off. On the pipe, the threads seems okay. The unit threads into a fitting that is part of a copper line. Since the spec sheet for its replace is 3/4" I assumed it was a 3/4" thread and wanted to put a plug in to keep it from dripping for the next couple of days. That 3/4" plug is too big. Meanwhile A 1/2 plug is too small. Just for testing purposes it seems to sort-a thread to a flared 5/8" but in looking a common plumbing threads, this 5/8" is not common for water use and it never gets tight anyway. Any idea what I am dealing with?