Water Heater Expansion Tank replacement issue

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?

It looks like part of the expansion tank threads are still in that fitting. Look at it close, you might have to dig them out. Just don't damage the threads.

Thanks very much. When I gave it all a fresh look, It was really hard to read but the fitting is stamped as a 3/4" and I see that now. Before seeing that I had already ordered a 3/4" copper Tee with threaded middle connector. Not a stock item at Home Depot or Lowes, but I found it on the web easily and it arrived a day after the expansion tank. And I replaced the fitting by sweating in a new one. The threads on the original was just too corroded and it wasn't up for the big show any-more. Problem solved. Thanks.