Last year I bought s heat exchanger for my above ground 12 foot paddling pool. I also bought a pair of 'gee' one and three quarter thread pva plastic adaptors to push in 50mm plain blank tapered holes. I can't find the invoice or who i bought them from I want to upgrade my Heath Robinson, Rube Goldberg tape and glue circuit with something a bit more click and connect maybe with fewer hose clips. I searched 'G1 3/4' i searched G7/4 inch thread I searched G one and three quarter thread to no avail. Where the hell have they gone? Are they extinct? Is there an exclusive club with a special handshake I have to join? Please help.
I believe G denotes British standard pipe thread, so they will be more difficult to find. See here, https://en.wikipedia.org/wiki/British_Standard_Pipe
If you can't find local and you live in the US, try a supplier website like McMasterCarr or Granger, etc.
Could this be what you are looking for? https://www.mcmaster.com/british-standard-thread-pipe-fittings
Well, if you cannot find them then your other option is to find a machine shop.
Any decent machine shop can make you two from brass stock with the other end machined to match the copper pipe or other fitting you need to adapt to.
What supplier did you use for the heat exchanger as they usually carry fittings as well. Perhaps worth contacting them.