Looking for People with experience with older Elos 120 tank

[DxSOR25]

You can guarantee it’ll be metric, so finding something to fit it is going to be challenging. That’s another pro to stepping it up and rethreading, you can change it to something available local. Since you're considering cementing a fitting in, another option would be to step the hole up and leave it smooth to match the outside diameter of the next size pvc pipe or threaded nipple.. something like this with one end cut off, then cemented i

YES! thats what I was thinking. I was telling my uncle i rather drill smooth hole without getting too close to the middle hole and cement in a smooth end and having an adapter for the return pump. I dont care about making it permanent. I will measure it and most like draw up to make the return an imperial system size. I will let you know how it goes. Thanks again for the help and sorry to the OP for hijacking his thread. Hard to find much about these plumbing systems.