I now have metal tank adapters!
I was at the Milwaukee Electric Car Club EV Build Day today. A while back I gave my original electric water heater tank elements to two of the guys to see what they could come up with to re-use the heating elements to make an adapter for standard piping to go into the water heater tank.
One member took the element, cut it out on the lathe, and tapped out the middle to 3/4ths" female pipe thread. I should be able to screw some standard pipe right into that.
The other member modified the element to sort of a scooped-out cone shape. That way we could weld a 1" pipe directly to it to be able to keep the larger diameter pipe, which may still allow for thermosiphoning from the tank, through the heater exchanger and back. I don't know how well this really would work or not, but I do know that bigger pipes are required for it to work.
It's pretty interesting how you can give two people the same problem and limitations, and they come up with completely different answers.
Either style connector will still use the original flat rubber washer to seal the adapter to the water heater tank, just like the original heater element was.