I think he means that in my photo, on the right side of the spring it went through the cutout in the plastic housing. The tube does need to go through the spring, but the spring needs to be a bit more compressed to seat in the plastic rim. Imagine the spring in my photo to be compressed to about half its current length and it's still touching the left white plastic part.

Alex, I haven't measured or taken pictures. The ring is approx 8 mm in diameter (from memory!). Don't know is this is of much use for you, though.

Good luck!

Thank you! Great addition.

I noticed the water in between the plastic and metal part. Only later I was able to take the heater apart with force, the leak was inside, in de middle.

The black part twists a bit to one side that way you can get it off. Needs quite some force, though!