In article , Guy King
writes
The message
from "Vass" contains these words:

I bought this fitting, which fits, but presumably I use PTFE tape on the 2
touching brass threads
and make sure the rubber washer sits inside?

The rubber will bear directly on the nose of the inner thread, so
there's no need to seal between the two brass threads.

I'd say the adapter on the left is designed to mate and seal with the larger
threaded piece on the right, it may incidentally seal the smaller pipe too
but that's not guaranteed, seal the threads too.
--
fred
Plusnet - I hope you like vanilla