For now:
1. Boil some water
2. Dip the end that has the thread INSIDE it (the piece that the other piece screws into) into the water and leave it to heat up for 30 seconds or so.
3. Take the whole thing out and quickly dip the other end in cold water, dry it fast on a towel and turn it again.
The idea is that you get the outside thread to expand while keeping the inside thread small.