I would have thought the cap would have been made to fit all the way to the end, but that's as far as it goes. It doesn't feel as secure as it should to be honest.
I'm guessing it's another cost cutting exercise like the actual pipe, making one part to fit different engines, this cap can probably be used on other parts.
And why is it green?!
It will be interesting to see if this fixes both our leaks, I have my doubts about mine!