Hi John, i think its 12mm, if you look down the port side of your engine you will see a piece of blue drain down pipe with an end stop put on it, cut a few mm say 10mm off, and then replace the end cap on the existing pipe. You will then have an off cut to take with you around plumbing shops. remember to also buy a couple end caps (stop ends) one to replace the old one, and a spare.

    good luck Ray