water in the oil

[Geert Mullens]

My Flavia Coupé 1.8 has water in the oil. In the collectors guide from Wim H.J. Oude Weernink, I read that you don't have to worry immediately. That it is possible that the alumium casting plugs in the cylinder head are leaking. So I checked it. And indeed on my right cilinderhead, there is a plug leaking. But what is a casting plug, my english is not that good to understand it? How can I cure this problem.
I thank you.
Geert