Paul8, your terminology has me somewhat confused. A couple of pictures would help to show what you are referring to. But I will hazard a guess.
There is a coolant drain cock on the left hand side of the engine when facing the belt cover that can be loosened off with a standard box end wrench or socket. I believe it is a metric 10. There is also a coolant drain plug fitted on the output of the heat exchanger. This plug requires an allen wrench (or hex key). It takes a fairly large one and I believe it is a metric 6 give or take 1 size (5, 5.5 or 7). I loosen off the drain cock then open the rad cap when I replace the coolant every 2 years. I have never opened the drain plug but I'm sure that others will chime in as to when it is used.
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