The fact that it's in the cylinder means that it could be a head gasket that slipped when you installed the head. I'm afraid that head has to come off, but before it does, pull the other spark plugs and see if there is any coolant anywhere else. If there isn't, than I would suspect a damaged head. If the source isn't obvious when you pull the head, get out your magnifying glass, or go get a magnaflux kit.
Take care,
~Chris
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