It looks like the fill plug Tom illustrated is one that has a "T" handle with threads which compress the rubber gasket against the inside of the fill tube when the handle is turned clockwise. Some engines will build enought pressure to blow a fill plug out creating other issues. So if yours is that type it's a good idea to make sure it is well secured when you finish the fill.
Sandy and Bob McBratney
Previous owners of 2007 Newell #1202 "Boomer"