A word of warning, do not leave popcorn or butter or anything liquid on the roof of the machine, on the gears for the stirrer, or on the screws on the roof. I am getting a lot of my great northern popper replaced for free by the company because it rusted. The roof rusts, the gears rust and the screws rust all on the inside ceiling. If you clean religiously after use you may get some good mileage out of the machine, but if you do not the parts will all rust and the machine will become unusable extremely quickly.
To reiterate, the ceiling, gears and screws on the inside roof of the popper are NOT made of stainless or rust-resistant material and there are no upgrade parts available. Clean religiously or lose your machine. If it happened already, great northern may replace the parts (they did for me, my popper was bought in April of this year).