Now that I am back on a computer I've had another look, and you have other problems. You are using layers incorrectly. Layers in Sketchup should only be used to control visibility and as such only groups and components should be assigned to different layers. All raw geometry needs to be on Layer0, once you have grouped it you can assign it a layer. To put it simply, if you explode everything down to raw geometry nothing should disappear when you turn a layer off.

keknor, did dukejazz's suggestions help you? You may have to type the file extension [.stl] at the end of the filename after you export it, since the exporter doesn't always do that automatically.

@box said: You need to rethink your modelling workflow. If the rest of your engine is built the same way your model will be so bloated as to be unworkable and unprintable. You have used 360 segments in a circle that is meant to be 3.3mm that isn't going to work. A default circle is 24 segments, 50 is pretty damn smooth, 100 is overkill, 360........... Anyway, the segment count is creating your tiny face issue and the huge files size. You need to work out how you want to 3d print this and the tolerances involved with the specific printing method. Work with fewer segments, work with all geometry on layer0 and move groups to layers for visibility not construction. Always keep layer0 as the active layer. When you look in the entity window of an unopen group it can say Layer63, but when you click into the group and look at the raw geometry it needs to say Layer0. That's all I have time to write just now. Thank you "Gilles" and "Box" for your advices. I will work on it and report here about my results.