As an extension from this question, is there any reason that you would not be able to use 1.75mm filament in a printer that takes 3mm filament? I know you would have to change the filament size in the slicing of prints but would there be any other problems?
Also, would using 1.75mm filament be possible if the nozzle diameter was greater than 1.75mm but less than 3mm?
It may work for a short time but you're going to fill the melt chamber quickly and possibly overflow to a point where the filament isn't constrained causing a messy jam. All the molten plastic will likely flow backwards to a point where the diameter isn't 3mm any longer (probably next to your drive gear) but depending on the length it may just flow up, cool down and jam the extruder.
No, the filament would just pass through unheated and not do anything.