only the 3mm bowden has a partial lining. 3mm direct has no lining. the bore on the hexagon is tight. it will will be more prone to jamming. from what i read its 1.8mm. you have to keep in mind also that plastic expands when it gets hot so 1.75mm filament for example very well might just make it down the bore without a jam. if you hit a spot that is 1.77-1.78 your done for. im not sure why they made it so tight. most hot end bores are 2mm. of course i know you have 3mm but i would assume the same would hold true with that.