A DSE901 cart does fit in a carto w/ the mouthpiece removed, but it will need to be trimmed a little so it doesn't hit the filler. I just tried it to make sure it will work, and it does. But it does affect draw slightly. I'd say I trimmed maybe 2-3mm off of the "skirt" that goes into the cart. Kinda nice actually. I have been meaning to try it. I think I am going to hollow the cart out and see if it works better. Best price I have seen on 901 carts is from strictlyejuice. Might as well get a 901 atty as well, then the shipping will be free. Then you can try out 3-piece.