Technically the stem thread attaches to the pipe body. The cartos that come with the pipe seal up against the top of the inside of the stem when fully attached. Since the provided cartos suck, I took boge and some other misc cartos and stuck them in. Since they were a tad shorter, I took a regular thick carto cap condom, trimmed a bit off the length and punched a small hole in the top. When I stuck it in the pipe stem, it stuck inside a small cutout area near the inside's top. When I used shorter cartos and regular attys with shorty drip tips attached, it kept the draw from being too light. Dripping proved to be somewhat of a pain so I switched entirely over to cartos in the pipe. Only downside was that there was no reliable way to know how topped off a non-clearo CE2 type carto was. So once I use up my stash of boges standard cartos and this pack of CE2s I'll start with whatever clearos I can find that fit.
Did you take the little rubber piece inside the stem out? Been wanting to try some different carto's on this pipe without changing the draw on it that much.