The reason I'm asking is an engineering issue. I'm already noticing after vaping for a few weeks that there are threading issues with a large, heavy clearomizer or tank and the smaller 510 threads. I first noticed this after getting a Mistic e-cig, which is KR808D-2 threaded: the threads are beefier and I've had slightly fewer issues unscrewing the cartomizer. With the larger clearomizers using 510 threads, I'm starting to notice a little wear and loose fitting on my eGo batteries (I'm thinking of using a little thread treatment, like Loctite). IMO, this is really only an issue with anything heavier than the CE4/Stardust clearomizer.
The H2 I have is eGo threaded but its horrible for gurgling/spitting juice. But it does have a nice tight, supported fit on the eGo battery.