If you're familiar with the design of the eGo you know it has that cone that
threads over the atomizer but also
threads further down the eGo battery than the atomizer. Both the eGo mega-mega atomizer and the mega cartomizer have 510 style
threads but also have a decorative 'collar' that matches up with where the cone threads. This means this collar or skirt extends below where the 510 atomizer threads are. You can either cut off enough of the collar or skirt to thread it onto the ST (its just plastic) or you can also do something like a pair of adapters first a 510->901 then a 901->510 adapter to get extra distance above the ST so you can thread on the mega carto.