If any of you have the SMK 510-T tank cartridges you know they are a bit different than the other 510-T cartridges . They have a small wad of filler at the end of them under which is two holes for filling . You can also remove the cap to fill . I find that the wad is not the same size on all of the cartridges and is glued so you have to move your syringe around the outside of the wad to find a way into the holes . What I did was just remove that wad and scrap out the glue . Then I put a small piece of blue foam in to replace it . It works really well . The old wads seem to come apart anyway and I find little bits of fiber from them plus they don't lie down again very well .