I can't screw it onto an SVD because the threads don't seem to catch. It makes contact (despite receding the pin all the way) and goes all the way down so that its base is flush with the SVD, but it pops right off. On the other hand, it screws perfectly onto a K-100 mech. All my other toppers from clearomizers, AGA-T, Fogger, Igo and trident drippers screw on both the SVD and K-100 just fine. What is also odd is that an old 510 extender I have screws fine onto the KFL, but doesn't screw on to the SVD. So my question is this, did I get a KFL with defective 510 connector threads, or is there something I don't know about different 510 thread widths out there? Otherwise, this tobeco KFL clone is excellent.