if you have the nickel collector tank it should fit the all mods cap. if you upgraded an older GGTS like #306 with an older aluminum collector tank, 4-in-1 adapter, adjustable draw cap, etc. then you'll need an updated collector tank for the all mods cap to properly thread on. i don't know if there is any real science to it, but i've noticed that my all mods caps only fit the collector tanks i have with the 4 air holes lined up in a row on one side, but not the ones with the four air holes spaced out around the tank. check with imeo or bruce if you know you have a newer tank and the cap doesn't fit, because there are collector tanks available to fit those caps.