usually, the measurement in millimeters is used to describe the distance across the well (the bottom). All threads for vapes should match, 99.9 percent of the time they are all 510 atomizer threads.
here is what I use.
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It’s helpful to remember that the legality of Vaping is constantly changing, so it can be hard to get replacement parts. I would def suggest using geekvape, because the mods are built to take a beating, and they ship coils direct from China regardless of the legality lol.
one last thing. If you enjoy your sanity, don’t buy a dripper ( RDA = rebuildable dripping atomizer). Buy something with coils you can exchange when they burn out. They constantly need to have e-liquid applied, and are terribly finicky. You will find yourself fixing the fucker way more than you vape it. If you absoloutely feel the need to build coils and want to learn, I would probably suggest a Rebuildable tank atomizer, that will at least spare you from carrying a bottle of juice around.