I have had problems with all three.
One doesn't fire at all, because the 510 connector sits too deep and it is a fixed pin so non of my toppers make contact ( chi you antique copper ).
That's a pretty easy fix ...
1. Remove the top cap of your mod from the tube
2. Then you just screw this top cap into the 510 connector of your atomizer
3. Then get a little screwdriver and screw in the pin (inside the top cap) until it stops ... This way, the pin will be contacting the bottom of the 510 connector of your atomizer.
If the little screwdriver slot in the pin is facing the wrong direction (on top of the top cap instead of on the bottom of the top cap) ... just remove it and turn it around before installing the top cap to the atomizer and tightening down the pin.
4. After the top cap is installed correctly on the bottom of the atomizer, then install the tube on the top cap, then install the battery in the tube, and finally the bottom cap / fire button assembly.
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Once all put together, if the battery rattles around in the tube, you will need a longer pin ... If there is a gap between the bottom of the tube and the bottom cap once tightened up, then you need a shorter pin.
You can buy a threaded M5 pin ("setscrew") at Lowes, Home Depot, ect ... Just cut it and file it to the length you want (a Dremel works great for this).
Top Cap Pin (aka: "Setscrew") ..
PIN ADJUSTMENT VIDEO:
http://www.youtube.com/watch?v=ogqDuoxyAvk