I had the same problem with my toptank mini.My wife's Istick100 is beginning to wear out, the 510 threads are getting tired and I have airflow adapter from back in the day to help save her threads. I got her a P3 and received it last week so I hope this should tie her over for the next many years.
My disappointment in the adjustable 510 that is a oring that squishes a bit with a load of pressure, way more pressure you'd use threading on a atty...that I can get past. Its with a kanger subtank mini and the gap from the ring to the atty is what I can't get past. I bought a extra P3 connector and the gap is even wider. Now I have to manufacture a something to fill the gap and hold solid cause I can't backthread the ring to touch the atty cause my wife will surely loose it filling it with juice. I expected a bit of gap but not like this.
Glad I spent only the hundred bucks on it rather than the 250 I was almost willing to spend when they came out.
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Solved it by pulling out the positive pin and sanding it shorter gradually checking as I went.
Sits perfectly flush now