Well, I should have written that I rebuild the Aspire Atlantis heads with vertical coils, while the Kanger Subtank heads get rebuilt with horizontal coils.
The Aspire verticals are trickier than the Kanger horizontals. Basically, the Aspire Atlantis head is a double-tube: a metal outer tube (where the holes are that allow juice to enter) and a second metal inner tube (inside of which sits the vertical coil wrapped in cotton/rayon wicking). In the small space between the two tubes is a strip of sheet-like fabric wicking, which regulates (restricts) the flow of juice to the coil. Without that strip of wicking fabric, the head floods like crazy and is useless. If, in removing the old coil from the head, that fabric comes out, it's a PITA to get back in. The fabric wicking strip will degrade eventually, so I presume the number of rebuilds per factory head to be limited. I don't know how many times exactly, but I'd guess no more than three or four rebuilds per head before that strip is shot. I bought ten clone heads, so I have more than enough to last for quite awhile.
The Kanger Subtank heads are much more straightforward to rebuild with horizontal spaced coils. They're simply larger versions of Kanger's older EVOD-style heads, which I've been rebuilding for the past year and a half.
I watched YouTube vids to learn. I generally do that with every new RDA/RTA I buy just to make sure I don't miss some tip or trick that's helpful or necessary for a specific atomizer.