I have used maybe 10 smok coil heads the whole time I vaped. 3 were junk clones that got sold as real (still have 2 unused boxes of these clone coils), and 2 authentics had issues too, one had a bad insulator ring, I don't remember what was wrong with the other one. So basically half of the coils I used was junk. Being the price they are, instead of buying more I spent that cash on a rebuildable, wire, and cotton. Best decision I have made in vaping.