I don't know about the thread on atty resistance.. but based on my experience with my 510 units, the nominal resistance is about 2.2 Ohms.  I have never seen one (new) higher than 2.3 (on a quality Fluke DVM).
The one that came in at 1.9 Ohms has substantially different vaping characteristics than my other units. But that make sense. On a 3.7VDC source, a 2.2 Ohm atty will draw 6.2 Watts. The 1.9 Ohm atty will draw 7.2 Watts! Hard on batteries and it runs hotter... but there is a marked difference in vapor production.. as one would expect.
	
		
			
		
		
	
			
			The one that came in at 1.9 Ohms has substantially different vaping characteristics than my other units. But that make sense. On a 3.7VDC source, a 2.2 Ohm atty will draw 6.2 Watts. The 1.9 Ohm atty will draw 7.2 Watts! Hard on batteries and it runs hotter... but there is a marked difference in vapor production.. as one would expect.
				
		
  p.s. the power went out the other night and I freaked out thinking I might run out of batteries!!8-o  I figured out then I need some way to charge this thing in the car (which I know is available)