Bottle thread answer doubt I'm trying to follow the following answer from the site:
How to model the bottle threads?

I could do almost everything, but when I have to do the (spiral) loop cut the program doesn't allow me and I don't know why and in the answer there is no clarification of how it's done. 



Additional info:
Before I remove the screw (even after doing the knife intersect) I can do the spiral loop ring as the picture shows. 
When I delete it I cannot do the spiral loop cut nor select the linked objects all the spiral with CTRL+L and I have to do it manually.
I tried it myself.  The problem is that not all of the vertices line up perfectly, so you end up with some rogue unwelded verts, like so:



See those little evil guys?  

To fix it, select all verts and choose Mesh->Vertices->Remove Doubles.  Then adjust the threshold until they all nicely merge.



Once all the verts are properly welded your spiral select will work.