Drawing a threaded object

I have tried repeatedly to draw a screw (bolt) but when attempting to change the contour of the body of the screw (dual circular lines around a cylinder) by scaling one of the spiral lines - the shape does not follow the scaled line. For some reason which I cannot figure out - the lines and the cylinder are not linked. I may not be clear in explanation but the lines and object seem to be separate elements and not tied to each other when scaling the object. Only the line or the cylinder move with the scale rather than the surface moving with the line. Perhaps there is something that will explain how to be certain the lines and cylinder can be connected for distortion purposes.

Recommended reading:

SketchUp Sage: Helical forms - spirals, screw threads and more