vieucaillou -
Drawhelix13 is the version I used. Now I can't reproduce the dimensional inaccuracy I mentioned earlier. I started these models in SketchUp7 but I had a lot of trouble with that version so I uninstalled it and went back to SketchUp6. Maybe something was happening there. But I just opened Sketchup and asked drawhelix13 to make me a helix with a pitch of 180" and I get a helix at 180"
Confusing to a novice.