Spirale però!
6mm https://www.google.it/search?site=&tbm=isch&source...
8mm https://www.google.it/search?site=&tbm=isch&source...
6mm
http://www.google.it/imgres?sa=X&biw=1280&bih=596&...
8mm
http://www.google.it/imgres?biw=1280&bih=596&tbm=i...