tapa bajo la cabeza

16:03 May 13, 2011 

One of the photos accompanying the document clearly shows a bolt that has shiny metal under the head and is also shiny along the first third of the shaft of the bolt, before reaching the thread. This must be the "adjustment distance" mentioned. However, I have left the phrase out and simply described what the photo shows. The main thing is that if the metal is shiny here then it is proof that it was rubbing on something before the bolts in question broke. In this case I went for "the surface just under the head".