Are you talking about the plastic preload bolt? once the preload has been set, and the 2 pinch bolts done up, this bolt should stay in place, mine has done a few thousand Km since last refitted, and is still there. if you mean the pinch bolts, these do have a torque rating (12-14Nm). the specs with torque rating are here
http://techdocs.shimano.com/media/te...9830750016.pdf