Further to what FB states, when you install the stem be very careful NOT to bottom it out into the tapered section at the bottom of the steerer. If you expand it into the tapered section it can let loose without warning exactly at the wrong time. See this reference http://sheldonbrown.com/handsup.html#threaded and scroll down to the "Too Low, Danger!" section for an explanation and a diagram, which includes a cutaway diagram of your cone type stem. Handlebars coming loose is only funny in the cartoons.