Originally Posted by Jeff Wills

The stem you have on your POS mountain bike has a double clamp for a straight mountain bar. It only works with handlebars that do not have a bulged center section, and will only clamp bars that are 22.2mm (aka 7/8") in diameter. I have never seen a drop handlebar that will fit that stem.

The "normal" mountain bike handlebar's clamp area is 25.4mm (aka 1") in diameter. Most road "drop" handlebars are 26.0mm or 26.4mm in diameter. Some older Japanese drop handlebars and most cheaper steel drop bars have a clamp area that is 25.4mm in diameter.