OK I take it you got a new stem. Here is the formula for fit. When you put the stem on press down on it engaging the fork. Now put the star nut in and screw it down till you get resistance not much though. Now with your bars in the stem turn the bars sideways. Now grab hold of the bars and pull on one side and push down on the other, as if you are trying to rip it off. Do you feel any slack? If so then tighten down the star nut some more and repeat. Do this until you get no play in the headset. Then tighten down the lower nut on the stem and then the top. If you do not have two, then just tighten the stem down. If this does not work you need to check your bearings to make sure they are seated correctly. The lower bearings should be facing up and the top facing down. If this did not fix it make sure there is no gaps between the headset cups and the head tube on the frame. If so then a block of wood and a hammer will seat them in the frame. Hope this helps.