I got some mini ape style bars from a buddy of mine. I gave them to my dad for his bike, but we cant get them to snugly fit into the stem. They dont have the raised center section like his other bikes, or the "threaded" area for grip. We tried to space the gap with rubber...no dice, the bars just moved since they are slick. Tried to build up tape right there in the center, bars still moved. Any other ideas short of buying a new stem?