FSA OS 99 CSI Carbon Stem. The FSA OS 99 CSI Carbon Stem is the perfect means to help lighten and stiffen your bike's cockpit. FSA truly pulled out all the stops with the utilization of their Carbon Structural Integration CSI technology, the fusion of carbon fiber and aluminum, in the construction of this stem. A structural outer layer of carbon fiber is wrapped around an aluminum core to not only increase stiffness but also create an elegant carbon aesthetic. Simply put, you've got a rock solid platform that's featherweight to boot. You can perfectly dial in your position with the 6 degree rise and its 31.8mm clamp diameter is compatible with a wide range of handlebars.