FSA OS-99 CSI Stem

The Carbon Structural Integration (CSI) process bonds a carbon weave to an aluminum core. This allows the stem to benefit from the stiffness and vibration-damping properties of carbon while maintaining the structural reliability and precise tolerances of a CNC-machined aluminum core.

The OS-99 CSI is designed for standard 1-1/8 inch steerer tubes. It is also compatible with 1-1/4 inch steerer tubes, making it a versatile choice for many modern road bike setups.

Yes, titanium bolts are prone to seizing if not installed correctly. We recommend using a high-quality anti-seize compound on the threads and always using a calibrated torque wrench to ensure you do not exceed the manufacturer's torque specifications.

This stem has a stack height of 41mm. This measurement is important because it dictates how much steerer tube length you need above your headset; if you are swapping from a stem with a different stack height, you may need to adjust your headset spacers to maintain your current fit.

While engineered for high-performance road cycling, the OS-99 CSI is stiff and durable enough for light gravel use. However, if you are riding aggressive, rough terrain, you might prefer a stem specifically designed for the higher impact loads of mountain or gravel biking.