PRO Tharsis XC Stem - 90mm - 31.8mmPRO Tharsis XC Stem 31. 8mm The new Tharsis XC stem is the next evolution of headset design. Unique pinch bolt design with headset preload ring enable the headset to be adjusted without the need for a top cap or star nut leaving the fork steerer tube free to take an internal Di2 battery or simply as a method to save weight over a traditional system. The stem has been designed to take an SM JC41 Di2 junction box internally and is machined to allow