Every Hollowtech II crankset installation depends on this small plastic bolt doing its job correctly. The crank arm fixing bolt threads into the left crank arm and sets the bearing preload before you tighten the pinch bolts that clamp the arm onto the spindle. Skip this step or lose the bolt, and the left crank arm won't stay secure no matter how tight you torque those pinch bolts. If yours went missing during a service or broke from overtightening, this is the direct replacement.

The installation sequence matters here. Thread this bolt into the left crank arm using the TL-FC16 tool or a coin inserted into the splines. Tighten just until the lateral play disappears from the crank spindle, then back off a quarter turn to avoid loading the bottom bracket bearings. Once you've found that sweet spot, tighten the two pinch bolts to spec. The fixing bolt then stays in place, functioning as both a preload adjuster and a dust cap for the spindle interface.

Compatibility extends across a wide range of Shimano Hollowtech II road cranksets. This bolt fits the FC-6800 Ultegra and FC-5800 105 it's named for, plus the newer FC-R8000 Ultegra, FC-R7000 105, FC-R7100, FC-4700 Tiagra, and FC-RS520 cranks. The thread pattern remained consistent across these generations, so one part number covers multiple groupsets. Mountain and gravel cranks like the GRX and XT series use a shorter version with the same thread, so verify which length your crankset requires before ordering.