Losing a left crank arm clamp bolt during a service is one of those small problems that stops a repair cold—you can't ride the bike, you can't finish the job, and the bolt isn't something you'll find at a hardware store because the M6 x 21mm sizing and Shimano's Hollowtech II clamping geometry are specific to the crankset. The Shimano Crank Arm Clamp Bolt is the genuine replacement, sold individually with the washer already installed, and it's the same fastener Shimano specs across a wide range of Hollowtech II cranksets from 105 and Ultegra road groups through XT and STEPS E-series e-bike drive units.

The bolt lives on the non-drive side of any Hollowtech II crank, where two opposing bolts pinch the left crank arm onto the splined spindle. Torque on these bolts is what keeps the arm from developing play or creaking, and the washer distributes the clamping load across the arm's slotted interface so the aluminum doesn't get chewed up over repeated installations. If one has gone missing, gotten stripped, or come out looking corroded during a bottom bracket service, replacing it with the correct Shimano part keeps the interface healthy for the next install.

Confirmed compatibility spans FC-R7000 105, FC-R8000 Ultegra, FC-E8000 and FC-E8050 STEPS crank arms, and a long list of other Hollowtech II cranksets. If you're working on a Shimano crank that uses two pinch bolts on the left arm, verify against Shimano's exploded parts diagram for your specific FC number before ordering, but this is the bolt that shows up on most of them. Sold individually—if you're replacing both, order two.