Plastic crank arm fixing bolts crack, strip, or go missing more often than you'd expect, and finding the right replacement requires matching the thread depth to your crankset generation. The Shimano FC-T661 Crank Arm Fixing Bolt carries an 8mm thread depth that fits a specific range of Hollowtech II cranksets spanning road, trekking, and cyclocross applications. If you're running 10-speed Ultegra, 105, or Tiagra cranksets, this is the correct preload cap. The same applies to LX, Deore, and Alfine cranksets from the same era, along with several utility and cyclocross models in Shimano's lineup.

The bolt threads into the left crank arm and provides bearing preload adjustment before the pinch bolts get tightened. Shimano uses plastic construction intentionally here rather than metal, which prevents overtorquing and protects the bottom bracket bearings from excessive preload. The 25mm cap diameter interfaces with Shimano's TL-FC16 tool, giving you controlled rotation without slipping. Once you've dialed in the preload and secured the pinch bolts, the fixing bolt's job is done until the next time you pull the crankset for maintenance or replacement.

Compatibility matters with these bolts because Shimano changed the thread depth when transitioning to 11-speed road cranksets. This 8mm version works with Ultegra 6700, 105 5700, and Tiagra 4600 series cranksets, but it won't work with Ultegra 6800 or R8000, 105 R7000, or Dura-Ace 9000 and later. Those 11-speed cranksets use a longer bolt with deeper threading that engages more material in the spindle. If you're not sure which version you need, pull your existing bolt and measure the threaded section before ordering. The 8mm thread length on this bolt is the giveaway that tells you whether you've got the right part for your crankset.