The Shimano Crank Arm Fixing Bolt is an upgrade or replacement for your Shimano crankset, there to keep the crank arm bolts from backing out in high stress situations such as cornering.
Available in a choice of colours that will add flair to your drivetrain, it also works to adjust the side load on the bearings and hold the arm firmly in place as you tighten your crank cinch bolts.
Choose from a long or short fixing bolt, depending on the thread length of your existing bolt.
Installation:
Thread the fixing bolt into the crank arm, adjust it until all the lateral play in the crank arm and spindle has disappeared. Back off the bolt a quarter or half turn to make sure there is no sideload on the bottom bracket bearings.
You're looking for the sweet spot between not being able to rock the cranks back and forth in the bottom bracket and not torquing it down tightly, which would cause friction and premature wear in the bearings. After this, tighten down to the cinch bolts to the recommended torque, alternating between the two bolts.