Any time you tighten a loose crank bolt you need to remove the crank bolt, apply Loctite® 242 on the crank bolt threads, reinstall, and tighten to the appropriate torque (22 to 25 ft-lbs).
If the crank bolts continue to loosen up you need to replace the crank arm. The steel crank arm is made of a slightly softer material than the hardened steel spindle and if the bike is continued to be ridden with loose crank bolts the crank arm will eventually “egg out” due to the excessive play between the crank arm and spindle.
For detailed instructions please visit the Parts section of our online Owner’s Manual.