Originally Posted by cxwrench

The only bike in the world that should have the headset top tightened to a specific value is the Trek Domane w/ front Iso Speed. You NEVER should tighten a normal headset to any specific torque value. You properly adjust it...that is all. I NEVER use a torque wrench on a headset unless it is the aforementioned Domane. Manufacturers put max torque on parts like that to keep idiots from breaking them or making them so tight they won't turn. The top cap is cracked because you're over tightening it.
ETA: This thread should obviously be in 'bicycle mechanics' not GD.