Originally Posted by Retro Grouch
If the crank bolt isn't tight enough, the crankarm will eventually work itself loose and may even come completely off.
Yes - BUT:
After a first-time installation with the proper torque, you may find that after riding a few 100 miles, the bolt will appear to have loosened. In fact, the bolt did not loosen, but the crank will have worked it's way further in. This is especially true if you greased the spindle square. (IMHO the only thing that goes against greasing!)
If this is the case, do not
retighten the bolt with the original torque - just retighten it to the point where you'd expect it (the bolt, not the crank!) not to come loose. Check repeatedly over the next few 100 miles.
Quoting Jobst Brandt:
>Failure from "over-tightening" is caused by repeated re-tightening of properly installed cranks. In use, an aluminum crank squirms on its taper and, because the retaining bolt prevents it from moving off the taper, it elbows itself away from the bolt and up the taper ever so slightly. The resulting loss of preload, after hard riding, can be detected by how easily the bolt can be turned.
Mechanics, unaware of why crank bolts lose preload (and commensurate crank tightening), have re-tightened bolts until cranks split. No warnings against re-tightening properly installed cranks are evident although it is here where the warning should be directed rather than at lubrication.<