Originally Posted by
terrymorse
If the fixing bolt is over tightened, which happens a lot, the crank arm "walks" up the taper. The crank arm develops a crack from being overstressed, then it breaks off without warning—usually when the rider is standing on the pedals—sending the rider to the ground.
The crank arm "walks up" the taper with every installation, even if it was torqued properly. This is a form of long-term wear for square-taper cranks. But, it typically takes a very large number of installations for a failure to occur, either from a bottom-out of the taper or a fatigue crack.
Excessive torque will move the starting point for that installation cycle slightly farther up the taper, which means that the crank will suffer more of that permanent wear than it should have. In today's cartridge-BB world of very infrequent re-installations, this almost never becomes a problem in the first place.
To cause a problematic amount of added walk-up on a single installation from overtorque, you'd have to go really nuts. Like "bust out a huge cheater bar for no reason" nuts. The kind of behavior that will wreck any kind of crank interface.
(What
is a big problem for taper interface wear is when someone misinterprets the walk-up as loosening of the fixing bolt, and repeatedly re-tightens the bolt after every few rides. Each time this is done, it restarts the walk-up cycle and puts a re-installation worth of wear on the interface.)