Originally Posted by
Bike Gremlin
Some MTB friends had their cranks working themselves loose without that plastic thingy.
I wouldn't bet my life on whether the (left) crank pinch bolts also prevent the plastic nut from unscrewing, but it does seem to hold the left crank in place and prevents it from gradually sliding off.
I agree about the bearings - Hollowtech II BBs don't have a real option for setting the bearing preload, I too don't think that cranks tightening "thingie" makes any difference there.
There is no way I'm going to believe that plastic part has the strength to hold on a crankarm. If the crankarms fell off, someone simply installed them wrong. Probably ignoring the indications of the stopper plate. Even if one where to get some made in metal, that'd be a poor fix for otherwise sloppy installation. It wouldn't solve a wobbly crank arm, it'd just let it stay on longer so the person ignorant of what is happening can completely ruin the crank arm and end of the spindle.
There was recall for a certain model Specialized bike recently for crank arms falling off. Don't remember it this was specifically Shimano cranks that were involved. But the failure was more the bike manufacturers fault than the crank arm itself... I think.