Originally Posted by
Duragrouch
I'm not sure I understand the break. I'm seeing one half of the master link still on the chain, intact, and the other half loose above it, intact. I see part of a broken link on the chain; Where is the other half of that? If that's on the underside of the pin on the loose link, I still don't see how it all went together, and I use master links. I see two completely intact outer plates of the master link.
Beyond that, not surprising to fail at master link, each pin is only press fit (or welded) into one side so will not stay as straight, and bending force on that pin (instead of pure shear) will bend and fatigue the outer plates. Replace link.
It certainly seems possible the interface between a master link and standard outer plate to inner plate would be different and might be the source of different stresses.
When you say "replace link" are you suggesting replacing the broken link and continuing to use the chain? That's not something I'm going to do. 4,000 miles is plenty out of a $50 chain. Not worth the risk to me of another failure somewhere else in the chain.
Something else I've wondered is if the plate was stressed when the chain was broken by the bike manufacturer (Trek in this case). I'm sure they get the chain in long spools and break it to length. Versus a retail chain cut to length by Shimano.