thats interesting, didnt know that.
and I have a 8 sp and a 9 sp bike (8 speed bike went years with same chain, so I've really only changed the chain once)

kinda supports the theory of not clicking the quicklink together properly and some movement wearing away at the shape until it couldnt do its thing anymore.

maybe not squeezing it together enough when doing the "pull" to lock it in place? I invariably get a bit of skin stuck in it when I do it properly.

In >200K miles of riding I've only broken chains I installed poorly. Consider finishing pin installation with a vise-grip: that can get it even when it's just a bit off, and makes mis-installation less likely.

have to disagree, but perhaps others that do this a lot more than me (the mechanics who have posted here) can chime in. I find that using my fingers I can be sure and feel that it seats properly and goes in well. I know there is a doohickey that makes it easier, but I change chains so infrequently that I havent bothered getting it.

I'd be concerned that vice grips could force it if it isnt properly positioned, ie squeezed inwards properly before pulling back.

That's not a broken chain... This is a broken chain. http://i167.photobucket.com/albums/u...r/DSCN5286.jpg

This is likely the problem I had that caused the failure. Maybe for some reason I didn't get it back together that time.

I have a $20 pair of reversing snap ring pliers. The "jaws" are thin pins meant for snap rings and circlips. A lever reverses the linkage so that squeezing the handles can move the pins together, or push them apart. The pins are thin enough to fit in the link of a chain. This makes installing and removing quick links easy.

I wrote 'finishing': the pin is already in the outer links fully and one can no longer miss. I've had occasions when I couldn't get the pin inserted perfectly, the smallest turn of the chain-tool that would move the pin would move it too far - perhaps there was something wrong with the feeling in my fingers, but I didn't think so. (I re-used pins: this may have been the problem. It also could have been a failing chain-tool: I've broken a few.) When this is the case a vise-grip can even it out. It now looks as though this guy merely didn't install his special link correctly, a different problem.