LV2TNDM

This is why:
"With the understanding that modern chains have plates thinned to the point of having minimal safety margin, we can expect greater instances of breakage owing to variables."

Crankycrank

There is a good probability the OP will never know what caused the break. Some good suggestions have been made but no way to test any of them really. IMO, get a new chain, enjoy the ride and every so often check the chain for peace of mind but probably not necessary. IME plates breaking are extremely rare and usually from installation error but once in a great while a mfr. will have a stinker such as Shimano did years ago with the 6700 Ultegra chain which had a few side plate breakages but the chain was re-designed with a new 6701 model number. Changing the chain at every 3k mile intervals before it's actually measured worn is overkill. If you want to spend the time, do some searches for CN-M8100 chain breakages/failures since if this is a common thing it will show up on the internet.

sweeks

The Shimano gauge (on the left) is positioned incorrectly. The "split" end is supposed to be inserted first, then the other end is observed to see if it fits down all the way into the chain. If it doesn't, the chain is still good. The split end corrects (theoretically) for looseness in the rollers, allowing elongation between the pins to be more accurately assessed.

sweeks

Pursuant to my last entry (and not intending to start a chain-checker war)...


Shimano Chain Checker TL-CN42

Using this tool is a 2-step process...

Step 1 is inserting this end into the chain; it's a "snap fit". This takes up slack in the rollers that might interfere with measurement.


Step 2 involves seeing if this end will drop into the chain. This must be done with almost no pressure (see next image); if it's pushed, the spring at the other end of the tool will allow it to drop in (false reading).


Instructions: Use light pressure ("Open Circle"); Don't force.

mpetry912 

hey @sweeks, thanks buddy, some real information as opposed to speculation, appreciate it !

Mtracer am I to understand that that the link that failed was immediately adjacent to the removeable link, do I see that right from the photo in the original posting ?

if so, that fact may be a clue to the cause of failure. I wonder what your disassembly process is. Do you have a chain plier to set those links so they are properly snapped closed ?

again, the fact that the failed linkis right next to the removeable link may be a clue.

/markp

sweeks

Here's some more on the Shimano chain checker...


Here's the tool in a chain that's approaching the end of its service life. When the tool drops easily into the chain, it's time to replace. Up until then, though, you're pretty much in the dark as with any "Go-No Go" measurement tool.


Here's a Park Tools CC-2 on the same chain. This gives a somewhat more quantitative estimate of chain wear, but may include roller wear. It tends to *overestimate* chain wear a bit.



A close-up of the CC-2 display. This indicates that the chain is nearly worn out. Park Tools told me that this tool is calibrated at 1.0% elongation; it's less accurate at lower wear readings, and actually won't fit into a brand new chain.