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Ultegra crankset SG-X 53B 53/39 with rear 27/12 10 speed Shimano 105 cassette. Chainstay length 41 cm.

Road bicycle had KMC X10 with 55 links. Measuring biggest cog in front with biggest one in rear the correct length is 54. So I removed one link. After test drive chain broke by hitting front derailleur: pin of one of the links just cracked open. Kinda confused. Is it the chain itself or miscalculation of correct length.

54 link chain as in big/big enter image description here

54 link chain as in small/small enter image description here

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    If the derailleur was taking up the slack in small/small, there's no need to shorten the chain.
    – Noise
    Jun 8, 2022 at 10:59
  • @JoeK Can you please elaborate on "taking up the slack". English is not my native tongue...
    – 01e5Dk
    Jun 8, 2022 at 11:08
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    How did you rejoin the chain? With a new pin, reuse an old pin, or a quick/master link ?
    – Criggie
    Jun 8, 2022 at 11:18
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    I don't have an answer for the chain, but your rear bottle cage is upside down
    – Andy P
    Jun 8, 2022 at 15:09
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    @01e5Dk It was not the masterlink that broke. Stuff happens. Could have been a manufacturing defect. KMC chains are generally fine. Maybe you got a counterfeit - where did you get your chain? Jun 8, 2022 at 20:34

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I suspect you'd somehow not rejoined the chain completely. I had a quicklink fail this morning after removing that chain for waxxing. Likely I hadn't clicked both sides home completely.

In the past I've had lower-speed chains where pin pushing and reinserting is the accepted way to open and close a chain. It is possible for a side plate to be poorly retained, because a pin is pushed too far in or not far enough. This is unlikely to be the cause, because your chain is 10 speed which is too thin for pin reuse. Instead you have to use a new "replaceable pin" every time, or use a quick-link, which look something like this:

enter image description here

You say it broke away from a master link - so the other option is a weakness in the chain. If it was new, could have been a manufacturing tolerance. If the chain was used it could have suffered damage in an accident or derailleur crash.

I had a chain once which got munched in the front disk brake (yes, really) and there was a 30 degree rotation in the chain for about 15 links. It was impossible to see normally, but gave strange clanking only under load, and occasional poor shifting.

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    I know some of this answer doesn't apply now you clarified it was a master link and the chain broke elsewhere, but I'd already written it and this could help a future reader.
    – Criggie
    Jun 8, 2022 at 11:27

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