Zenith

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I hope this gets fixed for you soon. I haven't seen that kind of breakage before.

... it's an older watch and I doubt the manufacturer will fix it for free.

Unless something dramatic happened - like the watch fell to a concrete floor from 6 feet (or something like that) - Zenith should fix this for free. Inform them that a steel chronograph part like that should never break simply from normal use. I once had ...  

Something wasn’t right regarding the clearance, and over time, with usage, it’s suffered material fatigue. There’ll be a fair bit of material debris in there, and it’ll accelerate wear on pivots, etc – so don’t even be tempted to continue wearing it. ...