The one rule that is not a myth: never reset the chronograph while it is running. The reset hammers slam the heart-piece cams to snap the hands to zero — doing that with the train engaged can damage the hour hammer, shear screws, or knock hands out of alignment. Always: stop, then reset. (The cal. 9904 Moonphase physically blocks the reset pusher while running; the 1861/3861 family does not — discipline is on you.)
Can I leave it running?
- Tolerable, not ideal. The 1861/3861 use horizontal coupling: with the chrono running, the coupling wheels stay engaged with added friction.
- Two costs: extra wear on the chrono works, and a noticeably shorter power reserve — owners report roughly 39+ hours with the chrono running versus 48 stopped.
- Running it to time things is what it is for. Running it 24/7 as a sweep-seconds hand just burns reserve and adds wear for no benefit.
- Parking is harmless: leaving the chrono hand stopped at 6 o'clock for looks, or stopped-but-unreset, does no damage. (Minor caveat: a sharp shock can make unreset hands jump.)
Pushers: technique matters
- Reset with a quick, firm, sharp press — then stop. Don't lean into the pusher with heavy force: the hammer can distort the hole in the brass main plate where the hour-recorder wheel post sits, causing permanent hand-misalignment.
- Don't repeatedly re-reset just to keep the hour hand parked at 12. That wears the hour-recorder cam — a part watchmakers replace "quite often" for exactly this reason.
- Start and stop can be pressed with normal, confident pressure. It is the reset-while-running and the gorilla-press reset that cause the damage.