What each symptom actually points to
A key that turns without resistance means the cylinder no longer drives the mechanism: a broken cam, a linkage that has come apart inside the case. It is a clean failure, and it usually leaves the door shut on one of those narrow Demi-Lune landings where very little gear can be set down. The job is planned before going up.
A key you have to hunt for is a different signal. In the houses of Le Bourg, cylinders take in dust from streets with no pavement and the damp that lingers in the back courtyards; elsewhere it is a poorly cut spare, copied in a hurry, that wears the barrel out early. The symptom settles in slowly, which is why it tends to be noticed too late.
Having to lift or push the door to lock it does not implicate the lock. On the sloping plots of Alaï and Le Méginand, a few millimetres of settlement are enough to shift the keep; on the solid timber doors of Le Bourg, sharp winters and dry summers do the same work in the other direction. Either way it is the door that is put right, not the mechanism.
Repair, replace, and two things never to do
A single part can be changed: a spring, a cam, a handle spindle, a roller, a keep to reposition, rods to reset. On a cased multipoint in a Demi-Lune flat the faceplate is never removed blind, because the rods are under load and refitting them means setting them accurately again. Those parts remain findable for as long as the model is still in production.
Replacement is the right call when the case is distorted, when parts fail one after another, or when the model has been discontinued. Before anything is ordered the door is adjusted: in Le Bourg as at Vaubonne, hanging the leaf square in its frame cures more faults than a new mechanism does. A new lock on a door that sits crooked wears out exactly like the last one.
Two habits make everything worse. Forcing a key that resists, which snaps cleanly inside the barrel. And flooding the lock with general-purpose penetrating oil, which leaves a film that traps dust and turns light fouling into a seizure. A dry lubricant once a year, plus a check on the faceplate and keep screws, keeps a Tassin door working.
How does this service work?
Techniques, typical situations, standards and things to watch out for: it is all on our locksmith repairs page. This page focuses on what the service involves in Tassin-la-Demi-Lune.