die Demontage
dismantling
The process in which a machine, system, or structure is taken apart into its individual components.
Die Demontage des alten Krans dauerte mehrere Tage.
The dismantling of the old crane took several days.

dismantling / takedown
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dismantling
The process in which a machine, system, or structure is taken apart into its individual components.
Die Demontage des alten Krans dauerte mehrere Tage.
The dismantling of the old crane took several days.

'Demontage' (dismantling) is the opposite of 'Montage' (assembly). The prefix 'de-' here means 'away' or 'un-'. This logic of taking apart applies both to technical devices and to a person's reputation.
Nach der Montage der neuen Küche folgte die Demontage der alten.
After the assembly of the new kitchen came the dismantling of the old one.
The noun 'die Demontage' (dismantling) describes the process or result of the action expressed by the verb 'demontieren' (to dismantle).
Der Mechaniker wird den Motor morgen demontieren.
The mechanic will dismantle the engine tomorrow.
You can recognize the meaning from the object: If it is about physical things like machines, furniture, or industrial facilities, the literal meaning is intended. If it is about abstract things like reputation, standing, power, or a myth, the figurative meaning is intended.
Die Demontage des Gerüsts beginnt nächste Woche.
The dismantling of the scaffolding begins next week.
In the figurative meaning, the word is often combined with adjectives such as 'öffentlich' (public) or 'politisch' (political). For example, people speak of the 'öffentliche Demontage eines Stars' (public takedown of a star) or the 'politische Demontage eines Gegners' (political takedown of an opponent).
Die Presse betrieb die systematische Demontage des Ministers.
The press carried out the systematic takedown of the minister.
expansion / removal
'der Ausbau' and 'die Demontage' can both mean that something is removed from a machine, system, or structure. 'der Ausbau' often means taking out a specific component, for example a battery or an engine. 'die Demontage' is usually broader: a machine or structure is taken apart into individual parts. Learners may confuse the words because both have to do with removing and taking things apart. If only one part is being taken out, 'Ausbau' often fits. If the whole thing is being dismantled, 'Demontage' is more precise.

Omar hilft in der Werkstatt bei der Demontage der alten Maschine.
Omar helps in the workshop with dismantling the old machine.
Vor der Reparatur ist der Ausbau der alten Batterie nötig.
Before the repair, the old battery has to be removed.