Meshing old with new gears

Meshing brand new gears with worn and used ones is not recommended. Of course everything depends on the application, load being carried and the amount of wear already sustained by the given component.  Gears in a particular mesh create an unique wear-in pattern after time. This is strictly associated with the housing tolerance stack-up, centerline alignments, deflection under load, individual gear lead and involute quality and the tooth surface hardness. As the gears mesh under load, tooth profiles and leads change wearing each other in. This process starts at the high points of contact  on the tooth surface (high asperities) and spreads along the tooth width on both members. Over time a custom pinion-gear pair is created.