I was servicing a vintage Rolex this week when the most peculiar thing happened. I had cleaned and oiled everything and I was making adjustments to the timing. One minute it had a healthy amplitude around 270 degrees but was gaining about 15 seconds a day. I took it off the timing machine adjusted the timing screws and when I put it back on the timing machine it had an amplitude of about 230 degrees.
First, I looked for some lint on the balance or hairspring, or anything else I may have contaminated it with, but nothing. I removed the balance to examine the pivots: good. What could it be? I actually set it aside and went to lunch. When I came back the problem became apparent. I had pulled out the stem, out of habit (because with hacking movements it will stop the balance, which is useful for adjusting the timing nuts/screws, but I didn’t push it back in. This can be a problem because now, the mainspring is turning all the gears in the going train and the motion works, the escapement and the balance, but also the setting wheel, the sliding pinion and the stem & crown. As it turns out these parts don’t turn very efficiently in most watches and this will cause a great drag on the movement.
Moral of the story: if you notice a large drop in amplitude, you just might make sure the setting train isn’t engaged before worrying about any damage you may have caused or problem you may have overlooked. This is a very common problem when working on vintage pocket watches, by the way, not so much with modern wristwatches, because most of them have hack levers.
That’s my story and I’m sticking to it.