I was just wondering just what the average run time can be expected on a "Good" or "new" mainspring. I have not always replaced the mainspring on a watch I get if it will run and keep time for at least 24 hours. What do you think?
I was going to say 24 hours but. How about the watch should keep reasonable time over at least 24 hours. That means it has to run past 24 hours. But a lot of this depends on the particular watch. Usually for better timekeeping because timekeeping is related to amplitude a longer run time is desired.
I have only worked on Elgin 12's and 16's as a hobbyist. However after servicing with NOS blue steel springs the run times have been between 31 and 32 hours. Each watch that I finish, I will use a timing machine to check the general health of the watch and the quality of my work. Then each one is checked in six positions for 24hr to monitor the fluctuation during the day. I have noticed that some will be as much as a minute off somewhere in the middle, but only seconds off from it's start point at the 24hr mark. Others have little fluctuation throughout the day.
I have about 40 South Bend watches and they run anywhere from 30 to 42 hours on a wind. Some size springs are fairly scarce so I don't always change mainsprings either when I clean a watch. I do always remove the barrel cover when cleaning. Visually inspect the spring and if it appears clean, no corrosion, no "crimps" or other deformities I leave it alone. Of coarse if the watch didn't run before cleaning you won't know how long it might run. However if after reassembly it becomes obvious that there is a spring problem, it is not too difficult to replace the spring later.