I have a number of 50s era U.S. Time watches with #21 movements, all of which have been cleaned and relubricated. The problem I have with several of these is that they keep terrific time for the first 12 or so hours after full winding, but then rapidly loose time after that. The movements may loose as much as 5 minutes during the next 12 hours after winding. I suspect the problem relates to the mainsprings, which I believe are all original. I assume that the springs have lost much of their spring over the years. Does anyone have a different idea on the cause of this problem? Also, if the problem is the mainsprings, does anyone know if there are non o.e.m. replacement springs that will fit? While I have no problem getting these movements apart and back together, fitting a main spring into the barrel is beyond my ability. But I would be willing to send out a set of main barrel assemblies for new springs if it could be done and if I could find somebody willing to do that.