Hairsprings and Grease Don't Mix

There was grease, that had turned to putty as grease does, packed into the inner most coils of the hair spring.  Meticulous cleaning by hand is the only way to get it out.

If the coils of the hairspring do not expand and contract evenly and smoothly, if they touch at all, then the spring becomes effectively much shorter.  The spring behaves as a pendulum, so a much shorter spring reduces the period of the balance wheel and the watch runs very fast.  Magnetism can do this to a spring too by causing coils to stick to each other when they're close.  Gaining several minutes per hour is not unusual when this is the trouble.

This is an Elgin grade 345, a 12 size, 17 jewel movement.  This example was made about 1920.