As J.Peter alluded to in his post on mainsprings, replacements for many older mainsprings are no longer being produced and the reserve stock available through parts suppliers is beginning to wane thin.
I happened to have just such a mainspring, for an old chronograph, come across my bench several days ago for which I was unable to find a replacement. The hook that locks into the wall of the barrel had broken off and a new hook had to be fabricated. Using the steps described by Donald DeCarle in chapter 8 of his book, Practical Watch Repairing,that illustrates the best method of repair for this particular type of mainspring, I was able to successfully proceed with the repair and complete the work with a minimal loss of time.
To begin, I first annealed the original hooked end of the mainspring for a length of about 10mm. I then introduced a slight bend into the metal using a broad, stout pair of tweezers. The tweezers that I used were actually a pair of old number 5 tweezers, cut short and modified to create a number 8 style of tweezer that was well suited to the task. I continued the bend, folding the metal over into a gradual “C” shape, heating in the mean time as I felt necessary in order to relieve stresses in the steel and keep it malleable. Once a suitable “C” shape was formed, I then pinched this form into a more acute bend using the same modified pair of tweezers. In order to keep from pinching to far, DeCarle recommends inserting a piece of old mainspring of the desired height into the bend as a safety. I simply used the old hook that had broken off and employed this piece, as well, to form the new hook by firmly clamping the newly formed bend down onto it. Altogether the repair of the mainspring only took about 10 minutes or so.
Here is the finished result:
To add your own insight or bits of wisdom to this method of repair, feel free to edit this article on repairing a mainspring hook that I created on Alliance Horlogère.
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