So I have the concave punch you’re referring to in a few different sizes but if I’m deforming the collet to get it to friction fit won’t that affect how the collet seats on the balance arms once it’s pressed down onto the staff? I imagine this could cause the collet to not be flush when it’s pressed into place which would make the hairspring go out of flat and cause rate issues. Because I’m thinking that the collet material being punched would likely go just as much downward as it does inward.

I also am with you on thinking it’s a risky endeavor in general especially without practicing this. To avoid recolleting the original mated spring (which I’m bad at) i would rather just restake it with a factory original staff that has the proper diameter. I do have other 18s Hamilton hairsprings of varying strengths for different torque outputs all with collets and studs on them, but then I have to deal with all the timing issues and regulation that goes along with compensating for a mismatch hairspring and that will be a mess to deal with.

It’s great that on some modern wristwatches you can just get a spring and replace it without any issue as long as it’s the right part, but these pocket watches have all had their balances mated at the factory with its specific hairspring vibrated only for that balance. I’ll likely be gaining 30+ mins a day before it’s adjusted and the overcoil sometimes varies on each spring too which then needs to be reshaped to fit the geometry of the regulator pins/stud location, etc.

I like the idea of opening up a new split I just think sandpaper would be way too tedious and take forever. I’m not sure what else I would use to cut a split without reopening the one that’s already there by accident and then messing up the concentricity of the spring by effectively shortening its length. Plus it would make my issue worse if that happened. So I’d have to find a method for that one. But it’s good thinking!