This was what heppened to an "NOS" blue steel Mainspring within the first 24 Hours after it was installed. The left view is what you see when removing the barrel cover, just think of the power it released. The safety pinion saved the watch wheel train and the safety Barrel acted to save the watch intact.

O My, that is wicked in a good way. I see the safety features are in place for a reason.
I have had NOS springs break when installing them, but never like that.

Dave, I have had 2 brand new unbreakable alloy mainsprings do the same recently within the first few days of use.

Severe thunderstorm?

Regards, John

Hard to believe until you see it.
Maybe you guys just aren't living right? ha ha

Maybe your spring is still under warranty?!

Seriously, though- would you say its risky ever to use an old NOS mainspring? I think the problem is called stress-corrosion cracking: given enough time, every metal (other than gold, maybe)is chemically unstable when exposed to air, oxygen, chemicals and humidity. Tiny spots of surface corrosion combined with stress can cause catastrophic fracture of the crystalline structure of the metal and -- boing! Do you have a rule of thumb about the age of springs you are willing to install in an antique watch?

Dennis, in concert with Chris Abell's comment above, I have ALSO had "Unbreakable white steel springs" delaminate and fail in their first 24 hours or so. The thunderstorm theory that John refers to was actually proposed by the watch industry in the early 1900's as an observed "reacton" to "Brand New" watch mainsprings failure in the timing rooms where they were being adjusted. Notwithstanding the questionable statistical evidence to that claim, there are many reasons why things break. My wife puts it all in a nutshell; "If it is Man-Made, it WILL break". Personally, I just run a freshly serviced watch a few days to make sure the thing keeps working!

Regarding Dennis' post about warranty, that Elgin ad reminds me of (from the early 70's heady days of Computerdom) the Memorex advert. guaranteeing "error-free" Memory Disks. If you had one with an error, Memorex replaced it free! Neat Gamble.