Would you have replaced this Omega mainspring?

[mikepilk]

When I buy a watch and clean it, I almost always fit a new mainspring. But with this Omega 1012 the spring looked good to me, so I didn't bother.

I ended up taking this watch apart three times trying to discover why I couldn't get the amplitude above 220 deg. 

All the pivots and jewels looked good under the microscope. I was stumped. I'd run out of things to try, so decided to fit a new mainspring

.... and now I'm getting about 270 deg amplitude.

Apart from the shape, are there any ways to tell if the steel has lost its elasticity ?

 

 

[oli]

Sorry I cannot answer your question directly, but I thought I would share my experience as it could be relevant.

Very much like you, I once worked on a watch which I could not get to run properly. I spent ages looking for the causes, no obvious issue with escapement, pivots, jewels, train was nice and free etc... disassembled, cleaned again, re lubricated, no more luck. After I changed the mainspring the watch worked perfectly. It turned out that the wrong mainspring had previously been fitted (slightly greater width) hence the power was not transmitting properly. 

It might be worth checking this - never take it for granted that a watch has the correct parts before you start working on it!

It may not be an elasticity issue after all!

 

[StuartBaker104]

The only thing I see is that it doesn't appear to lie flat on the desk, which could cause it to rub on the barrel cap / base

 

[rodabod]

Remember thta for automatic mainsprings, the bridle shape is important as it defines how much tension can be wound into the spring. You should get somewhere in the region of 5 turns of the barrel before slipping occurs.

[mikepilk]

Good point about the bridle. I'd like to see what a fully wound automatic spring looks like in the barrel.