Replacement mainsprings are categorized by three main factors:
A: Height or width in mm.
B: Strength or thickness 1/100mm.
C: Diameter of barrel in mm.
Matching the length is a risky way to choose a replacement. It's common for springs to break near the tail and an experienced repairer can make such a good job of forming a new hole further up the spring that it can be almost impossible to tell. If someone has removed a 'Day's Worth' of material you will not want an identical replacement. The clock that stopped on day 6 will still stop on day 6.
Always look carefully at an old mainspring before ordering a new one. The range of replacement springs has increased enormously over the past 20 years. It could be the original has been replaced before, using the nearest that could be obtained at the time. During two world wars springs were hard to obtain. A broken end was often discarded and a new eye made in the remainder.
The length of a spring can be calculated as follows:
The spring should occupy 1/3 to 1/2 of the free space inside a barrel. This calculation assumes 1/2. 80% of the length calculated will be sufficient for the clock to run.
Length = (((3.142 x (D/2) x (D/2)-(3.142 x (d/2)* (d/2)))/2)/t
Where D=Internal barrel diameter. d=arbor diameter and t=thickness.
If the nearest spring you can obtain is too long, you can shorten it. Heat the point to shorten it at to red heat in a flame. This will soften it. When cool, simply cut of the surplus and fashion the end and the eye to match the original. Heating the right amount of material to the correct temperature does require some expertise and should only be done when absolutely necessary. If not softened enough, it may fracture. If too soft it will tear. Always finish off the curves of the eye well, as a hairline fracture here will eventually tear.
In the same manner, you can make a new end on an old spring that has broken off at the outer end. Experience shows that once a spring reaches the stage when it breaks in use, it may do the same again. As a mainspring breakage can cause so much damage, it is not good practice to re-make the ends unless a replacement is unavailable.
Repairing Your Own Clocks by Mervyn Passmore