enlarging inner coil of mainspring

[Fleslie]

Is there a proper way to slightly enlarge the inner coil to fit the arbor. I cannot get the arbor edge in to get it started. However I do not want to damage the mainspring while trying to further open up the inner coil in order to place the arbor.

[oldhippy]

Is it a watch m/spring? if so is it new? are you sure you ordered the right one? New watch m/springs do not like to be tampered with, opening it up to fit the arbor it might break or split. The best way is to use round nose pliers, but be very careful. 

[mikepilk]

As @oldhippy says, round nose pliers are the way to do it. But, I have broken many springs trying to change the inner coil. Sometimes you cannot get the correct size, and you have no option but to tweak it. Opening out is easier than reducing the inner coil diameter. I have an Eterna where the inner coil is MUCH smaller than you can get with a standard spring. I broke two springs before finally getting it right.

[nickelsilver]

An inner coil that is too tight is a much better situation than one that is too large. If you grab the arbor with a pin vice, on the cap end, you can twist it into the inner coil, turn the opposite way that you would to wind the spring, give a little tilt to get it to start; it will cause the inner coil to open and accept the arbor. No need to actually bend anything- when you remove the arbor (same technique) the inner coil will probably be its original size.

 

When it's too big then you need to actually deform the metal to change its shape (with convex-concave pliers or round nose), and this is always tricky to do without breaking the spring.

[praezis]

There are springs that have a much narrower inner diameter than the traditional 1/3 barrel. There even your trick will not work.

I open them with flat nose pliers and gentle pressing, similar to hairspring bending. But the noses must have straight, not tapered edges - else the coil will leave its level. At least on Inox springs I had no breaking-issues.

Frank

[Fleslie]

I was able to use the pen vice a gently rotate the arbor into place. Thanks to all for the recommendations.