Mainspring arbour won't catch

[p2n]

Been trying to fit the mains spring on my Elgin B&B. I broke the last one.

I've hand fitted the spring but the arbour won't catch. A close look shows this seems to be because the hole in the end of the spring is further back on the new spring than the old one.

All these photos are the same scale. The red lines indicate the problem measurement - the old spring and the arbour match, but the new spring is bigger.

I can't change the new spring so is it acceptable to file the arbour to make that space a little longer?

[rogart63]

Should work i imagine. But what to do if it doesn't work? Can you find a new arbor? That is how i usually think when i  modify parts? Can i find a new or used parts i go ahead and learn something on the way. 

Edited June 2, 2018 by rogart63

[p2n]

I did think of that but I've still not found a good source of parts for these old pocket watches - I've only been watch bothering for 3 months.

I'm guessing there was no such thing as standardized sizes back then?

[rogart63]

Could you remove some from the end of the mainspring. If you could reach down there with a file without damage anything else? A thick paper behind and file enough material to get the mainspring down in the groove. Can show you tomorrow what i mean. 

[clockboy]

I would adjust the spring by filing. Just use the old spring as your template.

[p2n]

I was just concerned I'd weaken the spring - but I suppose if the original was like that...

Ok - will try and file the mainspring end.

[clockboy]

It's just the basics of horology you always just adjust/repair the faulty part. By altering the arbour (if it works) it will only work with that specific spring. 

[praezis]

52 minutes ago, clockboy said:

It's just the basics of horology you always just adjust/repair the faulty part. By altering the arbour (if it works) it will only work with that specific spring. 

Thank you! It is important nowadays to repeat this basic rule now and then.

Frank

[p2n]

Makes sense.

[p2n]

12 hours ago, rogart63 said:

Can show you tomorrow what i mean. 

Any tips would be welcome. My needle file seems huge compared to the size of the spring.

[Tmuir]

You may have trouble filing it with a regular file being the spring will be hardened. Have a look at getting yourself a small diamond file. I've bought a couple and really do love them, but like most things the more you spend the better it will be. The $2 diamond files aren't really worth the time

[rodabod]

Mainsprings are not fully hardened at the end; they are annealed. That’s why you can easily modify their profile to match the curve of the arbor easily. Either file or punch it. If you file it, make sure your file has suitable safe-edges, and modify the file if need be.