Old mainspring without an inner loop

[ifibrin]

I am currently servicing an AS 1012 movement, and the old mainspring in it doesn’t have a loop. Is it supposed to be like that?

[jdm]

Surely not, who had worked on the watch must have discarded the broken piece when the repair was not made.

A broken mainspring is easy to diagnose from the outside because the watch winds effortlessly and forever. You would then start work as a repair, not just servicing. 

[ifibrin]

1 hour ago, jdm said:

Surely not, who had worked on the watch must have discarded the broken piece when the repair was not made.

A broken mainspring is easy to diagnose from the outside because the watch winds effortlessly and forever. You would then start work as a repair, not just servicing. 

There is still a hole in the mainspring to engage the hook on the arbor, just that there is no loop!

[jdm]

1 hour ago, ifibrin said:

There is still a hole in the mainspring to engage the hook on the arbor, just that there is no loop!

The mainspring broke at the point of maximum stress, typically where curving is tighter. If you inspect the edge you could be able to see signs of fracture instead of a manufacturing cut. 

[watchweasol]

I think we can concur what happened is what jdm has said the spring broke at the most stressed point, a previous repairer has removed the spring and re punched the hole but not bothered to reform the coil as it would require the  spring to be annealed and reformed, a cheap repair that would work untill the spring slipped the arbour and would not re engage. a new mainspring is required to complete the job.

[jdm]

29 minutes ago, watchweasol said:

 a previous repairer has removed the spring and re punched the hole but not bothered to reform the coil as it would require the  spring to be annealed and reformed, a cheap repair that would work untill the spring slipped the arbour and would not re engage.

I see what you mean. As soon the mainspring is coiled a little it keeps the hook in the hole. Maybe the recommendation at delivery was not to ever let the watch unwind completely 

[watchweasol]

Probably so, that way the spring would always remain attached. Once released there would be no chance of re attachment other than physically removing the barrel to re engage.  A bad  repair.