What is this part of the mainspring?

[0812]

I opened up a mainspring barrel, unwound the MS and noticed this little piece fall out. I am not sure if this a piece that broke off from the MS, or if it's supposed to be a separate part. Any idea? In the picture that I took before unwinding it appears that it sits where I circled it in the picture. Thanks a lot!

 

 

[HectorLooi]

That is the bridle from an older type manual wind mainspring. It is also a common method of repairing a broken mainspring. The broken end is squared up, annealed and bent into a loop to hold a short length of bridle.

[0812]

8 hours ago, HectorLooi said:

That is the bridle from an older type manual wind mainspring. It is also a common method of repairing a broken mainspring. The broken end is squared up, annealed and bent into a loop to hold a short length of bridle.

Thank you. Do you recommend I try to put it back in or should I get a new mainspring instead? Then a follow up question would be, how do I determine which MS to use. 

[HectorLooi]

That would work fine. Just clean up the old spring and put it back the way you found it.

[haratua]

On 4/2/2023 at 1:06 AM, 0812 said:

Thank you. Do you recommend I try to put it back in or should I get a new mainspring instead? Then a follow up question would be, how do I determine which MS to use. 

Mainspring comes in a number of bridle (for example see charts). 
Then, either it’s for automatic or non-automatic.

After that, it’s by its measurements: height x thickness x length

And if you’re servicing an old watch that may not necessarily have the right mainspring fitted, then measuring it may not be useful to you. If that’s the case there’s a method you could use to determine the mainspring size; Alex explains it here in this video: