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Which main spring to buy?


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I looked at some webshops but there are so many different main springs to choose from and I have no idea what all the numbers and abbreviatons mean e.g. strength I was hoping someone could help me out.

I'm working on a vintage watch which is over 40 years old and belonged to my grandfather. I can't pin point the movement based on the markings (maybe someone else can?). It's a 21j hand wind movement which looks and disassembles simular to an ETA 6497. It's only smaler and I don't think it's Swiss. The main spring meassured a little more than 30 cm in lengt and aprox. is 1mm wide. The barrel has an inner diameter of aprox. 4mm.

I'm scared if I buy the wrong one the strength might damage the gear train.....

So if you have any tips ore suggestions, they are very welcome :)

Movement21j.jpg

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Measure the mainspring dimensions, length, width and thickness with digital calipers. You need 1/100 mm accuracy.

Any good supply house should be able to match those dimensions to the Dennison tables and provide a replacement spring. Mention the speed of the movement as well and this helps with selection, ie 18,000 BPH, 19,800 BPH etc...

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Measure the mainspring dimensions, length, width and thickness with digital calipers. You need 1/100 mm accuracy.

Any good supply house should be able to match those dimensions to the Dennison tables and provide a replacement spring. Mention the speed of the movement as well and this helps with selection, ie 18,000 BPH, 19,800 BPH etc...

Thanks Zig! For Dennison I need to measure strength. Also the original spring was broken and disformed so measuring it opposes a difficulty <_<

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