Junghans j97 mainspring

Emmy (https://www.emmywatch.com/db/movement/junghans--j97/) has a mainspring listing and suggests:

GR4915 (slightly thicker, slightly longer): Will increase amplitude... reduce total wind.
GR4906 (Slightly shorter, on a smaller 'barrel') Will reduce total wind only.
GR4907 (slightly shorter) Reduce total wind.
GR4895 (thinner, shorter, smaller barrel) decreased amplitude, reduce total wind
GR4933 (a bit thicker, shorter, smaller barrel) increase amp, reduce total wind.

source: https://www.emmywatch.com/db/mainspring/1_7-0_11-285-9_5-man/junghans--j97/
Note, smaller barrel just means the little aluminum circle it comes on will be smaller, so no functional difference.


IF it were me, I'd go for the GR4915. Cousins has it for ~8 GBP (https://www.cousinsuk.com/product/170mm-height?code=GR4915).

With old pocket watches, you can often tell based on the serial number what model it is, but you've cut off the serial in your photo!

I guessed at the serial number and came up with this which seems right: https://pocketwatchdatabase.com/search/result/omega-watch-co/3381883 (looking at your new photos, I guessed right!).

I don't see any info on model/etc, so you might just have to measure it.

The numbers are sold as:

Height in MM (long ways of the cross section) x thickness in mm (short ways cross section) x total length in mm x barrel diameter (optional).

At that point, you'll need to know what sort of 'end' is on it (the bridle), this page has a picture of what they look like: https://www.cousinsuk.com/category/mainsprings-by-list-watch-pocket



For example, see: https://www.emmywatch.com/db/mainspring/1_7-0_11-285-9_5-man/junghans--j97/

1.70 x 0.110 x 285 x 9.5mm.

Height: You can't go any higher as it probably wont fit in the barrel then. You could possibly go slightly smaller though. Reducing height reduces overall power.

Thickness: You can go either direction a little bit, thicker is increased amplitude, thinner is reduced.

Length: This determines the total wind of the watch. Longer it is(up to a point), the longer the watch will run. Shorter will mean you h ave to wind more often.

Barrel Diameter: This one matters less?? IF you get this too high, you'll not be able to press it in off the little ring, as it'll be too large and you'll need a mainspring winder of some sort.

That said, perhaps someone who has a better idea of how to identify these will show up, you can sometimes do it by keyless works.

You shouldn't adjust them via changing the mainspring, I was pointing out the problems it could cause if you went too high/too low. And yes, increasing the thickness of the mainspring can put additional wear on the gear train, but the REAL problem is that your balance can have too high amplitude and 'knock' on the other side of the pallet fork, which can cause havok/breakage.