How do you figure out the correct height of a mainspring if the one currently in the watch is incorrect

[GregG]

I have a watch where the mainspring in it is definitely not the original.  There is so much vertical play in the arbor, and you can feel the mainspring knock against the top and bottom of the barrel if you grab and shift the arbor.

I was thinking maybe shove a piece of Rodico in the barrel, and close it.  Then open it, measure it, and subtract maybe 0.1 or 0.2mm to give it some headroom.

[lexacat]

I'd suggest you could just measure the barrel wall height and subtract about 0.1mm to allow for some headroom. Assuming the lid doesn't protrude into the barrel when fitted. If it does, subtract that amount (or a good estimate of it) from the total also.

[Klassiker]

I agree with lexacat. You will get much more accurate measurements directly from the spring and barrel than from a piece of Rodico. If you can identify the movement, there are lists of suitable springs. Also, the arbor should not have vertical play, even when the spring is missing.

[GregG]

I remembered my calipers have a depth gauge, so I used that.  The distance from the very top of the barrel wall to the floor of the barrel is 2.14mm.  The vertical dotted lines are the inner shimmed wall of the barrel that the cap fits onto.  The horizontal dotted line is the height of the wall.

The edge of the barrel cap is 0.35mm.  The center of the barrel where the arbor fits into is 0.55mm.  Otherwise, the thickness of the barrel is 0.25mm.  Which measurement should I use to calculate the height I need?  I'm thinking that since the mainspring doesn't actually interact with the arbor pivot or outside wall, I would use 0.25mm, making the height ~1.9mm (minus some amount of tolerance).

The original replacement I purchased was a GR3355 (1.3x0.14x320x11), but it looks like I can use a GR5322 (1.8x0.14x340x11).

Edited January 29, 2023 by GregG

[Klassiker]

1 hour ago, GregG said:

it looks like I can use a GR5322

It looks like that spring will fit in the barrel, but what makes you think it's the right spring material thickness and length for your movement?

[GregG]

7 minutes ago, Klassiker said:

It looks like that spring will fit in the barrel, but what makes you think it's the right spring material thickness and length for your movement?

Ultimately, it is a guess, but it's best to have an educated guess.

EDIT: Just realized you had actually given my post a thumbs up back when I posted it.

 

Edited January 29, 2023 by GregG