Identifying replacement Mainsprings for Pocket Watches

[ama269]

Hello!

I’ve done some searching and can’t seem to find what I’m looking for to answer my question.

Finding replacement mainsprings seems pretty straightforward to me except for identifying the mainspring strength.

I’m working on a Hampden 18s pocket watch (full plate, 17j) and know I need 21 & 3/4 inch 2.9mm T mainspring but am stuck at the strength. There are two offered: .21mm and .23mm.

Any advice?
Art

[stevefitzwater]

Strength is thickness of the mainspring

if your not sure what thickness you need, normally the thicker springs go on lower quality movements (i.e. lower jeweled movements, as there is more resistance)

[ama269]

Thanks Steve. If I don’t have a caliper to measure the thickness, is there another way to go about identifying?

To be honest- I don’t know if 17j is a higher or lower amount of jewels.

[stevefitzwater]

a 17j watch is considered a higher quality, with 7 jewel being considered lower, the jewel surfaces give less resistance to the parts and thus require less energy to move..

[ama269]

Well I just ended up getting a caliper.

Was a bit difficult to get completely accurate down to such a small measurement, however I think it’s .23mm.

[Author]

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