Replacing window balance springs

A quick learner from Seattle WA says:
Trying to replace a BSI 2490 balance spring. 25" long, 5/8" wide.
Thumbnail of the uploaded file named image.jpg
Thumbnail of the uploaded file named image.jpg
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Emily from SWISCO responded:

Hi there!

It's hard to make out the end fittings for this balance but it looks like they may be non-winged, like our S351-25 or S390-25 . These two balances may look the same at first glance but each has a different bottom fitting size. The Series 351 comes attached with a 1" deep fitting, and the Series 390 comes attached with a 1-1/8" deep fitting. You'll need to measure as shown in the image below to see which is the best match. You will also need to weigh your sash and refer to the chart under the product description on the S380 and S385 store pages to determine the appropriate spring strength option for your balance.

Comparison of the S351 and S390 balance shoes.
Comparison of the S351 and S390 balance shoes.
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