Balance spring

A quick learner from Ashford CT says:
This spring broke on the left land side of a window I had installed by "Window World of Connecticut a cuple years ago. I am unable to locate the part locally.

In one of the pictures there is a very small etching on the white plastic that .reads "13". This part appears to be a single spring and it broke in the middle By my count it appears there a 6 winds in one haf of the broken part and 7 winds in the other half (matching the number 13 on the part. There are no other exterior markings..
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Bob from SWISCO responded:

See our 15-177. This pivot shoe is compatible with our Series 152 Coil Springs. I'm afraid that the stamp number on the plastic housing isn't particularly reliable. The only stamp numbers that matter would be on the coil spring itself. If you can't find any springs, or are uncertain, give us the exact weight of your window sash, and we'll work out what you need.

A quick learner from Ashford CT says:
Sash is exactly 17.0 lbs Bob.
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Bob from SWISCO responded:

Thanks for providing the weight. In that case, I'd go with the S152-9 9 lb coil spring, one for each side of the sash. This equals 18 total lbs of support--being 1 lb over the sash weight shouldn't make a difference. Now this means I think it would be wise to replace the coil springs on either side of the sash--if we don't know what the spring weights are on your old coils, we don't want to risk having the weight support on each side of the sash being uneven, as this can be harm the window's function. Let me know if you have any other questions.

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