Certainteed coil balance assembly replacement

A home owner from 20653 says:
February 13, 2022
I need assistance in finding the right part for the coil balance assembly to a Certainteed window. The sash no longer stays up when the window is open lifting the sash. The each coil has a number 6 written on it and it is and half inch wide and the sash weight about 20.2 lbs. I am not sure which parts to order.
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Casey from SWISCO responded:
February 15, 2022

Hello,

Your pivot shoe resembles the 15-192 made by Certainteed, which uses the S170 and S171 springs. 

The 6 stamped on your springs would normally lead me to believe that you would need the S170-406 and S171-406 coils, but that would support more than 20 pounds. If you're confident in your sash weight, you may be better off using one S170-405 and S171-405 on each side to support exactly 20 pounds. Thanks for posting. 
CertainTeed Pivot Shoe
CertainTeed Pivot Shoe
15-192
$6.39
406 Series 171 Coiled Balance Spring, Long
406 Series 171 Coiled Balance Spring, Long
S171-406
$6.55
406 Series 170 Coiled Balance Spring
406 Series 170 Coiled Balance Spring
S170-406
$6.52
405 Series 170 Coiled Balance Spring
405 Series 170 Coiled Balance Spring
S170-405
$6.52
405 Series 171 Coiled Balance Spring, Long
405 Series 171 Coiled Balance Spring, Long
S171-405
$6.55
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