window won't stay open

A quick learner from Labadieville, LA says:
November 10, 2017
I have what appears to be a 3/8"series 900 spiral balance for my windows. The windows are unbranded and over 20yrs old and the business where they were purchased no longer exists. A google search window parts brought me to your site, lucky for me!! I have measured the length of the balances to be 28 1/2" long. You offer the 27" and the 28" models. I am thinking that the 29" model would probably be the one to get as I am thinking it is better to be longer than shorter. If this is the correct replacement please let me know, if not please offer some guidance on the proper replacement. Thank you.
Sincerely and Respectfully,
Brian
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Paul from SWISCO responded:
November 14, 2017
I would go with a longer length, the S900-29, which was your suspicion. I pulled up some media guides on the right to further help you.
Spiral Balance Charging Tool for Tilt Windows
Spiral Balance Charging Tool for Tilt Windows
75-006
$9.97
29" Series 900 Spiral Balance
29" Series 900 Spiral Balance
S900-29
$9.15
How to replace a tilt window spiral balance.
How to replace a tilt window spiral balance.
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How to Identify a Replacement Spiral Balance
How to Identify a Replacement Spiral Balance
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How to adjust a spiral balance on a tilt window
How to adjust a spiral balance on a tilt window
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How to choose the proper spiral balance tip color
How to choose the proper spiral balance tip color
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