Spiral balance for old wood window.

A home owner from Brookfield, IL says:
Have two wooden sash windows with broken old spiral balances. Would like to replace. Windows have a routed channel 5/8" on both sides. Windows are 24" high approx. 8 lbs ea.
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Tom from SWISCO responded:
Could you use our Series 500 spiral balance? This has been used on wood windows for decades. You would also need a bracket like our 17-012 or 17-007.
A home owner from Brookfield, IL says:
here are some shots.Would like to replace old spiral balances on two no tilt windows 28x24high. 8lbs ea. Old spiral balance 5/8"slot-over all length 22.5" less the mount pins 21.5. Please suggest size, bracket
User submitted photo of their window balance.
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Paul from SWISCO responded:
Wow, these are very old balances. Your glass height, based on your photos, looks like 20". The formula we used to calculate length of Series 500 balances for wood sashes is simple: we simple add 1" to the visible glass height for the top sash, and 3" for the bottom sash. So, for your top sash, I recommend the S500-21 and for the bottom sash, I recommend the S500-23. Plese confirm! I also recommend a Red Tip for both, which can hold an 8 pound sash. For the balance bracket, I like to use our 17-012 on the bottom of the sash and secure it with two screws.
A quick learner says:
I'm only replacing the bottoms. The window (glass) potion is 20" (including the stile ) the actual window measures 23.75 inside front 24 backside.
Bottom is beveled. Still go with a S500-23. Thanks
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