window spiral balance

A contractor from new hope Pa says:
Hello I'm a contractor and will like to retrofit an old single hung wood window with spiral spring balances. The window measures 38" wide by 70" tall. The top wood sash is fix and circular at top, and the bottom operates part way until it coincides with curve of top.

I would like to remove old bottom sash and install the 5/8"spiral balance. the bottom sash measures 33inches from bottom rail to top rail.

What type and length of balances should I get?
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Paul from SWISCO responded:
Seeing how the top sash curves and the bottom sash only goes up partially, I recommend our S500-32 spiral balance with a 17-007 or 17-012 bottom bracket. The weight of the bottom sash will determine what color bearing you will use for its balances. Two S500-32 balances with Red Tips can support a window sash weight range of 7 through 29 pounds, which may be enough for you. You might want to double check just in case.
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