Sash balance for old Milgard garden bay window

A home owner from San Anselmo, California, USA says:
I have an old Milgard garden bay window. Millard doesn't have any information on the window and doesn't have any spiral sash balances for it.

It is a black aluminum frame and the spiral sash balances of both windows are broken. The weight of each of the two windows is about 4.5 pounds (actually 4 pounds, 7.25 ounces). There is one sash balance per window. There was no color coded piece on the sash balance and no manufacturer label or identification of any kind. The diameter of the sash balance is 13/32
spiral balance
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Paul from SWISCO responded:
Before we proceed, what is at the end of the spiral? How does it attach to the window sash?
A handy person from San Anselmo, California, USA says:
The end of the spiral is missing (broken off). I believe there were two holes drilled in the flat end of the spiral. The part that attached the drilled holes to the bottom of the window is also missing, but from marks on the window, I believe it was a right angle piece of metal that was attached to the flat part of the spiral and then to the bottom of the window via the square drive machine screw that holds the bottom of the window to the side of the window.

I have enclosed photos of one of the windows and a detailed photo of the bottom corner of the window with the square drive screw. If there is no stock piece of metal that will attach the bottom of the spiral via the two holes to the to square drive screw on the bottom of the window, I could probably fabricate such a piece and attach it to the two holes in the bottom of the spiral via rivets or vey short machine screws and locking nuts.

As long as the sash balance is the right diameter and length and can hold up the window, I can probably find a way to attach it to the window, which will be better than sticks propping the window open, which is what I have to do now.

Thanks,
Dennis J. Finn
User submitted photos of window hardware.
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Paul from SWISCO responded:
Thank you! For old wood windows like this, I would recommend our S500-24 non tilt window spiral balance, as well as a 17-012 bracket. See what you think.
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