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Thanks for writing in! You coil balance shoe resembles the 15-180. This shoe is compatible with the Series 190 Coil Balance, attached to the shoe using the 33-073 screw. The strongest spring we have is the S190-940, which is a 9lb spring; this will support up to 18lbs but I do believe it should be fine in your window.

To replace the support, you'll need the 15-186 Coil Balance Clip and the 15-182 Balance Bushing.

Alternate image for 15-180

It would be best to replace the balances on both sides to ensure that there is an equal amount of tension. Each piece is sold individually.