Paul from SWISCO responded:
September 6, 2011
If your window sash requires a coil balance where the springs are doubled up on each side of the window sash you would use both the S171 and S170 springs. The series 171 has a longer tail, so it would be used on the top and the shorter S170 would be used on the bottom. The spring on top needs a longer tail to reach pivot shoe.
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