Difference between the S170 and S171 springs

A quick learner from Havertown, PA says:
On the site I was looking for a #407 Spring replacement for a certainteed window.
The broken one I have has the number 407 stamped on the end.
On you site, the 407 spring comes up with two versions (S170-407 & S171-407), same number, virtually same description, but different part numbers. Could you please explain what the difference is as I want to make sure that I order the correct part.
Thank you for your help
Roger D.
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Paul from SWISCO responded:
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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