540 vs. 640

A handy person from Michigan says:
June 21, 2010
My window has a S190-540 and a S190-640 on the same shoe. Can I just replace them both with two 640s? What is the difference and effect of a 540 vs. a 640?
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Paul Sr. from SWISCO responded:
June 21, 2010
The window manufacturer is balancing out the weight of the window with one heavy and one lighter spring. You should replace exactly what you have. The 640 is stronger than the 540.
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