Certainteed Pivot Shoe dilemma.

A quick learner from Blaine says:
I have Certainteed DBL Hung Vinyl, sash is 18x24" installed 2003. One pivot shoe is white w/ s170/335 coil and the other is light gray plastic w/336 coil. The installer had a habit of breaking stuff so I'm not surprised one is a replacement. W/O having to remove another pivot and coil I'm wondering which to reorder, as I'm getting extra as spares. Are the lengths the same? Is it just a weight thing? Are 336 a better option? Looks like old style 15-155 pivot and H65 32/3M is stamped on the other thing that holds the spool and coil.
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Paul from SWISCO responded:
The coil balance springs are the same length, but the strengths are different. The S170-335 can carry 5 pounds each while the S170-336 can carry 6 pounds each. You would add these strengths together for each spring in the window to determine how heavy a sash they are meant to carry.
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