Which S770 option do I need for 3500 bsi lg?

A home owner from Matthews, NC says:
I need to replace four balances and they look exactly the same as the S770 series. The only thing that is writen on my original balances is 3500 bsi lg. I know i need 35" S770. I only need to figure out which one i need B,C or D. Can you please help?

Thank you,

Profile picture of Donna
Donna from SWISCO responded:
Hi Dino,
The letters indicate different balance strengths. To figure out the needed stamp option, please remove the window sash and place it on a scale to get the weight in pounds.

35" Series 770 balance with stamp option...

35B will carry a window sash weight range of 12 through and including 22 pounds

35C will carry 22 through and including 27 pounds

35D will carry 27 through and including 32 pounds

This information is based on 2 balances per sash. Balances are designed assuming friction in the jamb weather-strip will carry a portion of the sash weight. Suggested sash weights are approximate.
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