What is the difference between the S770 options?

A handy person from Evans, GA says:
April 13, 2011
I want to order two of these balances. I'm asked whether I want option D, C, or B. I can't find any explanation of what the different options are. Can you explain?

Thanks,
Richard P.
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Donna from SWISCO responded:
April 13, 2011
Hello Richard,
The stamp options indicate different strength balances, normally the stamp is indicated on the original balance.

If you cannot locate the stamp on the Series 770 balance you are trying to replace, please remove the window sash and place it on a scale. You will need the weight of the window sash to determine the proper stamp option.

I would be happy to provide the weight ranges for each option, just need the balance length.
A handy person from Evans, GA says:
April 13, 2011
Great, thanks. The balance length is 34". Please advise with weight ranges.

Richard
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Donna from SWISCO responded:
April 13, 2011
Hi Richard,

The 34" Series 770 with stamp option...

34B will carry a window sash weight range of 16 through 22 pounds
34C will carry 22 through 27 lbs
34D will carry 27 through 32 lbs

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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