4 digit number on channel balance

A home owner from Cincinnati, Ohio says:
What if my guide does not have the 4 digit number? What is the rule-of-thumb for choosing the 4 digit to order by? I am ordering the S390-17
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Donna from SWISCO responded:
If you do not have a 4 digit number stamped on your 17" Series 390 window balance, please remove the window sash and place it on a scale. You will need the weight of the window sash in pounds to determine the proper stamp option, the stamp numbers indicate the weight tolerance.

1610 will carry a window sash weight range of 5 through 8 pounds.
1620 will carry a window sash weight range of 8 through 12 pounds.
1630 will carry a window sash weight range of 12 through 14 pounds.
1640 will carry a window sash weight range of 14 through 19 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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