window balancer #'s

A contractor from Jacksonville FL says:
What is the difference between the numbers 2810 - 2840 for the S351-29? Is there different spring tensions?
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Donna from SWISCO responded:
Yes, the stamp numbers indicate different strengths. It is best to match up to the original stamp but if you cannot read it off the original remove the window sash and place it on a scale. We need the weight of the window sash in pounds to identify the proper stamp.

29" Series 351
Stamp Option 2810 will carry a window sash weight range of 7 through 12 pounds.
Stamp Option 2820 will carry a window sash weight range of 12 through 16 pounds.
Stamp Option 2830 will carry a window sash weight range of 16 through 23 pounds.
Stamp Option 2840 will carry a window sash weight range of 23 through 28 pounds.
Stamp Option 2850 will carry a window sash weight range of 28 through 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.
A home owner from San Francisco CA says:
What's the difference between series 351 and 390 channel balances. They both have the same tops and shoes. I need a replacement balance for an International Window.
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Dave Sr. from SWISCO responded:
The 351 series has a 15-002 shoe, whereas the the 390 series has a 15-001 shoe. A lot of the balances have subtle differences because the same way window manufactures make subtle differences in their window.
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