Series 770, what stamp? Weigh sash?

A home owner from Manahawkin NJ says:
Hi, I have a number of channel balances that need to be replaced. I have taken a few out and the only thing stamped on them is "2800." They are definatey the 770 series although I don't see any 28B,C,D etc. Would this mean I have to weigh the sash? P.S. Im replacing all of these because the spring ripped right through the plastic end. All of the windows have the rivets set for maximum tension. Should I lower these ones a notch or two to prevent this from happening again?
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Paul from SWISCO responded:
Thanks for posting! Yes, i would like to know the weight of your sash, as well as the length of your metal channel. This will help us determine what stamp you should choose for this Series 770 channel balance.

As for your balances ripping through the plastic end--it sounds like you didn't have the correct option for your balances installed in the first place. I don't recommend adjusting the height; hopefully, figuring out the proper stamp option will solve the problem.
A handy person from Manahawkin NJ says:
Hi. My balances are 28" long and the sashes weigh 20.8 lbs. thanks.
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Paul from SWISCO responded:
Thank you. Please look at our S770-28 with the 28C stamp option. This option can carry a window sash weight of 18 through, and including, 23 pounds.

Please keep in mind that the weight range 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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