390 Window Balance Length vs Stamp Question

Question

I have identified the required part as the 390 model. Here's the question. the metal balancer is 18", but stamped on the old part is 17A, indicating 17". Any idea as to why the variance?

Bob · Accepted Answer

Thanks for posting. Good question! The difference is simply a feature of the rating system these channel balances go by. Generally, for the Series 390 the first two digits of that stamped number is 'one less' than the actual length of the metal channel; exceptions occur with custom balances or where the original installer modified the balances to fit his or her needs. In any event, the stamped code actually refers not to the balance length specifically, but rather to the weight of the window sash the balance is meant to support. So if you are looking for a S390-18, your stamped code of 17A correlates with our listed stamp option of 1710, which is for a window sash weight range of 5 through 8 lbs.