Window Coil Balance

I agree, the S140-8 looks like a good choice. Don't worry about the number of springs, focus instead on how much weight they support. It doesn't matter if a balance has four springs or just one, as long as they hold up the same amount of weight.

For example, two S140-8 balances supporting the same sash can carry 15 through 16.99 lbs. As long as your sash falls in that range, and as long as the thickness and width of the S140-8 matches your original hardware, then I think it will work well.