Hello there. Your old pivot shoe isn't made any longer, but the 15-177 should work as a substitute. This setup uses Series 152 Coil Balances, sold separately. If the window definitely weighs 17-18 pounds, then two of the S152-9 9 pound coil springs will work, one for each side of the sash. This will equal 18 pounds of support overall. That overall weight must be within 1 pound of the exact sash weight, or else your window will not function correctly, so be sure to double-check the sash weight on a scale if you have not done so.
