160 or 165 coil

Thanks for posting. Good question. The Series 165 is 3/8" wide, whereas the Series 160 is 1/2" wide. Measure the width of your old spring's coil tape to determine which you need. As far as the spring weights you need, it will depend on what coil spring Series you have. For an 18 pound window, you could use either (2) S160-09 9 pound springs; or, the S165-5 5 pound spring and S165-4 4 pound spring, a pair for each side of the sash. Let me know if you have further questions.