Help needed

Hello there. It looks like you need a new spiral balance. First you have to determine the diameter of the outer tube. It should either be 3/8" or 5/8". Based on your first photo I would lean towards 3/8", but pictures of measuring tape can be deceiving, so I'd double and triple-check to be absolutely sure. If it's 3/8", you need a Series 900; if it's 5/8", a Series 600. Next, determine the length of the outer tube, up to and not including the colored tip. It looks to me this is roughly 28", but again, please confirm. That would mean you need either a S900-28 or S600-28. Finally, you have to select a color tip option, which designates the strength of the balance. It's possible the color on your spiral has faded from age. The best thing to do is to obtain the exact weight of the window sash. You can then match that exact weight with one of the weight ranges listed alongside each color tip option under the product description on the spiral balance's product page. Let me know if you have further questions.