new balances, window is very tight.

I'm not sure what the issue could be. A 29 5 option usually corresponds to our 2950 option. I would weigh your sash just to confirm. We have a chart on the S385-30 store page that shows you how much weight each option can support. There is a chance that you need another option.

Another possibility is that the Series 385 is not a good match for your old balance. If you'd like, you can upload pictures of your old balance here for us to consider. We would need to see one clear picture of the top nylon attachment and one clear picture of the bottom nylon attachment.

Thank you. I have received your pictures, and can't see any reason why the S385-30 couldn't work. If your sash is exactly 36 pounds, though, then the 2950 may not have been the best match, since it stops short of 35 pounds. The 2965 option can carry 35 through 39 pounds, so that may be better.