What weight spring do I need?

Hello,

If you could flip over the pivot shoe and coil in your second photo to show us the front, that should give us a better idea of what you need. 

There should also be a stamp at the end of the coil where it attaches to the shoe. If not, you'll need to weigh the window sash and divide the weight by the number of coils in the window to choose the correct strength option. Thank you!

Your pivot shoe resembles our 15-177, which is designed to work with our Series 152 coil spring. You can probably reuse your old shoe and cover, since they look to be in good condition, but you definitely want a new Series 152 coil.

To determine the strength option, you'll need to weigh the window sash as you mentioned and divide the weight by the number of coils on both sides of the window. 

Great, see our S152-7