Older Pella single hung window from Menards

Hi there. These old crossbow balances are no longer made, but one of our tilt window channel balances can replace it. To know which, can you tell me the diameter of the spring? That'll narrow it down. 

Thank you. I would recommend using our Series 795 channel balance. This isn't strictly made to work in your window, but we've found it to be a suitable alternative as long as you don't mind doing a little handiwork to get it functional. The video below will explain what needs to be done, step by step.