Old Wood window balance installtion

I suggest going back to the original balance system used by millworks when making wood windows: the Series 500 5/8" spiral balance with a 17-012 bottom bracket. You will need to rout out the side of the sash to take a 5/8" tube balance, however. Below are generic directions for these balances:

To install these balances, you would drop them from the top, on the side of the sash. Use 33-017 screws to attach the balance tubes at the top. Now raise the sash, and have someone hold it up while you attach the 17-012 bottom bracket with a 33-090 screw, while the U-shaped opening corrals the spiral. With your finger, bring the spiral up to the bracket, attach the 75-004 charging tool, and turn the spiral clockwise three turns before hooking it to the fork of the bracket. Repeat on the other side, and try the sash. If sash creeps down, raise and hold up the sash while giving the balances another turn or two.

Let me know if you need more information.

The rule of thumb when it comes to balance length for wood windows top sashes is to add 1" to the visible glass height, and for wood window bottom sashes to add 3" to the visible glass height. What is the visible glass height for the 34.5" sash?

Great! Thank you. I would round up the 33.5" to 34". With that in mind, I'd recommend our S500-35 for the top sash and S500-37 for the bottom sash. There's a little leeway with spiral balances, in terms of length, so you should be okay.