Eagle window (circa 2005) balance spring lower sash ( U or O B 26REG )

I believe you have a Marvin wood tilt window, some of our Marvin customers have been successful in replacing their damaged balances with our Series 751 channel balance. You'd need the S751-27 with a strength option of 26R. 

To replace your balance, you will need to raise the bottom sash up about six inches, tilt the sash in and twist it to release the bottom sash pivot bars from the terminal clips. Now, carefully remove the vinyl jambs from the window wood frame, they're held in by the pressure of the sash. After this, you'll lay the jamb liner down and slide up the broken balance from the vinyl track and remove the "S" clip from your old balance to install it on your new balance. You'll need to slide the new balance down the track while keeping the 15-014 terminal clip from catching inside the track. Then, hook the "S" clip on the track and pull down the terminal clip about 6 inches from the balance and lock it in place. Once you've replaced all the broken balances, you'll need to re-install the jamb liner and sashes.