Problem solved
I used a small saw and a chisel to cut a little notch in the sill, 3/8" wide and 1/4" deep. That was enough to allow the pin to clear the track at the bottom. It took all of 5 minutes. The sill is made of a wood-like material that was easily shaped. The window is now outside and I rewashed the window frame with vinegar as per someone's advice here. Thanks to all for your help!
|