In this video, This Old House general contractor Tom Silva shows how to install swap out broken or missing window security mechanisms.
1. Take photographs of the broken sash locks, and make a detailed sketch of the locks with key dimensions noted. Send photos and sketch to a window-parts dealer, who will provide you with exact-match replacement locks.
2. Remove the old sash locks by drilling out the rivets that hold the locking mechanism in place. There's no need to remove the keeper, unless it, too, is broken.
3. Set new locking mechanism in place on the sash and secure it with pop rivets. Drive the new rivets into the old rivet holes. Repeat for all other broken sash locks.