A well packer has a metal seal which seals between the packer mandrel and the casing. The metal seal includes a seal sleeve located on the exterior of the mandrel. The sleeve has an inner wall and an outer wall radially separated by a channel. The inner channel is initially separated from the exterior of the mandrel by a clearance. An energizing ring is sizably mounted to the mandrel for axial movement relative to the mandrel. Initially, the energizing ring will be located at the entrance of the channel. An actuating device moves the energizing ring into the channel, wedging the walls of the seal sleeve apart. The inner wall seals against the mandrel. The outer wall seals against the casing.