A wellhead seal assembly forms a metal-to-metal seal between inner and outer wellhead members. A metal seal ring has inner and outer walls separated by a slot. A lock ring has grooves for locking the inner wellhead member to the outer wellhead member during installation. An energizing ring has a C-ring captured on its inner surface. When the energizing ring is moved further into the slot, the C-ring is forced from its pocket and engages the surface of the inner wellhead member, locking the seal assembly to the inner wellhead member.