A metal insert is provided for use in forming an elastomer-tipped armature to a precise length. The insert is formed as an elongated member with a central hole passing through its length, and an elastomer element is molded within the hole. A circumferential groove is formed in the exterior surface of the insert to create a weakened "hinge" point in the insert wall such that, as mold elements for forming the elastomer element engage opposing ends of the insert, the insert may be compressed longitudinally at the groove to thereby decrease the overall length of the completed armature to within a predetermined tolerance.