This specification covers virgin, unfilled polytetrafluoroethylene (PTFE) in the form of extruded rods, tubes, and profiles. These products have been used typically for parts, such as bushings and insulators, requiring better chemical inertness and dimensional stability up to 500 °F (260 °C) than AMS 3657 and better mechanical and electrical properties than AMS 3656, but usage is not limited to such applications. For spplications such as bearings, seals, and back-up rings that do not require dielectric properties it is recommended to use AMS 3678/1 Grade B.