Back-up Rings are pure hard rubber piece with concave on one side relative to the o-ring shape, having no intended sealing function. Instead, as their name indicates, they are protective and supporting elements made from extrusion-resistant materials. They are installed in a groove together with an elastomeric sealing element preferably with a corresponding O-Ring, in both dynamic and static applications, to prevent extrusion of the O-ring when it is subject to high pressures or elevated temperatures. Elevated temperatures (160° F and higher) impose a severe burden on the physical and mechanical properties of rubber O Rings which further increases their tendency to extrude. Two back-up rings should be used with one on each side of the primary seal. Due to the tight fit of the Back-up Ring in the housing, they prevent extrusion of the pressurised elastomeric sealing element into the sealing gap.