Typically made of Hytrel® or polyurethane, crown seals (see Figure 144) are dynamic seals energized by an O-ring. The tough outer “crown” imparts wear and anti-extrusion properties even as the resilient inner O-ring enables the seal to handle low temperature and low pressure applications. The narrowness of the contact line and the shape of the cross-section also help crown seals resist spiral failure. Though rod configurations are available, crown seals are mainly used in piston designs. Crown seals cost much more than O-rings.