The 161 Series is a great replacement for standard large-diameter O-rings. This heavy duty static seal has a defined sealing lip manufactured from high-performance polyurethane. The square body profile prevents rolling in the groove, which is common with O-rings. The robust qualities of polyurethane allow the seal to be used without a backup ring. It is a substantial upgrade to the traditional O-ring and backup ring combination, which has been prone to rolling installation problems.