8. O-Ring, No Metal Detectable, No X-Ray Detectable, System of Measurement Metric, Cross Section Shape Round, O-Ring Material Viton, Durometer (Hardness) Rating Medium Hard, Nominal Inside Dia.4 mm, Color Black, Durometer 75 Shore A, Meets/Exceeds ASTM D2000, SAE J200, Nominal Outside Dia.13.