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The 9th Gen Civic Si's OEM radiator hoses are made from EPDM rubber. Unfortunately, in performance applications where coolant and under hood temperatures consistently higher than in normal driving conditions, EPDM degrades more quickly. Silicone is able to withstand temperatures over 100F higher than EPDM for sustained periods without losing strength or pressure holding ability. The upper hose features a unique design that provides additional clearance for use with an RBC or similar intake manifold where the throttle body is relocated and can interfere with the OEM upper radiator hose. These hoses feature 5mm walls with 4-ply polyester reinforcement. Hoses are SAE20R4# compliant.