DescriptionCummins 3935449 noise isolators set of six & 3921640 valve seal set of 12Cummins describes 3935449 as a “noise isolator” and it replaces the original O-ring that came on the factory-original bolt.This noise isolator mounts inside the valve cover (under) and serves as a bolt O-ring& helps isolate engine valve train noise from transferring through the valve cover.Cummins 3921640 Seal, Valve Stem 5.9L 6BT ’89-’98 12v