Exhaust Flapper With Clamp 4 in. | Sierra 18-4457Exhaust Flapper With Clamp 4 in. Sierra 18 4457 Sierra 4 in. Exhaust Flapper With Clamp 18 4457 Exhaust protector valve mounts on the outside of 4 in. exhaust tip to prevent backflow of water into the exhaust. For 4 in. diameter exhaust thru hulls Prevents backflow of water into exhaust system Stainless steel clamp included Designed to mount on the outside of exhaust tips Replaces Mercury 12644A1, 12644Q01 Replaces OMC Johnson Evinrude 509495 Replaces