![]() ![]() What makes these cast iron parts unique is that they are a double-walled pipe within another pipe. ![]() The exhaust hose is then attached to the aft end of the riser or elbow. Sometimes the riser slopes down from the end of the manifold, if the engine sits high enough above the waterline, in which case it’s often called an elbow. The riser, which is shaped like an inverted “U,” is sometimes located at the aft end of each manifold (e.g., on Chrysler engines), and sometimes it’s centered on top of the manifold (e.g., on MerCruiser engines). All V-8 engines, for instance, have a separate exhaust manifold along the side of each cylinder bank. Keeping the Water and Gas SeparatedĮxhaust manifolds and risers are large metal castings that carry hot exhaust gasses away from the engine block on inboard engines. Because the engine is often the single most expensive part of your boat, it makes sense to inspect or replace the risers and manifold periodically before an internal leak occurs, which is more a question of “when” than “if.” Once it happens, there is usually little or no warning before the engine is ruined. It can ruin your day, and much of your summer by the time busy mechanics get around to a total engine rebuild (if possible) or replacement. More likely, saltwater passed into the cylinders through a leak in either the raw-water cooled exhaust “manifold” or the “riser.” Once water gets inside the cylinders, the result is usually catastrophic engine failure. There are several possibilities, but if you guessed “Act of God,” try again. How did water get into the cylinders? (Hint: It’s not supposed to be there.) Later, when the engine is tornExhaust Riser down for inspection, it is discovered that the #4 piston connecting rod is bent and the cylinder walls are rusted due to water intrusion. Eventually he removes the spark plugs and discovers water in the # 2 and #4 cylinders (hmm…). He continues to run the engine briefly, and then shuts it down to investigate the cause. When it finally does start, he notices a distinct knocking noise which he’s never heard before. Pop Quiz: The owner of a 30′ powerboat hops aboard his boat one day to go for a cruise, but his inboard V-8 engine is slow to start. ![]() Keeping a Wary Eye on Raw-Water Cooled Manifolds and Risers Can Prevent the Premature Death of Your Engine ![]()
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