| Direct replacement for exhaust elbow/riser. Replaces MerCruiser part # B-76668A3, B-72964A1 and B-47877A4 (I/O applications) Exhaust Manifolds, Risers and Elbows Nearly every inboard and inboard/outboard (I/O) engine has exhaust manifolds and risers/elbows. Given the harsh environment of high temperature, saltwater and acidic exhaust gases in which these cast iron parts operate, their service life is often much shorter than most boat owners realize – often less than 10 years of regular engine use. Exhaust manifolds and risers are large metal castings that carry hot exhaust gases away from the engine on inboard engines. All V-8 and V-6 engines, for instance, have a separate exhaust manifold along the side of each cylinder bank. The riser, which is often shaped like an inverted U, is located at the aft end (or centered on top) of the manifold. 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 exhaust hose is then attached to the aft end of the riser or elbow. What makes these parts unique is that they're pipes within another pipe. This double-wall arrangement allows hot exhaust gases flowing in the internal pipe to be surrounded by cooling water in the external pipe, called a “water jacket”. At the aft end of the riser, water from the water jacket combines with and cools the hot exhaust gases before continuing out via the exhaust discharge. Without the cooling effect of the water, the exhaust gas would overheat the manifold and risers, then burn through the exhaust hose in short order. Keeping the water and exhaust gases separated until they exit the riser is crucial. If water finds its way into the gas-only chamber before the end of the riser because of a leak in the water jacket, it can seep into the cylinders when the engine is at rest and either seize the pistons with rust or create a "hydrolock" condition where the cylinders fill with water.
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