Acts as a check valve to keep following seas/water from washing back through exhaust to engine when engine is not running or at low RPM. Shutter is 2.48" diameter at cross pin (hinge pin). Stern drive exhaust water shutter 807166A1 is for use on MerCruiser stern drives to keep following seas/water from washing back into the exhaust system and into the engine when engine is not running or at low RPM. Shutter is 2.48" diameter at cross pin (hinge pin)