cylinder bank 17 connecting to a corresponding exhaust gas chamber 23 the exhaust gas chambers 23 open downwardly through openings 24 at the bottom of the block 11 and discharge into exhaust passages in the lower unit of the outboard motor not shown the engine block 11 is sand cast from aluminum using sand cores each exhaust gas chamber 23 and its corresponding port extender passages 21 are formed using a single core since operation of the engine 10 generates substantial heat a water cooling system is provided with cooling passages arranged to provide a relatively even temperature distribution throughout the engine block 11 and cylinder heads 12 as more fully explained in the copending application of the inventor entitled 34 v engine cooling system particularly for outboard motors 34 filed on the same day as this application and assigned to the same assignee the crankcase 14 is divided into compartments 25 one for each cylinder 16 by the crank disks 26 on the crankshaft 15 which support the crank pins 19 each compartment 25 is provided with its own inlet passageway 27 to supply air fuel mixture from carburetors not shown to be compressed in the crankcase compartments 25 from the crankcase 14 the air fuel mixture is directed to the cylinders 16 via the transfer ports 28 29 and 30 arranged to provide loop scavenging as taught in u.