12711 Mercruiser SHAFT

1.1 Engine Proper
(1) Combustion chamber The combustion chamber is made up of the cylinder head and piston top. The hole type injection nozzle as well as the nozzle tube is mounted on the cylinder head. The nozzle tube holds the nozzle in position and protects the nozzle from coolant. Since the outside of the nozzle tube is exposed to the water jacket, the top end of the tube is sealed off with an O-ring and the bottom end staked to prevent entrance of water.Combustion is accomplished by compressing the fuel directly injected into the combustion chamber.On 6D15 and 6D15-T engine, a valve recess is provided at the cylinder top to ensure clearance between piston and exhaust valve.(2) Valve mechanism (a) Both the intake and exhaust valves are made of surface-treated heat-resistant steel. The valve seat angle is 45° for both valves. Valve stem seal is mounted to the valve stem, which controls the amount of lubricant on the sliding surfaces between the valve and valve guide.A valve guide with carbon cutter is used for the exhaust valve.(b) Two unevenly pitched valve springs are used to prevent chattering at high speed. The inner and outer spring coil directions are opposite to each other.(c) The rockers are made of precisely forged carbon steel. The end sliding portions of the rockers has been quenched. The rocker shaft is a hollow round rod sealed off with expansion plugs at both ends. The inside of the shaft serves as an engine oil passage.(d) The push rod has a steel ball welded to its bottom end and a spherical or depressed piece welded to its top end. Both ends are carburized case-hardened. (e) The tappet is of cylindrical shape. Its mating surface with the camshaft is a large-diameter sphere to prevent local wear. The tappet can be removed by removing the cylinder head without removing the camshaft.(f) Excellent reliability at high speed has been achieved by adopting a high camshaft which permits the use of shorter push rods with high rigidity.(g) To facilitate the insertion and removal of the camshaft from the rear end of the case, the bearing I.D. is narrower toward the front.(3) Crankcase and cylinder liner (a) The crankcase is made of cast iron, provided with ribs to ensure high rigidity, and built with minimum stress concentration and deformation.(b) The oil main gallery is located on the right side as viewed from the front and supplies the engine oil that has passed through the oil filter and oil cooler to various sections. Engines other than the 6D14 and 15 series have an oil jet provided for each cylinder to cool the piston.(Refer to Group 12 LUBRICATION.)(c) Cooling water that has entered the water jacket cools the cylinders before it flows to the cylinder head. (d) The cylinder liner other than 6D16, 6D16-T engines is of the wet type that is removable. It engages with the crankcase at the top of the crankcase and at the bottom of the water jacket. An O-ring and rubber packing are used to prevent water leaks. 6D14 and 6D14-T engines have an O-ring groove provided on the crankcase side and 6D15 and 6D15-T engines on the cylinder liner side. The cylinder liner for 6D16 and 6D16-T engines is of the dry type that is mounted with the crankcase so that it can be easily removed.A size mark is stamped on the side of the cylinder liner collar to permit a correct fit with the crankcase.(4) Piston and piston ring(a) Piston The piston is made of an aluminum alloy casting with a trochoidal type combustion chamber (6D15-T, 6D16-T) or re-entrant type combustion chamber (other models) provided at the top.Engines other than 6D14 and 6D15 have a piston ring insert cast in the 1st ring groove for higher durability. The part No., weight, and size mark are stamped on top of the piston as shown above. A triangle mark indicating the piston mounting direction is also embossed on the piston.The piston pin is mounted using the full floating method, offset from the cylinder center. The pistons for the 6D16 engine have an internal strut to reduce piston slap noise.(b) Piston ring The piston ring set consists of three rings, namely, two compression rings and one oil ring, all these having chromium plated sliding surfaces for higher wear resistance.The shapes of piston rings are as illustrated.(5) Connecting rod and connecting rod bearing The connecting rod is a die-forged part with I-section. It has a lead bronze bushing press fitted at the small end and a split type plain bearing fitted at the big end of the connecting rod.This bearing is made of kelmet with a soft copper back plate whose inside surface has been plated with a lead, tin and copper alloy before plating the entire bearing with tin.The connecting rod and its cap are connected by knurled connecting bolts.For lubrication of the small end, the stem has a diagonally oriented oil passage. The connecting rod for 6D14-T, 6D15-T and 6D16-T has its small end wedge shaped to decrease piston boss plane pressure, thereby reducing mechanical stress in the piston.(6) Crankshaft and main bearing(a) Crankshaft The crankshaft is die-forged integrally with the balance weight and is supported by the main bearing in the crankcase.The pin and journal are induction hardened for higher wear resistance. An oil hole passes between the journal and pin, and part of the main bearing lubrication oil flows through the hole to the pin for lubrication of the connecting rod bearing.At the rear end of the crankshaft, the crankshaft gear is engaged to drive the timing gear.The crankshaft has oil seals fitted at its front and rear ends. The rear oil seal type is as follows. * Without flywheel PTO: Axial lip type* With flywheel PTO: Radial lip type The crankshaft has a pulley and torsional damper at the front end. The damper prevents torsional vibration of the crankshaft.(b) Main bearing The main bearing is a split type plain bearing made of the same