840838T 1 Mercruiser COVER ASSEMBLY, Top

Piston, Rings And Connecting Rods
One-piece aluminum pistons are used in most applications. Engines with higher cylinder pressures require one-piece steel pistons. Refer to the Parts Manual in order to obtain information about the type of pistons that are used in a specific engine.Aluminum and Steel One-Piece Pistons
The aluminum and steel pistons have an iron band for the compression ring. The iron band helps to reduce wear on the compression ring groove. The pistons have three rings:
Compression ring
Intermediate ring
Oil ringAll of the rings are located above the piston pin bore. The compression ring is a Keystone ring. Keystone rings have a tapered shape. The action of the ring in the piston groove that is tapered helps prevent seizure of the rings. Seizure of the rings is caused by deposits of carbon. The intermediate ring is rectangular with a sharp lower edge. The oil ring is a standard type of ring or a conventional type of ring. Oil returns to the crankcase through slots in the bottom of the groove.Oil from the piston cooling jets sprays the underside of the pistons. The spray lubricates the pistons and the spray cools the pistons. The spray also improves the piston life and the spray also improves the ring life. The aluminum pistons use a single jet. The steel pistons use two jets. Refer to the Specifications, "Piston Cooling Jet" for more information.The connecting rod has a taper on the pin bore end. This taper gives the connecting rod and the piston more strength. The additional strength is concentrated in the areas with the most load. Two bolts hold the connecting rod cap to the connecting rod. This design keeps the connecting rod width to a minimum, so that the connecting rod can be removed through the cylinder. The rod and the original cap must be kept together.Crankshaft
The crankshaft changes the combustion forces in the cylinder into usable rotating torque which powers the vehicle. A vibration damper is used at the front of the crankshaft to reduce torsional vibrations (twist on the crankshaft). That force can damage the engine.The crankshaft drives a group of gears on the front of the engine. The gear group drives the following devices:
Oil pump
Camshaft
High-pressure fuel pump
Gear-driven air compressorIn addition, belt pulleys on the front of the crankshaft drive the following components:
Radiator fan
Water pump
Alternator
Refrigerant compressorHydrodynamic seals are used at both ends of the crankshaft to control oil leakage. The hydrodynamic grooves in the seal lip move lubrication oil back into the crankcase as the crankshaft turns. The front seal is located in the front housing. The rear seal is installed in the flywheel housing.
Illustration 1 g01449227
Schematic of oil passages in crankshaft
(1) Oil gallery
(2) Main bearings
(3) Connecting rod bearings Pressurized oil is supplied to all main bearings from the oil gallery (1) through drilled holes in the webs of the cylinder block. The oil then flows through drilled holes in the crankshaft in order to provide oil to the connecting rod bearings (3). The crankshaft is held in place by seven main bearings (2). A thrust bearing next to the rear main bearing controls the end play of the crankshaft.Vibration Damper
The force from combustion in the cylinders will cause the crankshaft to twist. This force is called torsional vibration. If the vibration is too great, the crankshaft will be damaged. The vibration damper limits the torsional vibrations to an acceptable amount in order to prevent damage to the crankshaft.Viscous Vibration Damper
Illustration 2 g01449268
Cross section of viscous vibration damper
(4) Crankshaft
(9) Weight
(10) Case The viscous vibration damper is installed on the front of crankshaft (4). The viscous vibration damper has a weight (9) in a case (10). The space between the weight and the case is filled with a viscous fluid. The weight moves in the case in order to limit the torsional vibration.Camshaft
The camshaft is located in the upper left side of the cylinder block. The camshaft is driven by gears at the front of the engine. Seven bearings support the camshaft. A thrust plate is mounted between the camshaft drive gear and a shoulder of the camshaft to control the end play of the camshaft.The camshaft is driven by an idler gear which is driven by the crankshaft gear. The camshaft rotates in the same direction as the crankshaft. The crankshaft rotates in the counterclockwise direction when the engine is viewed from the flywheel end of the engine. There are timing marks on the crankshaft gear, idler gear, and camshaft gear to ensure correct camshaft timing to the crankshaft for proper valve operation. As the camshaft turns, each lobe moves a lifter assembly. There are two lifter assemblies for each cylinder. Each lifter assembly moves a pushrod. Each pushrod moves a set of valves (inlet or exhaust). The camshaft must be in time with the crankshaft. The relation of the camshaft lobes to the crankshaft position causes the valves in each cylinder to operate at the correct time.