Crankshaft; Piston And Connecting Rods; Camshaft And Drive; Cylinder Head - Mercury MerCruiser MCM 454 Mag MPI Service Manual

454 CID (7.4L) / 502 CID (8.2L)

Crankshaft

The crankshaft is supported in the block by five insert type bearings. Crankshaft end thrust
is controlled by flanges on the No. 5 bearing. A torsional damper on the forward end of the
crankshaft serves to help dampen any engine torsional vibration.

Piston and Connecting Rods

Piston pins are offset slightly toward the thrust side of the pistons to provide a gradual
change in thrust pressure against the cylinder wall as the piston travels its path. Pins have
a floating fit in the piston and a press fit in the connecting rod (to hold them in place).
Connecting rods are made of forged steel and are connected to the crankshaft through
insert type bearings.

Camshaft and Drive

Roller lifter camshafts are made of steel. All camshafts are driven at one-half crankshaft
speed by a timing chain and sprockets, or by timing gears, and are supported by five main
bearings, which are pressed into the block.
A helical gear on the aft end of the camshaft drives the distributor and oil pump.
On engines with cast iron camshaft and flat faced lifters, a taper on the lobes, coupled with
a spherical foot on the hydraulic valve lifters, causes the valve lifters to rotate, thus reducing
wear.

Cylinder Head

The cylinder heads are made of cast iron and have individual intake and exhaust ports for
each cylinder.
Stainless steel or graphite composition head gaskets are used to retard corrosion.

Valve Train

The valves and valve springs are of a heavy-duty design to withstand the high engine
speeds encountered. Valve tips have been hardened to extend valve life. Exhaust valve
rotators are used on 7.4L engines.
Hydraulic valve lifters ride directly on the camshaft lobes and transmit the thrust of the lobes
to the push rods which in turn actuate the valves through the rocker arm.
In addition to transmitting thrust of the cam lobes, the hydraulic lifters also serve to remove
any clearance (lash) from the valve train to keep all parts in constant contact.
The valve lifters also are used to lubricate the valve train bearing surfaces.

Lubrication System

The engine lubrication system is of the force-feed type in which oil is supplied under full
pressure to the crankshaft, connecting rods, camshaft bearings and valve lifters, and is
supplied under controlled volume to the push rods and rocker arms. All other moving parts
are lubricated by gravity flow or splash.
A positive displacement gear-type oil pump is mounted on the rear main bearing cap and
is driven by an extension shaft from the distributor which is driven by the camshaft. Oil from
the bottom of the pump in the rear of the oil pan is drawn into the oil pump through an oil
pickup screen and pipe assembly.
If the screen should become clogged, a relief valve in the screen will open and continue to
allow oil to be drawn into the system. Once the oil reaches the pump, the pump forces the
oil through the lubrication system. A spring-loaded relief valve in the pump limits the maxi-
mum pump output pressure.
Index
Page 3A-18
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SERVICE MANUAL NUMBER 23
90-861326--1 MARCH 1999