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Hydraulic
Lifters
Hydraulic lifters are used most often since they can reduce valve
train noise by maintaining zero valve clearance (no spacing between
valve train components.) The oil filled lifters adjust automatically
for changes brought on by temperature variations and wear of parts.
Engine motor oil fills the inside of the lifter, pushing the lifter
plunger up until all the play in the valve train is removed.
Mechanical Lifters
Mechanical lifters, also called solid lifters, simply transfer cam
lobe action to the push rod. They do not contain oil and are not
self adjusting. As a result, they require periodic adjusting. Valve
trains using mechanical lifters are prone to a clicking or clattering
noise as the valves open and close. This is why hydraulic lifters
are more common.
Roller Lifters
Roller lifters are either mechanical or hydraulic. Designed into
the lifter is a roller that rides the cam lobe, reducing friction
between the camshaft and lifter. Friction between these two components
is one of the highest friction points in an engine.
Spring
Retainer
The spring retainer is designed to hold in place the valve stem
tip. This allows the rocker arm to act directly on the valve.
Timing
Chain
Timing chains are becoming the standard for turning both the intake
and exhaust camshafts. The chains are located on the engine's front
end, and are pulled by a drive sprocket (1), which is turned by
the engine's crankshaft. Timing chains are required for both the
intake cam sprocket (2) and the exhaust cam sprocket (3). A chain
guide (4) is also provided. Some engines still use belts instead
of chains. In either case, excess slack or play will degrade engine
performance.
Timing
Belt
A timing belt instead of a timing chain may be used to turn the
camshafts. The inner side of the belt is designed with square (cogged)
teeth which prevent the belt from slipping. The belt should be checked
periodically for wear and proper tension.
Belt Tensioner
The belt tensioner is a spring-loaded wheel which keeps the timing
belt in tension and aligned with the cam sprocket. The smooth side
of the timing belt rides over the tensioner. The tensioner applies
a force on the backside of the belt. This keeps the belt in tension.
Whenever the belt needs to be removed, the tensioner can be pulled
away, freeing the belt.
Valves
Each
cylinder has at least one intake valve (1) and one exhaust valve
(2). Some engines are designed with two sets of valves per cylinder
as shown in the photo. The intake valve has a larger diameter than
the exhaust valve, which maximizes air flow to the cylinder. The
exhaust valve must withstand higher temperatures than the intake
valve since the air flowing past the intake valve keeps the intake
valve at a lower temperature. However, both intake and exhaust valves
must transfer their heat to the cylinder head otherwise they will
burn.
Valve
Springs
Valve springs (4) provide a resisting force that returns displaced
valves to their closed position. The spring may be a one-coil design
or a two-coil design which has inner and outer coils. The second
coil increases the amount of force holding the valve closed.
Sodium Filled Valves
Sodium filled valves are used when extra cooling is required. The
hollow valves contain sodium which melts during engine operation.
Valve action causes the sodium to circulate, removing heat from
the valve head. The heat travels up the valve stem (3) and is transferred
to the cylinder head. Coolant channels in the cylinder head (shown
in green) carry the heat away.
Stellite
Valves
Stellite valves have a hard metal coating that prolongs valve life.
Back in the days of leaded gasoline, the lead additives coated the
valves, providing added protection. With unleaded fuels now being
burned, the hard metal coating does the protecting.
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