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A valve is a device for controlling flow through an opening. The
internal combustion engine, which is basically an air pump, depends
on the efficient sealing of the valves in order to produce
compression. The timing of when the valves open, and the duration of
their opening, affects engine operation. The cam dictates these two
factors. The following terms describe the major components
associated with the valves.
"Valve clearance" is the gap between the end of the valve stem and
valve lifter or rocker arm to compensate for expansion due to heat.
Engines with hydraulic lifters often do not need valve clearance
adjustments because the lifters automatically take up the slack.
The "valve face" is the part of the valve which mates with and rests
upon some seating surface. The "valve head" is the portion of a valve
upon which the valve face is machined.
The "valve lock" (also called the key, keeper, or washer) is a device
which holds the valve spring in place on the valve stem.
"Valve overlap" is an interval which is expressed in degrees
where both valves (intake and exhaust) in each cylinder are open at
the same time.
The "valve seat" is the part of the cylinder head upon which the
valve face rests. These are precision ground to mate with the valve
face and thereby seal the cylinder.
The "valve spring" is attached to the valve to return it to its seat
after lift is released.
The "valve stem" is the longest portion of the valve which passes
through the valve guide.
The "valve guide" is the sleeve through which the valve stem passes.
It is pressed or threaded into the cylinder head, and is self
lubricated by the composition of its materials. Older cars depend on
the lead in "Regular" gas to lubricate the guides.
The "valve timing" refers to the relative position of a valve (either
open or closed) to the piston in its travel, in crankshaft degrees.
The "valve train" is the complete set of mechanisms used to transmit
the rotating motion of the engine crankshaft to the reciprocating
valve stem, causing the valves to open.
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