Lesson 1/Learning Event 1
Valves
When an engine is operating, the fuel-air mixture is continually entering the combustion chamber through
intake ports. The burned mixture or exhaust is expelled through exhaust ports. When combustion occurs, the
inlet and exhaust ports into the combustion chamber must be closed to prevent leakage of the expanding gases.
Metal doors called valves are placed in the port openings to automatically control the flow of gases and to seal
the port openings. These are opened and closed by the valve operating mechanism that will be discussed later.
Valve Construction. A valve is made with a stem and has a head to seal the round openings of the intake and
exhaust ports. The sealing surfaces of the valve and valve seat are accurately machined so it can make a
perfect seal.
Valve Spring and Guide. The stem of the valve is fitted into a guide to ensure the valve rests squarely on its
seat each time it closes. A valve spring encircles the valve stem and provides tension to hold the valve head
tight against its seat. The spring is attached to the valve by a retainer cap and valve spring locks or keepers that
fit in grooves in the end of the valve stem.
Piston Pin
To harness the force exerted against the piston, a connecting rod is coupled to the piston by means of a piston
pin. The pin passes through holes or bosses in the piston and a hole in the rod. Once connected, the rod is
free to swing back and forth under the piston by turning on the pin.
Connecting Rod
In addition to the piston-pin-hole at its top, the connecting rod has a larger hole at its bottom. The larger hole
is split so the bottom part or cap can be removed. This is so that it can be clamped around the machined
bearing surface on a shaft (crank-shaft) using bolts. The inside of the hole is generally machined to accept a
two-piece bearing.
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