ON 4 STROKE ENGINE (ANIMATION)

    4 STROKE ENGINE No.2 compression stoke and No.3 power stroke valves are closed.

No. 1 Intake
No. 2 Compression
No. 3 Power stroke
No. 4 Exhaust

     EXHAUST       SIDE ->

On No.2 compression and No.3 power stroke
the valves are closed. You will notice the
spark plug fires every other revolution.

You want the engine to be on No.3
at the end of the power stroke.
This way the piston is all the way down
and the valves are closed.

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An optional way to check that the valves are closed.
Have someone turn the engine over by hand with a 18mm socket from the front of the engine.
Turn the engine over until it is going up on the Compression  Place your thumb at the
top of the spark plug hole at the same time to block off the air. When you feel the engine
compression stop pushing air against your thumb the piston will be top dead center. Turn
the engine a little more to be going down on the Power stroke, both valves should be closed
at this point, and the piston should all the way down.

Information on today's combustion engines:
Today, internal combustion engines in cars, trucks, motorcycles, aircraft, construction
machinery and many others, most commonly use a four-stroke cycle. The four strokes refer

The cycle begins at top dead center (TDC), when the piston is
farthest away from the axis of the crankshaft. On the intake or
induction stroke of the piston, the piston descends from the top
of the cylinder, reducing the pressure inside the cylinder. A mixture of
fuel and air is forced (by atmospheric or greater pressure) into the
cylinder through the intake (inlet) port. The intake (inlet) valve (or
valves) then close(s), and the compression stroke compresses the
fuel–air mixture.

The air–fuel mixture is then ignited near the end of the compression
stroke, usually by a spark plug (for a gasoline engine) or by the heat
and pressure of compression (for a Diesel cycle or compression ignition
engine). The resulting pressure of burning gases pushes the piston
through the power stroke. In the exhaust stroke, the
piston pushes the products of combustion from the cylinder through an
exhaust valve or valves.