In a 2-stroke petrol engine, lubrication occurs by mixing oil with the petrol, so oil passages are not needed.
And if, like many 2-strokes, air-cooling is used instead of liquid-cooling, there is no need for coolant passages in the cylinder head.
Another difference between 4-stroke and 2-stroke petrol engines is how they deliver air-fuel mixture to the cylinder.
In a 2-stroke petrol engine, separate passages called ports let air-fuel mixture into the crankcase and cylinder, and exhaust gases out. They are covered and uncovered by the piston as it moves up and down the cylinder.
In the 4-stroke petrol engine, the inlet and exhaust ports are opened and closed by valves. These valves need a system to control how they work.
The valve is held in place by a valve guide, with a spring on its stem.
A rocker arm is attached to a stationary shaft that allows the arm to pivot. The rocker arm compresses and releases the valve spring, so that the valve opens and closes.
