Are you surprised to hear that the piston shape is not round, rather it is oval, which means the head head is smaller than the piston skirt.
The first time I heard this, I have to use a venier caliper to measure the piston head and the skirt, it was then I discovered that the piston head is smaller than the skirt. I know you will like to know why. Lets go to that below.
The piston travels inside the cylinder, the clearance between the piston and the cylinder must be accurate, not too much not too tight.
If the clearance is too tight then compression will leak downward and oil will also be consuming by the engine.
If the piston is too tight on the sleeve, it will also prevent smooth movement of the piston which can cause friction between the piston and the cylinder.
Due to the heat that comes to the piston during the combustion, the piston expands and becomes tighter in the cylinder.
But you should know that the piston head which is regarded as the piston crown receives more heat than the piston skirt. So the piston head expands more than the skirt when the engine is hot due to the amount of heat that the portion receives.
Therefore during the expansion, the piston head also becomes bigger and equals the size of the piston skirt.
Now let’s think; if the piston head and the skirt are equal size, before expansion, then when the piston becomes hot, the piston crown will becomes bigger than the skirt.
But because the skirt is bigger than the head before expansion, when the piston becomes hot the head expands the most and that makes the head and the skirt becomes equal.
You can now understand why the piston head is smaller than the piston skirt.
No wonder without the piston rings, if you put the piston inside the cylinder, it look smaller. When the engine start running and becomes hot, the piston expands and fills the circumference of the cylinder.