If That Is A Piston, What Is This?

Modern motorcycle pistons are thin, miniskirted wonders

Why have pistons atrophied to almost nothing? In this example from a Honda CRF450R motocrosser, all we have is a thin disc that is enough to locate the piston rings, two bosses for the wrist pin that connects the piston to the connecting rod, and little skirts.

The up-and-down motion of the piston produces a shaking force. You cannot balance all the shaking force of an engine by adding counterweights to the crankshaft. You can reduce the vibration, but you can’t make it go away.

Also, in a very high-rpm engine, moving up and down as it does, the piston produces very large bearing loads. Those bearing loads translate into friction, which is subtracted from the horsepower that your engine can send to the rear wheel.

Manufacturers therefore make pistons as light as possible, getting rid of everything that isn’t necessary. This particular example is large, but the center of the dome is quite thin. What happens to the heat of combustion that is collected by this surface?

With traditional pistons, that heat is conducted out to the cylinder wall. But this piston dome is large and that is a great distance for heat to flow. Normally, to conduct all that heat away from the center of the dome, the dome would be thick—a broad avenue for heat.

Instead, a small oil jet is placed in the crankcase, which sprays oil underneath the piston, hits one side, splashes across under the dome, is deflected down the other skirt, and falls back into the crankcase. That oil cools the piston dome, allowing it to be made thinner.