Anatomy Lesson: Motorcycle Pistons

Dissecting motorcycle components

Modern pistons are cast or, less often, forged from aluminum alloy. In low stress applications, a "eutectic" alloy (containing roughly 12% silicone) is used, but when something stronger is required, a "hypereutectic" version beefed up with 12.5 - 16% silicone is preferable. Forged pistons, which are stronger though slightly heavier, are often used in high performance/race applications

Piston Skirt
The skirt extends downward from the wrist pin boss, and prevents the piston from cocking in the bore.

Wrist pin bore
The wrist pin passes through this hole to connect the piston to the connecting rod. Small clips or buttons secure the pin in place. The thick area supporting the hole is called the wrist pin boss.

Piston Crown
The piston crown forms the bottom of the combustion chamber. The shape of the crown dictates the actions of the incoming fuel/air charge during the compression stroke so traditional flat top pistons have largely been replaced by dished, domed and contoured pistons that are better able to help promote "fuel swirl." Swirling the fuel provides better fuel atomization, which reduces emissions, while increasing power and economy.

Valve cutouts
To prevent contact between the valves and the piston, reliefs are cut into the piston crown.

Ring grooves
Machined cutouts where the rings ride. Typically, a four- stroke engine will have three grooves: two for the compression rings located near the top of the piston, and one for the oil control ring below them.

Oil return holes
These serve as return passages for any oil that the oil ring scrapes from the cylinder walls.

Anatomy of Motorcycle Pistons - Motorcycle Cruiser Magazine