Modern pistons like KTM pistons are cast and less often, forged from aluminum alloy. In low stress use, a eutectic alloy with about 12% silicone is used, but when we need something stronger, a hyper eutectic version with up to 13% silicone is required. Forged pistons are stronger but slightly heavier and often used in high performance applications like races.
Piston Skirt of KTM pistons
The skirt of KTM piston extends downward from the wrist pin boss, and prevents the piston from cocking in the cylinder bore.
Wrist pin bore
The wrist pin passes through this hole to connect the piston to the connecting rod. Small clips secure the pin in place so it can’t move and damage cylinder bore.
The piston crown forms the bottom of the combustion chamber in KTM cylinder. The shape of the crown dictates the actions of the incoming fuel and air charge during the compression stroke so traditional flat top pistons have largely been replaced by dished, domed and contoured pistons that are better able to help promote fuel swirl.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. Way to go!
To prevent contact between the valves and the piston, reliefs are cut into the piston crown. This is not so common and in used only for few KTM bikes.
…are cutouts where the rings sits. Typically, a four-stroke engine will have three grooves, two for the compression rings at the top of piston, and one for the oil control ring below the two compression rings.
Oil return holes
These serve as passage for any oil that the oil ring scrapes from the cylinder walls.