On the fourth stroke, the burned gases are sent out through the exhaust system. The wrist pin connects the piston to the connecting rod. The connecting rod comes up through the bottom of the piston. Pistons are made of aluminum, because it is light and a good heat conductor. Pistons perform several functions.
Pistons transmit the driving force of combustion to the crankshaft.
This causes the crankshaft to rotate. The piston also acts as a moveable gas-tight plug that keeps the combustion in the cylinder. The piston acts as a bearing for the small end of the connecting-rod. Its toughest job isto get rid of some of the heat from combustion, and send it elsewhere. It may be flat, concave, convex or any one of a great variety of shapes to promote turbulence or help control combustion.