Piston Crown

The piston crown is the top surface of the piston, which faces the combustion chamber and takes the full heat and pressure of combustion. Below the crown sit the ring lands with the piston rings, then the skirt.

Crowns can be flat, domed or dished, and often have valve reliefs, small pockets that give the valves clearance in four-strokes. The shape affects the compression ratio and how the flame spreads. In two-strokes, the crown is usually domed to match the cylinder head.

The look of the crown says a lot about engine health. A light, even carbon layer is normal; heavy oily deposits suggest oil burning, while pitting, erosion or a melted spot points to detonation or a lean mixture. Check the crown whenever the head is off, and don't scrape it with sharp tools.