Shown here are a Ford 6.0L diesel head and a 6.7L Cummins piston. Forgive our mixing of parts (that may seem an unholy union to some) in this image. We are only using the two parts to show how the combustion chamber in a diesel is in the piston rather than the head. Looking up from below, the head is flat where the cylinder would be. Looking down on the dished piston also shows some melting at the edge of the piston caused by overheating the engine and continuing to run it. That has nothing to do with our example, but it is a cool illustration of the extremes these parts will see within an engine.