A spark-ignition overhead valve type internal combustion engine has its cylinder or cylinders closed at the top by semispherical cylinder heads with the piston terminating in a crown of semispherical configuration sized slightly smaller than that of the cylinder head. Shallow recesses within the piston crown to opposite sides of spark plug apertures form dual combustion chamber with a recess within the piston crown aligned with and facing the cylinder head aperture, forms an initial ignition area, with shallow narrow grooves leading from the initial ignition area, within the piston crown, to the shallow combustion chamber defining recesses. The burning process of a fuel-air charge is slowed from the initial ignition area through the shallow grooves and then through the shallow combustion chambers to retard the explosion process.